Integrated Water–Soil–Nitrate Management Under Arid Conditions Using Mulching: A Composite Sustainability Index Approach

Abstract

Soil water availability, salinity dynamics, and nitrate transport are key factors controlling agricultural sustainability in arid environments characterized by limited water resources and high evaporative demand. This study evaluated the combined effects of soil texture, nitrate–nitrogen application, and sawdust mulching on soil water retention, evaporation losses, salinity redistribution, and nitrate movement in loamy sand and sandy clay loam soils under controlled greenhouse conditions. Results showed that soil texture was the dominant control on hydrochemical behavior, with sandy clay loam exhibiting higher water retention and lower drainage than loamy sand. Sawdust mulching significantly improved soil water conservation by reducing evaporation and stabilizing moisture distribution, while the 4 cm mulch treatment achieved the highest overall CSI performance. Evaporation strongly governed salinity accumulation in surface layers, whereas mulching reduced salt build-up and promoted a more uniform salinity profile. Nitrate transport closely followed water fluxes, resulting in higher leaching in loamy sand and greater retention in sandy clay loam. Increasing nitrogen application enhanced nitrate mobility and leaching in both soils. A Composite Sustainability Index (CSI) was developed to integrate soil water conservation, evaporation reduction, salinity control, and nitrate retention into a unified metric. Sensitivity analysis demonstrated that treatment rankings were largely unaffected by alternative weighting schemes, confirming the robustness of the CSI framework. The CSI identified mulch application, particularly the 4 cm mulch treatment, as the most effective management option based on overall sustainability performance. The CSI framework provides an integrated decision-support tool for evaluating coupled water–salt–nitrate interactions and improving water use efficiency and salinity management in arid agricultural systems. This study offers a novel integrated CSI-based framework for simultaneously quantifying hydrological and hydrochemical soil responses under mulch management in arid environments.

1. Introduction

Soil water, solute transport, and nutrient dynamics constitute the core processes governing agricultural sustainability in arid and semi-arid environments. These regions are characterized by limited precipitation, high evaporative demand, and fragile soil–water balance, making them highly vulnerable to salinization, nutrient depletion, and inefficient water use. Under such conditions, soil hydraulic properties and surface management practices become key controls of water availability and the fate of dissolved salts and nutrients within the soil profile [1]. Soil texture is a primary determinant of hydraulic behavior due to its influence on pore-size distribution, water retention capacity, and hydraulic conductivity. Coarse-textured soils, such as sandy loam, exhibit high permeability and low water-holding capacity, leading to rapid drainage and higher leaching risk. In contrast, finer-textured soils, such as sandy clay loam, show stronger capillary forces and greater retention of water and solutes due to higher microporosity and surface area [2,3]. These differences strongly influence water redistribution, evaporation dynamics, and solute mobility under both saturated and unsaturated conditions. Water retention and hydraulic conductivity functions are essential for describing soil hydraulic behavior under variable matric potentials. The van Genuchten–Mualem model provides a widely used framework for predicting soil water retention and unsaturated hydraulic conductivity [4,5]. These parameters are critical for quantifying water availability and simulating water and solute transport in agricultural systems, as they directly control evaporation losses, drainage behavior, and nutrient leaching potential.

Evaporation from bare soil is a major pathway of water loss in dryland agriculture and is controlled by atmospheric demand, soil hydraulic properties, and surface conditions. It typically occurs in two stages: an initial energy-limited phase followed by a diffusion-controlled phase as surface drying progresses. Capillary flow from deeper layers sustains evaporation while simultaneously driving upward transport of dissolved salts and nutrients [6]. This process contributes to reduced water availability and promotes salinization in the upper soil layers, limiting crop productivity in arid regions.

Surface mulching is an effective strategy for reducing evaporative losses and improving soil water conservation. Organic mulches act as physical barriers that reduce solar radiation, lower soil temperature, and limit vapor diffusion. These effects reduce evaporation rates and modify soil moisture distribution patterns. Previous studies have shown that mulching enhances soil water retention, particularly in coarse-textured soils where evaporation losses are high [7,8]. However, its efficiency depends on mulch thickness, material, and interaction with soil hydraulic properties, with diminishing returns beyond optimal application levels [9]. Soil salinity dynamics are strongly linked to evaporation and water fluxes. In arid environments, upward capillary flow driven by evaporation leads to salt accumulation in surface layers, forming salinity gradients that negatively affect germination and plant growth. In contrast, downward drainage can redistribute salts depending on soil permeability and water flux intensity. The balance between these processes governs salt distribution within the soil profile [10,11]. Therefore, reducing evaporation is a key mechanism for controlling salinity build-up.

Nitrate nitrogen is highly mobile in soils due to its high solubility and weak adsorption. Its transport is primarily controlled by water fluxes, making it highly sensitive to irrigation and drainage conditions. In coarse-textured soils, rapid percolation results in severe nitrate leaching and fertilizer losses, while finer-textured soils promote greater retention due to slower water movement and longer residence time [12,13]. This enhances opportunities for plant uptake and microbial transformations [14]. The coupling between soil water dynamics, salinity redistribution, and nitrate transport highlights the complexity of hydrochemical interactions in soil systems. Water flow acts as the main driver of both salt and nutrient movement, meaning that any modification in hydraulic conditions directly alters chemical redistribution patterns [15]. Numerical and mechanistic approaches such as convection–dispersion modeling have been widely used to describe these coupled processes [5]. Mulching further modifies these interactions by altering surface energy balance and hydraulic gradients. By reducing evaporation, mulching suppresses upward water fluxes and limits salt and nitrate accumulation at the soil surface. It may also influence downward percolation, thereby affecting nutrient leaching dynamics [16,17]. Thus, mulching represents an integrated soil management strategy that simultaneously affects physical, chemical, and hydrological processes.

Despite extensive research on evaporation control, salinity dynamics, and nitrate leaching as individual processes, limited studies have simultaneously integrated these interacting mechanisms within a unified soil–water–solute framework under arid conditions. There is still a lack of quantitative assessment tools that can effectively capture the coupled effects of soil texture, nitrogen loading, and organic mulching on water conservation efficiency and hydrochemical redistribution. This gap restricts the development of decision-support approaches for optimizing soil and water management strategies in dryland agricultural systems. Although the proposed framework provides an integrated approach for evaluating coupled water–salt–nitrate interactions, its application is currently based on the soil types, mulch material, nitrogen application rates, and controlled environmental conditions examined in this study. These factors define the scope of the present investigation and provide a basis for future evaluation of the framework under a wider range of environmental and management conditions. Therefore, the objective of this study is to investigate the combined effects of soil texture, nitrate nitrogen application, and sawdust mulching on soil water dynamics, evaporation losses, salinity redistribution, and nitrate transport under controlled arid conditions. In addition, a Composite Sustainability Index (CSI) is developed to quantitatively integrate these interacting processes and evaluate the overall sustainability performance of different management scenarios, with the aim of identifying optimal practices for improving water use efficiency and mitigating salinity and nutrient losses in arid and semi-arid soils.

2. Materials and Methods

2.1. Experimental Site

The study was conducted under greenhouse conditions at the College of Food and Agriculture Sciences, King Saud University, Riyadh, Saudi Arabia. During the experimental period, greenhouse air temperature ranged between 27 and 32 °C, with an average temperature of approximately 29.5 °C. Potential evaporation was determined using free-water evaporation columns identical in dimensions to the soil columns used in the experiment. The columns were weighed daily throughout the study period, and water loss was converted to equivalent evaporation depth (mm) based on the surface area of the columns. The average potential evaporation rate was estimated at approximately 2.56 mm day⁻¹. All experiments were carried out under controlled greenhouse conditions to evaluate the effects of soil texture, sawdust mulching, and nitrate application on soil water dynamics, salinity redistribution, evaporation losses, and nitrate transport under arid environmental conditions (Figure 1).

2.2. Soil Characterization and Water Quality

Two agricultural soils with contrasting textures were used in this study: a loamy sand soil (S1) and a sandy clay loam soil (S2). Soil samples were collected from the surface layer (0–30 cm) of the experimental farm of the College of Food and Agriculture Sciences, Dirab, Riyadh, Saudi Arabia (24°25′ N, 46°34′ E). The collected soils were air-dried, gently crushed, and passed through a 2 mm sieve prior to laboratory analyses and soil column packing. Soil physical and chemical properties were determined using standard laboratory procedures. Measurements included soil texture, bulk density, electrical conductivity of the saturated paste extract (EC_e), pH, soluble cations and anions, organic matter content, calcium carbonate (CaCO₃), and nitrate concentration. Soil analyses were conducted following standard methods described in Dane and Topp [18], and Luo et al. [19] and Parker and Patrignani [20] for hydraulic and soil water retention measurements.

The main properties of the studied soils are presented in

Table 1. Selected physical and chemical properties of the studied soils.

Property	Loamy Sand (S1)	Sandy Clay Loam (S2)
ECe (saturated paste extract), dS m−1	1.23	2.94
SAR, dimensionless	1.57	0.60
pH (saturated paste extract), dimensionless	8.48	8.36
Organic matter, %	0.69	0.85
Calcium carbonate (CaCO3), %	30	35
Ca2+, mmolc L−1	6.9	19.3
Mg2+, mmolc L−1	1.4	5.5
Na+, mmolc L−1	3.2	2.1
K+, mmolc L−1	0.6	2.3
CO32−, mmolc L−1	Nil	Nil
HCO3−, mmolc L−1	1.6	0.4
Cl−, mmolc L−1	4.9	11.0
SO42−, mmolc L−1	5.6	17.8
NO3−, mg kg−1	37.88	26.00
Sand, %	82	52
Silt, %	6	24
Clay, %	12	24
Texture class, dimensionless	Loamy sand	Sandy clay loam

.

Tap water was used throughout the experiment to supply the amount of water required to bring the soil columns to saturation prior to the evaporation phase. The chemical characteristics of the irrigation water used in the study are presented in

Table 2. Chemical properties of the irrigation water used in the study.

ECw, dS m−1	SAR	pH	Ca2+, mmolc L−1	Mg2+, mmolc L−1	Na+, mmolc L−1	K+, mmolc L−1	CO32−, mmolc L−1	HCO3−, mmolc L−1	SO42−, mmolc L−1	Cl−, mmolc L−1	NO3−, mg L−1
0.53	1.1	7.9	3.3	0.4	1.5	0.03	Nil	0.5	2.7	2.0	Nil

. The water exhibited low salinity and sodicity levels, with an ECw of 0.53 dS m⁻¹ and an SAR of 1.1, indicating its suitability for laboratory experimentation and irrigation purposes.

2.3. Experimental Design and Treatments

A greenhouse column experiment was conducted using a completely randomized design (CRD) with a factorial arrangement and three replicates per treatment. The experiment was designed to evaluate the interactive effects of soil texture, sawdust mulch depth, and nitrate–nitrogen application rate on soil water dynamics, evaporation, salinity distribution, and nitrate transport under controlled arid conditions. The study included two soil textures: loamy sand (S1) and sandy clay loam (S2). Three sawdust mulch depths were applied: 0 cm (M0), 2 cm (M1), and 4 cm (M2), corresponding to approximately 0, 28.41, and 56.82 Mg ha⁻¹, respectively. The sawdust mulch used in the experiment had a bulk density of 0.142 g cm⁻³ (142 kg m⁻³), and the mass-based application rates were calculated accordingly. The sawdust was applied as an untreated surface layer to evaluate its function as a physical barrier for regulating soil evaporation and water flux. Sawdust is a lignocellulosic by-product of wood processing characterized by a porous structure and heterogeneous particle size distribution, typically ranging from 0.2 to 5 mm depending on the milling process. Wood-derived residues are generally reported to contain high organic carbon content (approximately 40–50% on a dry weight basis) and exhibit a high carbon-to-nitrogen ratio (approximately 200:1 to 500:1), reflecting their low nitrogen availability and slow decomposition potential [21,22]. In terms of salinity, raw sawdust is generally characterized by low electrical conductivity (<1.0 dS m⁻¹), indicating minimal soluble salt content and negligible salinity risk. Sawdust also exhibits a high water-holding capacity (approximately 200–400% of its dry weight), which enhances its effectiveness as a mulch material by improving soil moisture retention and reducing evaporation losses [22,23]. Four nitrate–nitrogen application rates were used: 0 (R0), 200 (R1), 300 (R2), and 400 (R3) kg NO₃–N ha⁻¹, applied as potassium nitrate (KNO₃). Potassium nitrate was selected as the nitrate source; therefore, potassium ions were introduced simultaneously with nitrate during fertilizer application. The contribution of the accompanying potassium ions was considered when interpreting changes in soil electrical conductivity and salinity redistribution. The experiment was arranged as a factorial combination of two soil textures, three mulch treatments, and four nitrate–nitrogen application rates, resulting in 24 treatment combinations (2 × 3 × 4). Within each soil type, 12 treatment combinations were evaluated. Each treatment was replicated three times under a completely randomized design, resulting in a total of 72 experimental units. The experimental units were arranged in PVC soil columns, and each column was subjected to a controlled sequence of saturation, free drainage, and evaporation phases. The experiment was designed to evaluate the interactive effects of soil texture, sawdust mulch depth, and nitrate–nitrogen application rate on soil water dynamics, evaporation, salinity distribution, and nitrate transport under controlled arid conditions following standard factorial experimental design principles described by Montgomery [24].

2.4. Soil Column Preparation and Experimental Procedures

Soil columns were prepared using PVC cylinders with an internal diameter of 10.8 cm and a height of 60 cm. The columns were packed with loamy sand (S1) and sandy clay loam (S2) soils at bulk densities of 1.50 and 1.45 g cm⁻³, respectively, to represent field-relevant soil conditions and ensure comparable pore structure across treatments. Prior to packing, the required amount of potassium nitrate (KNO₃) for each nitrogen treatment was dissolved in irrigation water and uniformly applied to the upper soil layer to ensure homogeneous distribution of nitrate within the soil profile. Additional distilled tap water was then added gradually to each column until complete saturation was achieved. After saturation, the soil columns were sealed at the surface using Parafilm to prevent evaporation and were allowed to undergo free gravitational drainage. Drainage continued until water flow from the bottom of the columns ceased. The leachate collected during this phase was measured for volume and stored for subsequent chemical analysis, following standard soil column experimental procedures described in Dane and Topp [18] and Luo et al. [19]. Following the termination of drainage, the Parafilm cover was carefully removed, and sawdust mulch was applied uniformly on the soil surface according to the assigned treatments (0, 2, and 4 cm thickness). The columns were then transferred to evaporation conditions and exposed to atmospheric demand for a period of 83 days.

During the evaporation phase, each soil column was weighed daily to determine cumulative evaporation losses based on mass change. Cumulative mass losses were converted to equivalent evaporation depth (mm) using the surface area of the soil columns before statistical analysis and graphical presentation. At the end of the experiment, the soil columns were sectioned at 10 cm depth intervals to determine vertical distributions of volumetric water content, electrical conductivity (EC_e), and nitrate concentration. A companion set of identical soil columns was used to evaluate soil water, salinity, and nitrate distributions immediately after free drainage and prior to the onset of evaporation, following the same saturation and drainage procedures. All experimental activities were conducted under controlled greenhouse conditions where temperature ranged between 27 and 32 °C, with an average potential evaporation rate of approximately 2.56 mm day⁻¹.

2.5. Measurement and Calculations

This section describes the determination of soil water dynamics, evaporation losses, salinity distribution, nitrate redistribution, and the computation of the Composite Sustainability Index (CSI).

2.5.1. Soil Water Content and Evaporation Dynamics

Soil water content was determined gravimetrically by oven-drying soil samples at 105 °C for 24 h until constant weight. Gravimetric water content (θg) was first calculated according to Equation (1):

$${\theta }_g={\displaystyle \frac{M_w}{M_d}}$$

(1)

where θ_g is the gravimetric water content (g g⁻¹), M_w is the mass of water (g), and M_d is the oven-dry soil mass (g). Volumetric water content (θ_v) was subsequently calculated from gravimetric water content and soil bulk density using Equation (2):

$${\theta }_v={\theta }_g{\displaystyle \frac{{\rho }_b}{{\rho }_w}}$$

(2)

where θ_v is the volumetric water content (cm³ cm⁻³), ρ_b is the soil bulk density (g cm⁻³), and ρ_w is the water density (g cm⁻³). The density of water was assumed to be 1.0 g cm⁻³.

Daily evaporation losses (E, mm day⁻¹) were determined from changes in column mass (Equation (3)):

$$E={\displaystyle \frac{\mathsf{\Delta }W}{A\cdot {\rho }_w}}$$

(3)

where ΔW is weight loss, A is soil surface area, and ρ_w is water density. Soil water depletion under evaporation was used to evaluate the water conservation efficiency of the different treatments, following standard gravimetric and soil water measurement procedures described by Dane and Topp [18] and Luo et al. [19], and approaches commonly used in soil column studies [25,26].

2.5.2. Leachate Collection and Nitrate Losses

Cumulative leachate (L, mm) was calculated from collected drainage water volume (Equation (4)):

$$L={\displaystyle \frac{V_l}{A}}$$

(4)

where V_l is leachate volume. Nitrate mass loss (N_loss, mg) in leachate was computed from Equation (5):

$$N_{loss}=C_{N{O}_3}\cdot V_l$$

(5)

where C_NO3 is nitrate concentration in leachate. Nitrate transport in soil columns was interpreted as a function of water flux and solute displacement, consistent with vadose-zone transport theory and standard solute leaching methodologies in soil column studies [15,25,26,27].

2.5.3. Soil Salinity (EC_e) Determination

Electrical conductivity of the saturated paste extract (EC_e) was measured using a calibrated EC meter after equilibration of saturated soil paste. Salinity redistribution index (SRI) was defined from Equation (6):

$$SRI={\displaystyle \frac{({EC}_{e,i}-{EC}_{e,0})}{{EC}_{e,0}}}$$

(6)

where EC_e,i is salinity after treatment and EC_e,0 is initial salinity. Similar salinity redistribution approaches have been widely applied in arid soil column studies [25].

2.5.4. Soil Water Retention and Hydraulic Conductivity

Soil water retention characteristics were determined by using pressure plate apparatus at matric potentials ranging from −100 to −15,000 cm H₂O. The soil water retention curve was fitted using the van Genuchten model (Equation (7)):

$$\theta (h)={\theta }_r+{\displaystyle \frac{{\theta }_s-{\theta }_r}{{\left[1+(\alpha \mid h\mid {)}^n\right]}^m}}$$

(7)

where θ_r is residual water content, θ_s is saturated water content, h is matric potential, and α, n, m are empirical parameters [4]. Saturated hydraulic conductivity (Ks) was determined using the constant head method, consistent with standard soil physics methodologies [4,18], while soil water retention characteristics were determined using pressure plate and pressure-cooker apparatus following Parker and Patrignani [20].

2.5.5. Nitrate Distribution in Soil Profile

After sectioning soil columns at 10 cm intervals, nitrate concentration was determined using the modified Kjeldahl reduction method. Nitrate redistribution index (NRI) was calculated from Equation (8):

$$NRI={\displaystyle \frac{\sum N_i}{{\mathrm{N}}_{total}}}$$

(8)

where N_i is nitrate content at depth i. Vertical nitrate transport patterns were interpreted based on water-driven solute movement in unsaturated porous media [26].

2.5.6. Composite Sustainability Index (CSI)

To provide an integrated evaluation of the combined effects of sawdust mulching, soil texture, and nitrate application on soil water conservation and solute dynamics, a Composite Sustainability Index (CSI) was developed. The index combined the major hydrological and chemical indicators measured during the experiment into a single dimensionless sustainability metric.

The CSI was constructed using four key indicators representing the dominant soil processes controlling sustainability under arid conditions:

• Soil water conservation after evaporation (SWC),
• Cumulative evaporation (E),
• Surface salinity after evaporation (EC),
• Nitrate redistribution and accumulation within the surface layer after evaporation (NO₃).

Indicators associated with improved soil performance (e.g., soil water conservation) were considered beneficial indicators, whereas variables associated with degradation risks (e.g., evaporation, salinity accumulation, and nitrate accumulation) were considered non-beneficial indicators.

To allow comparison among variables with different units and magnitudes, all indicators were normalized to dimensionless values ranging between 0 and 1 using min–max normalization.

For beneficial indicators, Equation (9):

$$\begin{array}{cccc}& X_n={\displaystyle \frac{X_i-X_{\mathrm{m}\mathrm{i}\mathrm{n}}}{X_{\mathrm{m}\mathrm{a}\mathrm{x}}-X_{\mathrm{m}\mathrm{i}\mathrm{n}}}}& & \end{array}$$

(9)

For non-beneficial indicators, Equation (10):

$$\begin{array}{cccc}& X_n={\displaystyle \frac{X_{\mathrm{m}\mathrm{a}\mathrm{x}}-X_i}{X_{\mathrm{m}\mathrm{a}\mathrm{x}}-X_{\mathrm{m}\mathrm{i}\mathrm{n}}}}& & \end{array}$$

(10)

where X_n is the normalized value, X_i is the observed value for a given treatment, X_min is the minimum observed value, and X_max is the maximum observed value.

The Composite Sustainability Index was then calculated as the arithmetic mean of the normalized indicators (Equation (11)):

$$\begin{array}{cccc}& CSI={\displaystyle \frac{{\displaystyle {\sum }_{i=1}^n}{X}_n}{n}}& & \end{array}$$

(11)

where CSI = Composite Sustainability Index, X_n = normalized indicator value, n = number of indicators included in the index. Higher CSI values indicate greater sustainability performance through improved water conservation, reduced evaporation losses, lower salinity accumulation, and reduced nitrate redistribution toward the soil surface. The CSI approach was adapted from widely used sustainability assessment and multi-indicator evaluation methods in soil and environmental studies [28,29,30], as well as multi-criteria decision-making frameworks for sustainability assessment [29,30]. Equal weighting was adopted because the four selected indicators represent complementary and interdependent hydrological and hydrochemical processes governing soil sustainability under arid conditions, namely water conservation, evaporation control, salinity regulation, and nitrate retention. Since no objective or experimentally supported basis was available for assigning different weights to individual indicators within the scope of the present study, equal weighting was considered the most transparent and unbiased approach for integrating the selected variables into a single composite metric.

To evaluate the robustness of the Composite Sustainability Index (CSI), a sensitivity analysis was conducted using alternative weighting schemes. In addition to the equal-weight scenario (0.25, 0.25, 0.25, and 0.25 for soil water conservation, evaporation suppression, salinity control, and nitrate control, respectively), four alternative scenarios were evaluated by assigning greater importance to individual indicators: water-priority (0.40, 0.20, 0.20, and 0.20), evaporation-priority (0.20, 0.40, 0.20, 0.20), salinity-priority (0.20, 0.20, 0.40, 0.20), and nitrate-priority (0.20, 0.20, 0.20, 0.40). The resulting CSI values and treatment rankings were compared with those obtained under equal weighting to assess the influence of weighting assumptions on the overall sustainability assessment. The indicators included in the CSI were selected because they directly represent the major hydrological and hydrochemical processes investigated in this study, namely water conservation, evaporation losses, salinity redistribution, and nitrate dynamics. Collectively, these indicators capture the principal hydrological and hydrochemical processes governing system performance under arid conditions and therefore provide a concise basis for integrated sustainability assessment. Additional environmental indicators, such as soil organic carbon, nutrient-use efficiency, crop productivity, greenhouse gas emissions, and biological soil quality indicators, may also contribute to broader sustainability assessments and could be considered in future developments of the CSI framework.

2.6. Statistical Analysis

The selected level of replication is consistent with commonly adopted practices in controlled soil column experiments and was considered adequate for evaluating the main effects and interactions among soil texture, mulch treatment, and nitrate application rate under the experimental conditions. All measured variables were statistically analyzed using analysis of variance (ANOVA) within a factorial experimental framework under a completely randomized design (CRD) with three replicates. The experimental factors included soil type (S), mulch treatment (M), and nitrate–nitrogen application rate (N). The statistical model used for analysis is expressed from Equation (12):

$$Y=\mu +S+M+N+\left(S\times M\right)+\left(S\times N\right)+\left(M\times N\right)+\left(S\times M\times N\right)+ϵ$$

(12)

where Y is the observed dependent variable, μ is the overall mean, S represents soil type effect, M represents mulch treatment effect, N represents nitrate application rate effect, and ε represents the random error term. The significance of main effects and interaction effects was evaluated using the F-test. When significant differences were detected, treatment means were compared using the Least Significant Difference (LSD) test at probability levels of p ≤ 0.05 and p ≤ 0.01.

For evaporation dynamics, cumulative evaporation values were used for statistical comparisons among treatments to avoid potential temporal dependence associated with repeated measurements on the same soil columns over time. This approach ensured statistical independence by reducing autocorrelation effects inherent in time-series evaporation data. Soil profile data were analyzed separately at each depth interval (0–10, 10–20, 20–30, 30–40, 40–50, and 50–60 cm) to assess vertical variability in soil water content, salinity, and nitrate redistribution. This depth-wise analysis approach is commonly adopted in soil column studies to evaluate treatment effects across soil layers while maintaining consistency of independent observations within each depth class [31].

Prior to analysis, all datasets were tested for normality and homogeneity of variance to ensure compliance with ANOVA assumptions. Statistical analyses were performed using standard procedures widely applied in soil physics and environmental factorial experiments involving water and solute transport processes.

3. Results

3.1. Soil Hydrophysical Properties

The influence of soil texture on soil hydraulic behavior was evaluated through water retention characteristics and hydraulic conductivity parameters, which collectively determine water storage and movement within the soil profile.

3.1.1. Soil Water Retention Characteristics

The relationship between matric suction (ψ) and volumetric water content (θ) for the studied soils is presented in Figure 2. Distinct water retention behaviors were observed between the loamy sand and sandy clay loam soils, reflecting differences in soil texture and pore-size distribution. In both soils, θ decreased rapidly with increasing ψ at low suction levels, followed by a more gradual decline at higher suction values. This pattern reflects rapid drainage of macropores followed by progressive water release from smaller pores. The reduction in θ with increasing ψ was more pronounced in the loamy sand soil. At ψ = 300 cm, θ values were 0.0912 and 0.2217 cm³ cm⁻³ for the loamy sand and sandy clay loam soils, respectively. At ψ = 15,000 cm, θ further decreased to 0.0375 cm³ cm⁻³ in the loamy sand soil and 0.08395 cm³ cm⁻³ in the sandy clay loam soil, confirming the greater water retention capacity of the finer-textured soil throughout the entire suction range. Field capacity values were approximately 0.1875 cm³ cm⁻³ for the loamy sand soil (ψ ≈ 100 cm) and 0.2217 cm³ cm⁻³ for the sandy clay loam soil (ψ ≈ 300 cm). The soil water retention curves were described using the Van Genuchten model based on limited measured θ–ψ data across the suction range of 100 to 15,000 cm of water. The fitted parameters indicated that the model provides an excellent representation of the retention behavior for both soils. The Van Genuchten parameters (θ_s, θ_r, α, n, and m) showed clear differences between the two soils, where the loamy sand soil exhibited θ_s = 0.4146 cm³ cm⁻³, θ_r = 0.0390 cm³ cm⁻³, α = 0.0228 cm⁻¹, n = 2.0208, and m = 0.5052, while the sandy clay loam soil showed θ_s = 0.4614 cm³ cm⁻³, θ_r = 0.0690 cm³ cm⁻³, α = 0.0148 cm⁻¹, n = 1.5972, and m = 0.3739. These parameters reflect the stronger water retention capacity and smoother retention curve of the sandy clay loam soil compared with the loamy sand soil.

Likewise, the residual water content (θ_r) at ψ = 15,000 cm was lower in the loamy sand soil, indicating reduced water availability under dry conditions. The soil water retention curves were accurately described using the Van Genuchten model, with a coefficient of determination (R² = 0.999) for both soils. Lower fitted parameters (α, n, and m) in the sandy clay loam soil indicate a smoother retention curve and greater pore-water retention relative to the loamy sand soil. The differences in soil water retention behavior between the two soils reflected the strong influence of soil texture and pore-size distribution on water storage under unsaturated conditions. The sandy clay loam soil consistently retained greater volumetric water content across the entire matric suction range compared with the loamy sand soil, indicating higher water-holding capacity associated with its finer texture and greater proportion of micropores. In addition, the high coefficient of determination (R² = 0.999) obtained from the Van Genuchten model confirmed the excellent agreement between the measured and fitted soil water retention data for both soils. The results demonstrate the dominant role of soil texture in controlling soil water retention and water availability under unsaturated conditions.

3.1.2. Hydraulic Conductivity Functions

The relationship between unsaturated hydraulic conductivity (K) and volumetric water content (θ) for the studied soils is presented in Figure 3. Hydraulic conductivity exhibited a highly nonlinear response to changes in water content in both soils, varying over several orders of magnitude across the investigated moisture range. At low θ values, K remained extremely small, indicating limited hydraulic continuity under dry conditions. As θ increased, K increased sharply, particularly near higher water contents, reflecting progressive pore connectivity and enhanced water transmission through water-filled pores. The two soils exhibited distinctly different conductivity behaviors. At equivalent θ values, the loamy sand soil consistently exhibited greater K values than the sandy clay loam soil. For example, at θ ≈ 0.30 cm³ cm⁻³, K reached approximately 6 × 10⁻² cm h⁻¹ in the loamy sand soil, whereas the sandy clay loam soil exhibited a substantially lower value of approximately 2 × 10⁻³ cm h⁻¹. This behavior reflects the dominance of larger and more continuous macropores in the coarse-textured soil. In contrast, under comparable matric suction conditions (ψ), particularly at relatively high suction levels, the sandy clay loam soil maintained greater hydraulic conductivity than the loamy sand soil. At ψ ≈ 300 cm, K values were approximately 3.4 × 10⁻⁵ cm h⁻¹ in the loamy sand soil and 1.7 × 10⁻⁴ cm h⁻¹ in the sandy clay loam soil. The greater conductivity observed in the sandy clay loam soil under drier conditions reflects its greater water retention and improved hydraulic continuity at lower water potentials.

Differences in the curvature of the K–θ functions further reflected variations in pore-size distribution between the two soils. The loamy sand soil exhibited a steeper conductivity response, indicating rapid transition from low to high conductivity over a relatively narrow moisture range. In contrast, the sandy clay loam soil showed a more gradual increase in conductivity, consistent with a broader pore-size distribution and smoother desaturation behavior. The observed differences in hydraulic conductivity behavior between the two soils were strongly associated with differences in pore-size distribution and soil texture. The loamy sand soil exhibited greater conductivity at relatively high water contents due to the dominance of larger and better-connected macropores, whereas the sandy clay loam soil maintained relatively greater hydraulic continuity under drier conditions because of its higher water retention capacity and greater proportion of fine pores. The conductivity functions were accurately represented by the Van Genuchten–Mualem model. Lower fitted parameters (α, n, and m) in the sandy clay loam soil contributed to smoother conductivity behavior and greater water retention, whereas the larger parameter values in the loamy sand soil produced a sharper increase in conductivity with increasing θ.

Overall, the results demonstrate the strong influence of soil texture on unsaturated hydraulic behavior, with the loamy sand soil favoring rapid water transmission at higher water contents and the sandy clay loam soil maintaining greater hydraulic continuity under drier conditions.

3.2. Evaporation Dynamics

This section presents the effects of soil texture, nitrate application, and mulch depth on evaporation dynamics and soil water loss, highlighting the response of the different treatments under arid conditions.

3.2.1. Effect of Mulching on Cumulative Evaporation

The effect of sawdust mulching on cumulative evaporation from loamy sand (S1) and sandy clay loam (S2) soils is presented in Figure 4. The results clearly demonstrate that sawdust mulching substantially reduced evaporation losses in both soils throughout the experimental period. Cumulative evaporation increased progressively with time under all treatments; however, the magnitude of evaporation was strongly influenced by mulch depth and soil texture. In both soils, the non-mulched treatment (M0) consistently exhibited the highest cumulative evaporation values throughout the experiment, whereas the mulched treatments, particularly those receiving 2 cm (M1) and 4 cm (M2) sawdust layers, showed markedly lower evaporation rates and cumulative evaporation. Statistical analysis based on factorial ANOVA confirmed that mulch treatment had a highly significant effect (p ≤ 0.01) on cumulative evaporation across all soil depths and interactions (see Table S1 in Supplementary Materials). Differences among mulch treatments were significant at p ≤ 0.01 according to the LSD test, confirming the effectiveness of mulch application in suppressing evaporative water losses under arid conditions.

The evaporation curves of the non-mulched treatment displayed the typical multi-stage evaporation behavior characterized by an initially rapid evaporation phase followed by a gradual decline over time. In contrast, the mulched treatments maintained substantially lower evaporation rates throughout the experimental period, indicating a strong reduction in cumulative water loss from the soil surface. Evaporation reduction increased with increasing mulch thickness in both soils. In the loamy sand soil, cumulative evaporation after 83 days reached 122.8 mm under the non-mulched treatment, whereas it decreased to 36.1 mm and 27.1 mm under the 2 cm and 4 cm mulch treatments, respectively. Relative to the non-mulched control, cumulative evaporation was reduced by approximately 70.6% under the 2 cm mulch treatment and by 78.0% under the 4 cm mulch treatment. Similarly, cumulative evaporation in the sandy clay loam soil reached 134.9 mm in the non-mulched treatment and decreased to 40.8 mm and 28.2 mm under the 2 cm and 4 cm mulch treatments, corresponding to reductions of approximately 69.7% and 79.1%, respectively. These results demonstrate the strong effectiveness of sawdust mulch in suppressing evaporative water losses under both soil conditions. Cumulative latent evaporation reached 212.2 mm during the experimental period and greatly exceeded actual soil evaporation under all mulch treatments. Despite the lower evaporation coefficients observed under the 4 cm mulch treatment, the differences between the 2 and 4 cm mulch depths remained comparatively small. Although increasing mulch thickness from 2 to 4 cm further reduced evaporation, the additional reduction remained relatively limited compared with the substantial decrease achieved by the initial 2 cm mulch application.

The additional reduction achieved by increasing mulch depth from 2 to 4 cm was approximately 7.3 percentage points in the loamy sand soil and 9.3 percentage points in the sandy clay loam soil, indicating that most of the evaporation suppression was already achieved with the 2 cm mulch layer, indicating that most of the evaporation suppression was achieved with the 2 cm mulch layer. Clear differences between soil textures were also observed. The sandy clay loam soil consistently exhibited greater cumulative evaporation than the loamy sand soil under all mulch treatments. After 83 days, cumulative evaporation from the sandy clay loam soil exceeded that of the loamy sand soil by approximately 9.9%, 13.0%, and 4.1% for the M0, M1, and M2 treatments, respectively. The effect of soil texture on cumulative evaporation was statistically significant (p ≤ 0.01), with the sandy clay loam soil consistently exhibiting greater evaporation losses than the loamy sand soil throughout the experimental period.

The effect of soil texture on cumulative evaporation was statistically significant (p ≤ 0.01) according to ANOVA results (see Table S1 in Supplementary Materials).

Cumulative latent evaporation increased continuously throughout the experiment and reached 212.2 mm after 83 days (Figure 4). Actual cumulative evaporation under all treatments remained substantially lower than cumulative latent evaporation during the entire evaporation period. At the end of the experiment, cumulative evaporation represented approximately 57.9%, 17.0%, and 12.8% of cumulative latent evaporation in the loamy sand soil under the M0, M1, and M2 treatments, respectively. The corresponding values for the sandy clay loam soil were 63.6%, 19.2%, and 13.3%, respectively. Relative to cumulative latent evaporation, the 2 cm mulch treatment reduced actual evaporation losses by approximately 70.6% and 69.8% in the loamy sand and sandy clay loam soils, respectively, while the 4 cm mulch treatment reduced evaporation losses by approximately 77.9% and 79.1%, respectively. Figure 4 further shows that the differences between mulched and non-mulched treatments became progressively larger with time, reflecting the cumulative effect of mulch in limiting evaporative water loss during prolonged drying. The greatest cumulative evaporation was consistently recorded under the non-mulched treatment, whereas the lowest values were observed under the 4 cm mulch treatment in both soils throughout the experimental period.

Overall, the results presented in Figure 4 confirm that sawdust mulching was highly effective in reducing cumulative evaporation under arid conditions. The reduction was evident in both soil types and increased with increasing mulch thickness, with the greatest reduction observed under the 4 cm mulch treatment.

3.2.2. Effect of Nitrogen Application on Evaporation

The effect of nitrate nitrogen application rates on cumulative evaporation from the loamy sand (S1) and sandy clay loam (S2) soils is presented in Figure 5. Overall, nitrate nitrogen application had no significant effect on cumulative evaporation in either soil throughout the experimental period. Figure 5 shows that cumulative evaporation increased progressively with time under all nitrogen application rates in both soils. The cumulative evaporation curves corresponding to the different nitrogen treatments exhibited very similar temporal trends throughout the 83-day evaporation period. No consistent response to increasing nitrate application rate was observed under either soil type, or the evaporation curves remained closely grouped during the entire experiment.

In the loamy sand soil (S1), cumulative evaporation after 83 days reached 62.8, 61.8, 63.1, and 60.2 mm under the R0 (0 kg NO₃⁻–N ha⁻¹), R1 (200 kg NO₃⁻–N ha⁻¹), R2 (300 kg NO₃⁻–N ha⁻¹), and R3 (400 kg NO₃⁻–N ha⁻¹) treatments, respectively. Relative to the control treatment (R0), cumulative evaporation decreased by approximately 1.6% and 4.1% under the R1 and R3 treatments, respectively, whereas a slight increase of approximately 0.5% was observed under the R2 treatment. Despite these small numerical differences, no consistent trend associated with increasing nitrogen application rate was evident. A similar pattern was observed in the sandy clay loam soil (S2). At the end of the experiment, cumulative evaporation values were 68.6, 67.3, 68.0, and 68.0 mm under the R0, R1, R2, and R3 treatments, respectively. Compared with the control treatment, cumulative evaporation decreased by approximately 1.9%, 0.9%, and 0.9% under the R1, R2, and R3 treatments, respectively. The differences among nitrogen treatments remained small throughout the evaporation period.

Statistical analysis using factorial ANOVA confirmed that nitrate nitrogen application had no consistent significant effect on cumulative evaporation across most soil depths (p > 0.05 overall), although a limited depth-specific response was observed at the 20–30 cm soil layer (p < 0.05), as shown in Supplementary Table S1.

As shown in Figure 5, the evaporation curves under the different nitrogen treatments largely overlapped throughout the drying period in both soils. The absence of clear separation among the curves indicates that nitrate nitrogen application had only a limited influence on cumulative evaporation under the conditions of this study. Analysis of variance confirmed that nitrate nitrogen application had no statistically significant effect on cumulative evaporation in either soil (p > 0.05). In contrast, soil texture remained the dominant factor affecting evaporation behavior throughout the experiment. The sandy clay loam soil consistently exhibited greater cumulative evaporation than the loamy sand soil across all nitrogen application rates. After 83 days, cumulative evaporation in the sandy clay loam soil exceeded that in the loamy sand soil by approximately 9.2%, 8.9%, 7.8%, and 13.0% under the R0, R1, R2, and R3 treatments, respectively. A temporary increase in the evaporation rate was observed around day 31 for all treatments in both soils. This increase coincided with a rise in greenhouse temperature during the experimental period. However, despite this temporary increase, the overall evaporation pattern remained similar among all nitrogen application rates.

Finally, the results indicate that nitrate nitrogen application within the investigated range did not significantly influence cumulative evaporation under the conditions of this study. The absence of significant treatment effects (p > 0.05) suggests that evaporation dynamics were primarily governed by soil texture and atmospheric evaporative demand rather than by nitrate nitrogen application rate. The results presented in Figure 5 demonstrate that cumulative evaporation followed nearly identical temporal patterns across all nitrogen treatments in both soils throughout the experimental period.

3.3. Soil Water Distribution and Storage

3.3.1. Soil Water Distribution After Drainage

Figure 6 presents the vertical distribution of volumetric water content (θ) within the soil profiles immediately after free drainage in the loamy sand soil (S1) and the sandy clay loam soil (S2) under different nitrate–nitrogen application rates (R0, R1, R2, and R3). The results indicate that nitrate applications exerted only a limited influence on soil water redistribution following drainage. Volumetric water content values remained relatively similar among the four nitrogen application rates at all depths investigated in both soils, suggesting that nitrate addition did not substantially affect post-drainage soil moisture distribution. Statistical analysis confirmed that nitrate–nitrogen application had no significant effect on volumetric water content after drainage across most soil depths (p > 0.05). In contrast, soil texture exerted a highly significant effect on water distribution and storage within the profile (p ≤ 0.01), highlighting the dominant role of soil physical properties in controlling water retention after drainage.

In the loamy sand soil (S1), volumetric water content within the surface layer (5 cm) ranged from 0.32 cm³ cm⁻³ under R0 and R1 to 0.33 cm³ cm⁻³ under R2 and R3. Similar values were observed throughout the upper and middle portions of the profile, where θ generally varied between 0.32 and 0.35 cm³ cm⁻³. At 15 cm depth, values ranged from 0.32 to 0.35 cm³ cm⁻³, while at 25 and 35 cm depths they remained within a narrow interval of approximately 0.32–0.34 cm³ cm⁻³. A gradual increase in water content was observed toward the lower profile layers, where θ reached 0.34–0.36 cm³ cm⁻³ at 45 cm depth and increased further to between 0.36 and 0.39 cm³ cm⁻³ at 55 cm depth. The highest value recorded in this soil was 0.39 cm³ cm⁻³ under the R2 treatment in the deepest layer.

The sandy clay loam soil (S2) consistently maintained greater volumetric water contents than the loamy sand soil throughout the profile. In the surface layer, θ ranged from 0.35 to 0.37 cm³ cm⁻³ across the nitrogen treatments. Between 15 and 35 cm depth, volumetric water content remained relatively stable and varied between approximately 0.33 and 0.36 cm³ cm⁻³. As observed in the loamy sand soil, water content increased in the deeper layers, reaching values between 0.37 and 0.39 cm³ cm⁻³ at 45 cm depth and between 0.37 and 0.42 cm³ cm⁻³ at 55 cm depth. The maximum volumetric water content measured in the experiment was 0.42 cm³ cm⁻³ under the R0 treatment at the deepest layer, confirming the greater water retention capacity of the sandy clay loam soil.

When averaged across nitrogen application rates, volumetric water content in the upper 5 cm layer was approximately 0.33 cm³ cm⁻³ in the loamy sand soil and 0.36 cm³ cm⁻³ in the sandy clay loam soil. Corresponding values in the deepest layer increased to approximately 0.37 and 0.40 cm³ cm⁻³, respectively. The increase in water content with depth observed in both soils indicates that complete redistribution of water had not yet been achieved immediately after drainage. Greater water contents in the lower profile layers suggest that gravitational water movement remained more active near the bottom of the soil columns, whereas the upper layers had already undergone greater drainage and water depletion.

The observed differences between the two soils reflect the strong influence of soil texture on post-drainage water retention and redistribution. The coarser loamy sand soil exhibited lower residual water contents because its larger pore network promoted more rapid drainage and reduced water retention. In contrast, the sandy clay loam soil retained greater amounts of water throughout the profile owing to its finer pore-size distribution, higher specific surface area, and stronger matric forces. Consequently, the sandy clay loam soil maintained consistently higher volumetric water contents than the loamy sand soil at nearly all depths.

Overall, the results presented in Figure 6 demonstrate that soil texture was the primary factor controlling soil water distribution and storage after free drainage, whereas nitrate–nitrogen application had only minor effects on the vertical redistribution of soil moisture. The consistently greater volumetric water contents observed in the sandy clay loam soil confirm its superior water-holding capacity relative to the loamy sand soil and emphasize the importance of soil physical characteristics in governing post-drainage water dynamics.

3.3.2. Effect of Mulching on Volumetric Soil Water Content After Evaporation

Figure 7 illustrates the effect of sawdust mulch on the vertical distribution of volumetric water content (θ) within the profiles of the loamy sand soil (S1) and the sandy clay loam soil (S2) after 83 days of evaporation. The results clearly demonstrate that mulch application substantially influenced soil water conservation and residual moisture distribution throughout the soil profile. In general, the average volumetric water content in both soils decreased considerably compared with the Initial moisture distribution measured at the beginning of the evaporation period. However, the magnitude of moisture depletion varied markedly according to mulch treatment. Statistical analysis based on factorial ANOVA confirmed that mulch treatment had a highly significant effect on volumetric soil water content after evaporation (p ≤ 0.01), while nitrogen application showed no significant effect (p > 0.05). Soil texture also showed no significant interaction effect in most soil depths. The complete ANOVA outputs, including degrees of freedom, F-values, and significance levels, are provided in Supplementary Table S1.

Under the non-mulched treatment (M0), evaporation caused substantial depletion of soil water throughout the profile in both soils. In the loamy sand soil (S1), volumetric water content in the surface layer (5 cm) decreased from an Initial value of 0.36 cm³ cm⁻³ to only 0.08 cm³ cm⁻³ after 83 days of evaporation. Similar reductions were observed throughout the profile, with θ values increasing gradually from 0.10 cm³ cm⁻³ at 15 cm depth to 0.17 cm³ cm⁻³ at 55 cm depth. These results indicate severe drying throughout the profile and the development of an extensive dry zone extending through most of the soil column. The strong reduction in moisture content reflects the high evaporation losses that occurred in the absence of mulch protection. A similar trend was observed in the sandy clay loam soil (S2), although the moisture distribution pattern differed somewhat from that observed in the loamy sand soil. Volumetric water content in the surface layer decreased from an Initial value of 0.36 cm³ cm⁻³ to 0.07 cm³ cm⁻³ under M0. Water content remained relatively low within the upper portion of the profile, ranging between 0.11 and 0.15 cm³ cm⁻³ from 15 to 55 cm depth. The results suggest the formation of a dry zone within the upper half of the profile followed by a relatively wetter transmission zone in the lower layers. This pattern indicates the occurrence of at least two stages of evaporation decline, where water movement from deeper layers partially compensated for moisture losses near the soil surface.

Application of a 2 cm sawdust mulch layer (M1) substantially improved soil water conservation in both soils. In the loamy sand soil, volumetric water content within the upper 20 cm of the profile increased markedly relative to the non-mulched treatment, reaching approximately 0.21 cm³ cm⁻³ at both 5 and 15 cm depths. Compared with the field capacity value of approximately 0.1875 cm³ cm⁻³ for this soil, mulch application increased moisture content in the upper layers by about 12%. In the deeper profile layers, θ values ranged between 0.28 and 0.30 cm³ cm⁻³, representing an increase of approximately 55% above field capacity. The moisture profile under M1 was therefore characterized by a distinct transmission zone with substantially greater water storage than that observed under M0. The positive effect of mulching was even more evident in the sandy clay loam soil. Under M1, volumetric water content remained remarkably uniform throughout the entire profile, varying only between 0.28 and 0.30 cm³ cm⁻³. Compared with the field capacity of approximately 0.2217 cm³ cm⁻³, moisture content increased by nearly 31% throughout most of the profile. The relatively uniform moisture distribution indicates that mulch application effectively reduced evaporation losses and maintained favorable hydraulic conditions within the soil column. Unlike the loamy sand soil, which still exhibited some variation with depth, the sandy clay loam soil developed an almost entirely uniform transmission zone.

Increasing mulch depth to 4 cm (M2) provided the greatest moisture conservation in both soils. In the loamy sand soil, volumetric water content increased to 0.24 cm³ cm⁻³ in the surface layer and remained between 0.28 and 0.30 cm³ cm⁻³ throughout the remainder of the profile. These values were consistently greater than those observed under M1, although the differences between the two mulch treatments were not statistically significant. The absence of a pronounced dry surface layer under M2 indicates that the thicker mulch effectively suppressed evaporation and preserved soil moisture throughout the profile.

Similarly, the sandy clay loam soil under M2 exhibited the highest and most uniform moisture contents among all treatments. Volumetric water content ranged from 0.29 to 0.32 cm³ cm⁻³ throughout the profile, compared with Initial values ranging from 0.34 to 0.40 cm³ cm⁻³. Although some moisture loss occurred during the 83-day evaporation period, the reductions were substantially smaller than those observed under the non-mulched treatment. The nearly uniform distribution of θ throughout the profile indicates that the thicker mulch layer greatly reduced evaporation-driven moisture gradients and maintained a stable soil water regime.

Comparison of the mulch treatments with the Initial moisture distribution further highlights the effectiveness of sawdust mulch in conserving soil water. Initially, volumetric water content ranged from 0.34–0.39 cm³ cm⁻³ in the loamy sand soil and from 0.34–0.40 cm³ cm⁻³ in the sandy clay loam soil. After 83 days of evaporation, the non-mulched treatment retained only a small fraction of this moisture, whereas both mulch treatments preserved substantially greater amounts of water throughout the profile. The beneficial effect became more pronounced as mulch depth increased from 2 cm to 4 cm, reflecting the progressive reduction in soil surface evaporation.

Overall, the results presented in Figure 7 demonstrate that sawdust mulching exerted a highly significant effect on volumetric soil water content after evaporation. Mulch applications reduced evaporative water losses, increased residual soil moisture, and promoted the development of a more uniform moisture distribution throughout the soil profile. The beneficial effects were particularly evident when comparing the mulched treatments (M1 and M2) with the non-mulched treatment (M0). Although the sandy clay loam soil generally maintained slightly greater moisture contents than the loamy sand soil, both soils responded similarly to mulch application. Consequently, mulch thickness emerged as the dominant factor controlling soil water conservation during evaporation, while increasing mulch depth improved the ability of the soil profile to retain moisture and resist surface desiccation.

3.3.3. Soil Water Conservation

The effects of sawdust mulching on soil water storage after 83 days of evaporation are presented in

Table 3. Soil water storage after 83 days of evaporation as affected by sawdust mulch depth in loamy sand (S1) and sandy clay loam (S2) soils.

Soil Type	Mulch Treatment	Soil Water Storage (%)	Relative Increase (%)
Loamy sand	M0 (No mulch)	38.1	—
Loamy sand	M1 (2 cm mulch)	81.8	114.7
Loamy sand	M2 (4 cm mulch)	86.4	126.8
Sandy clay loam	M0 (No mulch)	38.9	—
Sandy clay loam	M1 (2 cm mulch)	81.5	109.5
Sandy clay loam	M2 (4 cm mulch)	87.2	124.2

, while the corresponding moisture distribution within the soil profile is illustrated in Figure 7. Statistical analysis revealed that mulch application significantly increased soil water storage in both soils (p ≤ 0.01), whereas nitrate nitrogen application showed no significant effect (p > 0.05). No significant interaction effects were observed between mulch and soil type or between mulch and nitrogen application rate.

As shown in Table 3, soil water storage in the loamy sand soil (S1) under non-mulched conditions (M0) after 83 days of evaporation was 38.1%. Application of sawdust mulch substantially increased soil water storage to 81.8% and 86.4% under mulch depths of 2 cm (M1) and 4 cm (M2), respectively, corresponding to relative increases of approximately 114.7% and 126.8% compared with the control treatment. Similarly, soil water storage in the sandy clay loam soil (S2) increased from 38.9% under non-mulched conditions to 81.5% and 87.2% under M1 and M2 treatments, respectively, representing relative increases of approximately 109.5% and 124.2%.

The moisture profiles shown in Figure 7 support these results and demonstrate that mulch application effectively reduced water depletion throughout the soil profile, particularly within the upper layers where evaporation losses were greatest. Under non-mulched conditions, severe drying developed near the soil surface, resulting in substantial reductions in residual soil water storage by the end of the experimental period. In contrast, the mulch treatments maintained considerably higher moisture contents at all profile depths. The beneficial effects of mulching can be attributed to the reduction in direct soil exposure to atmospheric demand, which suppressed evaporation and limited upward water movement toward the soil surface. Consequently, a greater proportion of the initial soil water remained stored within the soil profile throughout the evaporation period.

Although increasing mulch thickness from 2 cm to 4 cm resulted in additional improvements in soil water storage, the magnitude of this increase was relatively small and not statistically significant (p > 0.05). Therefore, the 2 cm mulch treatment appears sufficient to achieve most of the water conservation benefits associated with thicker mulch applications.

Differences between the two soils remained evident throughout the experiment. The sandy clay loam soil generally maintained slightly greater water storage than the loamy sand soil because of its finer texture and greater water-holding capacity. Nevertheless, both soils responded strongly to mulch application, confirming the effectiveness of sawdust mulch as a practical and sustainable soil water conservation practice under arid conditions.

3.4. Soil Salinity Dynamics

Salt transport and redistribution within the soil profile were strongly influenced by soil water movement during drainage and evaporation processes. Variations in soil texture, nitrate nitrogen application, and mulch treatment affected both downward salt leaching during drainage and upward salt accumulation during evaporation. To evaluate these processes, electrical conductivity of the leachate (EC) and soil saturation extract (EC_e) were monitored under the different experimental treatments. The obtained results provide insight into the combined effects of soil hydraulic behavior, evaporation dynamics, and mulching on salinity redistribution within the soil profile under arid conditions.

3.4.1. Salinity in Leachate

The electrical conductivity (EC) of the leachate collected immediately after free drainage is presented in Figure 8. The results revealed clear differences in leachate salinity between the two soil textures and among nitrate nitrogen application rates, indicating substantial variation in salt mobility and leaching behavior during drainage. As shown in Figure 8, the sandy clay loam soil (S2) consistently exhibited considerably greater leachate EC values than the loamy sand soil (S1) under all nitrogen application rates. In the loamy sand soil, EC values increased progressively from approximately 7.5 dS m⁻¹ under the control treatment (R0) to approximately 14 dS m⁻¹ under the highest nitrogen application rate (R3). Similarly, the sandy clay loam soil exhibited higher EC values ranging from approximately 18 dS m⁻¹ at R0 to more than 20 dS m⁻¹ at R3.

The increase in leachate EC with increasing nitrate nitrogen application reflects the greater concentration of soluble salts within the soil solution following fertilizer addition. Higher nitrate application rates increased ionic concentration in the soil water, resulting in greater salt transport during gravitational drainage. This trend was particularly evident in the loamy sand soil, where EC values increased sharply with increasing nitrogen application rate. The consistently greater EC values observed in the sandy clay loam soil indicate stronger retention of dissolved salts within the finer pore system and a greater concentration of salts in the drainage water. The finer texture and higher water-holding capacity of the sandy clay loam soil likely enhanced solute residence time and promoted greater salt accumulation within the soil solution prior to leaching.

Despite the higher hydraulic conductivity generally associated with coarse-textured soils under near-saturated conditions, the sandy clay loam soil produced more saline leachate, suggesting that soil texture influenced not only water movement but also solute concentration and redistribution processes during drainage. The results presented in Figure 8 demonstrate that nitrate nitrogen application increased leachate salinity in both soils, while soil texture exerted a dominant influence on the magnitude of salt leaching during free drainage. Statistical analysis showed that both soil texture and nitrate nitrogen application significantly affected leachate salinity following free drainage. Soil texture exerted a highly significant effect on EC values (p ≤ 0.01), with the sandy clay loam soil consistently producing more saline leachate than the loamy sand soil. Nitrogen application rate also significantly increased leachate EC (p ≤ 0.01), reflecting enhanced soluble salt concentrations in the soil solution with increasing nitrate addition. Moreover, the interaction between soil texture and nitrogen application (S × RF) was significant at several profile depths, indicating that the salinity response to nitrate application differed according to soil texture. The complete ANOVA outputs, including degrees of freedom, F-values, and significance levels, are provided in Supplementary Table S2.

3.4.2. Salt Distribution After Drainage

The vertical distribution of soil salinity immediately after free drainage is presented in Figure 9. The results indicate downward redistribution of soluble salts within both soil profiles following gravitational water movement. Soil texture exerted a strong influence on salinity distribution patterns, while nitrate nitrogen application affected both the magnitude and distribution of EC throughout the profile.

In the loamy sand soil (S1), EC values remained relatively low throughout the profile, ranging from 0.72 to 1.82 dS m⁻¹. The lowest values were generally observed within the intermediate depths (25–35 cm), whereas higher EC values occurred in the deeper layer (55 cm). Under the control treatment (R0), EC varied between 0.76 and 1.12 dS m⁻¹. Increasing nitrate application increased EC in the lower part of the profile, reaching 1.40, 1.60, and 1.82 dS m⁻¹ at 55 cm depth under R1, R2, and R3, respectively. These results indicate downward transport and partial accumulation of soluble salts in the lower layers during drainage. In contrast, the sandy clay loam soil (S2) exhibited substantially greater EC values, particularly in the deeper layers. EC values ranged from 1.06 to 10.59 dS m⁻¹ and increased markedly with depth under all treatments. In the upper layers (5–35 cm), EC values generally remained below 2.0 dS m⁻¹, whereas a sharp increase occurred below 45 cm depth. At 55 cm depth, EC reached 5.98, 6.82, 8.45, and 10.59 dS m⁻¹ under R0, R1, R2, and R3, respectively. The pronounced salinity buildup in the lower profile demonstrates considerable downward movement and accumulation of dissolved salts during free drainage.

The progressive increase in EC with depth, particularly in S2, reflects the leaching effect associated with gravitational water movement. As drainage water moved downward through the soil columns, soluble salts were transported from the upper layers and accumulated in deeper portions of the profile. This redistribution pattern was considerably more pronounced in the sandy clay loam soil than in the loamy sand soil, likely due to its greater water-holding capacity and lower hydraulic conductivity, which promoted salt retention and accumulation in the lower layers. Nitrogen application rate also influenced post-drainage salinity distribution. Although the effect was relatively limited in the upper layers, increasing nitrate application generally increased EC values in the deeper layers of both soils. The highest salinity levels were consistently observed under the R3 treatment, particularly at 55 cm depth, confirming that greater nitrate inputs increased the concentration of soluble ions available for downward transport and accumulation during drainage.

Overall, the results demonstrate that free drainage promoted substantial downward salt movement in both soils, with the magnitude of salt accumulation strongly controlled by soil texture and nitrogen application rate. Statistical analysis confirmed that soil texture significantly affected post-drainage salinity distribution within the soil profile (p ≤ 0.01), particularly in the deeper soil layers where salt accumulation was greatest. Nitrate nitrogen application also significantly increased EC values at several depths, especially within the lower portions of the profile (p ≤ 0.01). In addition, significant soil texture × nitrogen application (S × RF) interactions were observed for salinity distribution in some layers, indicating that the extent of downward salt transport differed between the two soils depending on nitrogen application level. The complete ANOVA results, including degrees of freedom, F-values, and significance levels, are provided in Supplementary Table S3.

3.4.3. Effect of Mulching on Soil Electrical Conductivity Distribution After Evaporation

Figure 10 shows the effect of sawdust mulch on the vertical distribution of soil electrical conductivity (EC) in the loamy sand soil (S1) and the sandy clay loam soil (S2) after 83 days of evaporation. The figure also includes the initial salinity distribution (Initial), representing the average EC_e values measured before the beginning of the evaporation period. Comparison between the initial and final salinity distributions clearly demonstrates the influence of evaporation and mulch application on salt redistribution within the soil profile. Following the cessation of free drainage and the onset of evaporation, salts remaining within the soil profile were redistributed upward due to evaporation-induced capillary flow. This process was particularly evident in the non-mulched treatment (M0), where substantial salt accumulation occurred in the upper soil layer. In both soils, the surface EC values under M0 were considerably greater than the corresponding initial values, indicating strong upward transport and concentration of soluble salts near the evaporating surface.

In the loamy sand soil (S1), the initial EC value in the upper layer was approximately 1.06 dS m⁻¹, whereas it increased to 4.64 dS m⁻¹ under the non-mulched treatment. Application of sawdust mulch markedly reduced this accumulation, with surface EC decreasing to 1.38 and 1.07 dS m⁻¹ under the 2 cm (M1) and 4 cm (M2) mulch treatments, respectively. These values were very close to the initial salinity level, indicating that mulch effectively minimized evaporation-induced salt accumulation and maintained a more stable salinity distribution. Furthermore, the EC profiles under M1 and M2 were relatively uniform throughout the soil column, suggesting that the salinity distribution approached a near-equilibrium condition by the end of the evaporation period.

A similar response was observed in the sandy clay loam soil (S2), although salinity levels were generally higher than those recorded in S1. The surface EC increased from an initial value of approximately 1.24 dS m⁻¹ to 6.88 dS m⁻¹ under the non-mulched treatment, reflecting severe salt accumulation near the soil surface. Mulch application substantially reduced surface salinity, with EC values declining to 2.08 and 1.88 dS m⁻¹ under M1 and M2, respectively. Despite this reduction, the salinity distribution in S2 remained less uniform than in S1, as EC values in the lower half of the profile were generally greater than those in the upper half. This pattern indicates that salt redistribution in the sandy clay loam soil had not yet reached equilibrium by the end of the evaporation period.

Comparison between the two soils revealed no significant differences in EC values at most profile depths. However, a significant difference was observed at the deepest layer (55 cm), where the average EC value was approximately 1.10 dS m⁻¹ in the loamy sand soil and 3.80 dS m⁻¹ in the sandy clay loam soil. The greater salinity observed in the lower layer of S2 reflects the higher water-holding capacity and stronger salt retention characteristics of the finer-textured soil.

The statistical analysis demonstrated a highly significant effect of mulch application on salinity distribution after evaporation (p ≤ 0.01). Significant differences were detected between the non-mulched treatment (M0) and both mulch treatments (M1 and M2) within the upper 10 cm layer in both soils, where salt accumulation was greatest. In contrast, no significant differences among treatments were observed in the remaining profile layers. Moreover, no significant differences were detected between the 2 cm and 4 cm mulch treatments, indicating that both mulch depths were similarly effective in suppressing surface salt accumulation. These results confirm that the statistical outputs correspond to Supplementary Table S3, which summarizes the full ANOVA results for post-evaporation salinity distribution.

The comparison with the Initial profile further confirms the role of mulch in controlling salinity development. While the non-mulched treatment produced a pronounced salinity gradient characterized by strong surface accumulation, the mulch treatments-maintained EC values close to their initial levels and reduced vertical salinity gradients throughout the profile. This response indicates that mulch reduced evaporation from the soil surface, weakened upward capillary flow, and consequently limited the upward transport of dissolved salts.

Overall, the results demonstrate that sawdust mulching effectively mitigated evaporation-induced salt accumulation and promoted a more uniform salinity distribution within both soils. The beneficial effect was most evident in the surface layer, where mulch reduced EC by approximately 70–80% relative to the non-mulched treatment. These findings highlight the importance of mulch application as an effective management practice for reducing salt accumulation and maintaining favorable soil conditions under arid and semi-arid environments.

3.5. Nitrate Dynamics and Transport

3.5.1. Nitrate Leaching

The amount of nitrate leached in the drainage water under different nitrate nitrogen application rates is presented in Figure 11. The results demonstrated significant differences in nitrate leaching between the two soil textures and among nitrogen application levels, indicating strong effects of soil hydraulic properties and fertilizer rate on nitrate mobility during free drainage.

As shown in Figure 11, the loamy sand soil (S1) consistently exhibited significantly greater nitrate leaching than the sandy clay loam soil (S2) under all nitrogen application rates. The amount of nitrate leached from the loamy sand soil exceeded that from the sandy clay loam soil by approximately 1.55, 1.25, 1.55, and 1.80 times under R0, R1, R2, and R3 treatments, respectively. These results indicate that the coarse-textured loamy sand soil was more susceptible to nitrate loss through leaching due to its larger pore system, greater hydraulic conductivity, and lower nitrate retention capacity. The greater nitrate retention observed in the sandy clay loam soil reflects its finer texture, higher water-holding capacity, and lower permeability relative to the loamy sand soil. These properties reduced downward water flux and prolonged nitrate residence time within the soil profile, thereby decreasing nitrate transport with drainage water. Nitrate leaching increased progressively with increasing nitrogen application rate in both soils. Relative to the control treatment (R0), nitrate leaching from the loamy sand soil increased by approximately 74.7%, 121.6%, and 158.8% under nitrogen application rates of 200, 300, and 400 kg NO₃⁻–N ha⁻¹, respectively. Similarly, nitrate leaching from the sandy clay loam soil increased by approximately 115.4%, 120.5%, and 121.8% under the same nitrogen treatments. The increase in nitrate leaching with increasing nitrogen application reflects the greater concentration of soluble nitrate ions within the soil solution following fertilizer addition. Higher nitrate concentrations increased the amount of nitrate available for downward transport during gravitational drainage, particularly under conditions of heavy irrigation and free drainage. The effect of nitrogen application rate on nitrate leaching was especially pronounced in the loamy sand soil, where rapid water movement through the coarse pore network facilitated nitrate transport beyond the soil profile. In contrast, the sandy clay loam soil retained a larger proportion of nitrate within the soil matrix, thereby reducing nitrate losses in the leachate.

The observed differences between the two soils are consistent with the previously described hydraulic behavior (Figure 2 and Figure 3), where the loamy sand soil exhibited greater hydraulic conductivity and faster water transmission, while the sandy clay loam soil maintained greater water retention and lower permeability. These contrasting hydraulic characteristics strongly controlled nitrate mobility and leaching behavior during drainage. The fraction of total nitrate lost through leaching also differed between the two soils and among nitrogen application rates. In the loamy sand soil, nitrate leaching fractions ranged from approximately 58.3% under the control treatment to approximately 69.7% under the highest nitrogen application rate. Corresponding values in the sandy clay loam soil ranged from approximately 55.9% to 45.7%. In general, nitrate leaching fractions were consistently greater in the loamy sand soil than in the sandy clay loam soil, particularly under the higher nitrogen application rates.

The results presented in Figure 11 demonstrate that nitrate leaching was strongly influenced by both soil texture and nitrogen application rate. The loamy sand soil showed substantially greater susceptibility to nitrate loss through leaching, whereas increasing nitrate application rates significantly enhanced nitrate transport in both soils. Statistical analysis using factorial ANOVA confirmed that both soil texture and nitrate nitrogen application rate had highly significant effects on nitrate leaching (p ≤ 0.01), and a significant interaction effect (S × R) was also detected, indicating that the response of nitrate leaching to fertilizer rate depended on soil texture (Supplementary Table S2).

3.5.2. Nitrate Distribution in Soil Profile

The vertical distribution of nitrate within the soil profile immediately after free drainage and after 83 days of evaporation is presented in Figure 12 and Figure 13, respectively. The obtained results indicate that nitrate redistribution was strongly influenced by water movement within the soil profile, soil texture, nitrogen application rate, evaporation, and mulch application. Distinct redistribution patterns were observed following free drainage and after the subsequent evaporation period. Figure 12 shows the nitrate distribution within the loamy sand (S1) and sandy clay loam (S2) soils immediately after free drainage under the four nitrate–nitrogen application rates (R0, R1, R2, and R3). The results indicate that nitrate distribution was relatively uniform throughout most of the soil profile in both soils, except within the deepest layer (55 cm), where nitrate accumulation increased markedly with increasing nitrogen application rate. Statistical analysis revealed no significant differences among nitrogen application rates throughout most profile depths in either soil, whereas significant differences were observed in the bottom layer (p ≤ 0.01).

In the loamy sand soil (S1), nitrate concentration at 55 cm increased from 27.11 mg under R0 to 48.86, 81.80, and 110.97 mg under R1, R2, and R3, respectively. Relative to the control treatment (R0), nitrate accumulation increased by approximately 75.9%, 194.5%, and 285.1% under R1, R2, and R3, respectively. Similar trends were observed in the sandy clay loam soil (S2), although the magnitude of accumulation was substantially greater. Nitrate concentration at the bottom layer increased from 14.18 mg under R0 to 53.16, 137.50, and 230.73 mg under R1, R2, and R3, respectively, corresponding to increases of approximately 211.7%, 798.5%, and 1289.5% relative to the control treatment. These results clearly demonstrate the downward transport of nitrate with drainage water and the accumulation of nitrate within the lower portion of the soil profile following heavy irrigation.

The results further indicate substantial nitrate leaching during free drainage in both soils. However, nitrate movement appeared more rapid in the loamy sand soil because of its higher permeability and lower water-holding capacity. At the same time, nitrate accumulation within the lower profile was more pronounced in the sandy clay loam soil, reflecting its greater capacity to retain drainage water and dissolved nitrate within the deeper layers. Consequently, the lower portion of the sandy clay loam profile acted as an important zone of nitrate accumulation following free drainage.

The effect of sawdust mulch on nitrate redistribution after 83 days of evaporation is presented in Figure 13. The results indicate that nitrate moved upward from the lower layers toward the soil surface during evaporation, particularly under the non-mulched treatment (M0). This upward redistribution was associated with evaporation-induced upward water movement and was considerably reduced by mulch application. In both soils, nitrate distribution remained relatively uniform throughout most of the profile, whereas nitrate accumulation increased substantially within the upper 0–10 cm layer.

In the loamy sand soil (S1), nitrate concentration within the surface layer reached 97.35 mg under M0, compared with 50.44 mg under the 2 cm mulch treatment (M1) and 33.06 mg under the 4 cm mulch treatment (M2). The corresponding Initial value before the beginning of evaporation was only 21.00 mg. Similarly, in the sandy clay loam soil (S2), nitrate concentration within the surface layer increased to 83.55 mg under M0, whereas values decreased to 44.10 and 40.36 mg under M1 and M2, respectively, compared with an Initial value of 22.81 mg. These results clearly indicate substantial nitrate accumulation near the soil surface during evaporation and demonstrate the effectiveness of mulch in reducing upward nitrate transport. Statistical analysis using factorial ANOVA confirmed that soil texture, nitrogen application rate, mulch treatment, and their interactions significantly affected nitrate distribution within the soil profile at different depths (p ≤ 0.01), as summarized in Supplementary Table S3.

Comparison of the mulch treatments with the Initial nitrate distribution further demonstrates the effect of evaporation on nitrate redistribution. In the loamy sand soil, the Initial nitrate concentration increased from 21.00 mg at 5 cm depth to 67.18 mg at 55 cm depth, whereas in the sandy clay loam soil the corresponding Initial values ranged from 22.81 mg at the surface to 108.89 mg at the bottom layer. Following evaporation, nitrate accumulated near the surface under M0, while the use of M1 and particularly M2 reduced this upward redistribution and promoted a more uniform nitrate distribution throughout the profile. These results indicate that sawdust mulch effectively suppressed evaporation-driven upward nitrate transport by reducing soil water losses and maintaining greater moisture contents within the soil profile.

Overall, the results demonstrate two contrasting nitrate redistribution processes. Free drainage promoted downward nitrate transport and accumulation within the lower profile, particularly under the higher nitrogen application rates (R2 and R3), whereas prolonged evaporation induced upward nitrate movement and accumulation near the soil surface, especially under the non-mulched treatment (M0). Mulch application substantially reduced this upward redistribution, with the 4 cm mulch treatment (M2) providing the greatest reduction in surface nitrate accumulation.

The significant effects of soil texture, nitrogen application rate, and mulch treatment, together with the observed redistribution patterns, confirm that nitrate movement within the soil profile was strongly controlled by the interaction between water flow processes and soil hydraulic properties.

3.6. Nitrate Balance

Nitrate balance was evaluated under both drainage and evaporation conditions to assess the influence of soil texture, nitrate application rate, and mulch treatment on nitrate retention and redistribution within the soil system.

3.6.1. Nitrate Balance After Drainage

The nitrate balance within the soil system immediately after free drainage is presented in

Table 4. Nitrate balance after free drainage in loamy sand (S1) and sandy clay loam (S2) soils under different nitrate nitrogen application rates.

Soil Type	Treatment	Nitrate in Leachate (mg)	Nitrate Retained in Soil (mg)	Total Recovered Nitrate (mg)	Initial + Applied Nitrate (mg)	Difference (mg)
Loamy sand	R0	181.86	128.67	310.53	312.10	−1.57
Loamy sand	R1	317.66	175.52	493.18	493.68	−0.50
Loamy sand	R2	403.02	181.15	584.17	585.16	−0.99
Loamy sand	R3	470.70	202.60	673.30	675.26	−1.96
Sandy clay loam	R0	117.61	88.78	206.39	207.10	−0.71
Sandy clay loam	R1	253.28	133.93	387.21	388.68	−1.47
Sandy clay loam	R2	259.32	218.64	477.96	480.16	−2.20
Sandy clay loam	R3	260.86	307.07	567.93	570.26	−2.33

. The obtained results provide a quantitative assessment of nitrate partitioning between leaching losses and nitrate retained within the soil profile following the drainage event. The balance calculations demonstrated that both nitrate leaching and nitrate retention increased with increasing nitrate–nitrogen application rate in the two soils. However, substantial differences were observed between the loamy sand soil (S1) and the sandy clay loam soil (S2), indicating a strong influence of soil texture on nitrate transport, retention, and overall nitrate recovery.

In the loamy sand soil (S1), the amount of nitrate recovered in the drainage water increased progressively with increasing nitrogen application rate. Nitrate leaching increased from 181.86 mg under the control treatment (R0) to 317.66 mg under R1, 403.02 mg under R2, and 470.70 mg under R3. Relative to the control treatment, nitrate losses increased by approximately 74.7%, 121.6%, and 158.8% under R1, R2, and R3, respectively. These results indicate that increasing nitrate application substantially enhanced nitrate mobility within the coarse-textured soil and increased the quantity of nitrate transported beyond the soil profile during free drainage.

At the same time, the quantity of nitrate retained within the loamy sand profile also increased with increasing nitrogen application rate, although at a considerably lower rate than nitrate leaching. Nitrate retained within the soil profile increased from 128.67 mg under R0 to 175.52 mg under R1, 181.15 mg under R2, and 202.60 mg under R3. Despite this increase in nitrate retention, the proportion of nitrate remaining within the soil profile decreased progressively as nitrogen application rate increased. Approximately 41.4% of the recovered nitrate remained within the soil profile under the control treatment, whereas this proportion decreased to approximately 35.6%, 31.0%, and 30.1% under R1, R2, and R3, respectively. Conversely, nitrate losses through leaching increased from approximately 58.6% of the recovered nitrate under R0 to approximately 69.9% under R3. These results clearly demonstrate that the loamy sand soil became increasingly susceptible to nitrate loss as nitrogen application increased, reflecting its coarse texture, high hydraulic conductivity, and limited capacity to retain dissolved nitrate during drainage.

A different response was observed in the sandy clay loam soil (S2). Although nitrate leaching increased with increasing nitrogen application rate, the magnitude of increase was considerably smaller than that observed in the loamy sand soil. Nitrate leaching increased from 117.61 mg under R0 to 253.28 mg under R1, 259.32 mg under R2, and 260.86 mg under R3. The large increase between R0 and R1 indicates a strong initial response to nitrogen addition; however, further increases in fertilizer application produced only modest increases in nitrate losses through drainage. This behavior suggests that the finer-textured soil possessed a greater capacity to retain additional nitrate within the soil matrix and thereby limit further increases in nitrate transport with drainage water.

The quantity of nitrate retained within the sandy clay loam profile increased markedly with increasing nitrogen application rate. Nitrate retained within the soil profile increased from 88.78 mg under R0 to 133.93 mg under R1, 218.64 mg under R2, and 307.07 mg under R3. The increase in nitrate retention between the lowest and highest nitrogen application rates exceeded 245%, demonstrating the substantial capacity of the sandy clay loam soil to retain dissolved nitrate within the profile. Unlike the loamy sand soil, the proportion of recovered nitrate retained within the soil increased with increasing nitrogen application rate. Retention represented approximately 43.0% of recovered nitrate under R0 and increased to approximately 34.6%, 45.7%, and 54.1% under R1, R2, and R3, respectively. Correspondingly, the proportion lost through leaching decreased from approximately 57.0% under R0 to approximately 45.9% under R3. These results indicate that the sandy clay loam soil retained an increasingly greater proportion of the applied nitrate as fertilizer application increased.

Comparison between the two soils clearly demonstrates the dominant role of soil texture in controlling nitrate fate during free drainage. Under all nitrogen application rates, nitrate losses through leaching were greater in the loamy sand soil than in the sandy clay loam soil. For example, under the highest nitrogen application rate (R3), nitrate leaching reached 470.70 mg in the loamy sand soil compared with only 260.86 mg in the sandy clay loam soil. In contrast, nitrate retained within the soil profile under the same treatment reached 307.07 mg in the sandy clay loam soil compared with only 202.60 mg in the loamy sand soil. These differences reflect the contrasting hydraulic characteristics of the two soils. The larger pore system and greater permeability of the loamy sand soil promoted rapid downward water movement and enhanced nitrate transport beyond the root zone, whereas the greater water-holding capacity and lower permeability of the sandy clay loam soil increased nitrate residence time and promoted retention within the soil profile.

The total recovered nitrate values showed excellent agreement with the total amount of nitrate initially present in the soil system plus the applied nitrate fertilizer. In the loamy sand soil, total recovered nitrate ranged from 310.53 to 673.30 mg, compared with expected values ranging from 312.10 to 675.26 mg. Similarly, in the sandy clay loam soil, total recovered nitrate ranged from 206.39 to 567.93 mg, whereas the corresponding expected values ranged from 207.10 to 570.26 mg. The differences between recovered and expected nitrate quantities remained very small for all treatments, varying from only −0.50 to −1.96 mg in the loamy sand soil and from −0.71 to −2.33 mg in the sandy clay loam soil. These small discrepancies indicate a high degree of nitrate recovery and confirm the reliability of the nitrate balance calculations. The close agreement between measured and expected values further suggests that nitrate losses through processes other than leaching were negligible during the drainage period.

Statistical analysis confirmed that both soil texture and nitrate–nitrogen application rate exerted highly significant effects on nitrate partitioning between leaching losses and nitrate retained within the soil profile (p ≤ 0.01). Nitrate leaching increased significantly with increasing nitrogen application rate, while nitrate retention was strongly influenced by soil texture. The loamy sand soil exhibited significantly greater nitrate losses through leaching, whereas the sandy clay loam soil retained significantly greater amounts of nitrate within the soil profile, particularly under the higher nitrogen application rates. Significant interactions between soil texture and nitrogen application rate were also detected (S × RF), indicating that the response of nitrate transport and retention to increasing fertilizer addition depended strongly on soil hydraulic properties. These findings demonstrate that the fate of nitrate following irrigation and free drainage is governed not only by the amount of nitrate applied but also by the capacity of the soil to retain water and dissolved solutes within the profile. The nitrate balance results demonstrate that increasing nitrate–nitrogen application enhanced both nitrate leaching and nitrate retention in the two soils. However, the relative importance of these processes differed markedly between soil textures. The loamy sand soil lost a larger proportion of nitrate through leaching, whereas the sandy clay loam soil retained a greater proportion within the soil profile. Consequently, soil texture emerged as the primary factor controlling nitrate partitioning and nitrate recovery following free drainage.

3.6.2. Nitrate Balance After Evaporation

The nitrate balance after 83 days of evaporation is presented in

Table 5. Net nitrate balance after 83 days of evaporation under different mulch treatments.

Soil Type	Mulch Depth	Net NO3–N Change (%)
Loamy sand	0 cm	+2.2 to +7.6
Loamy sand	2 cm	−4.3 to −5.9
Loamy sand	4 cm	−4.7 to −7.0
Sandy clay loam	0 cm	+3.9 to +20.3
Sandy clay loam	2 cm	−4.8 to −6.5
Sandy clay loam	4 cm	−5.4 to −6.8

. The results demonstrated clear effects of both soil texture and sawdust mulching on nitrate retention and loss within the soil profile following the evaporation period. Under non-mulched conditions, both soils exhibited positive nitrate balance values, indicating net increases in soil nitrate content during the evaporation period.

Net nitrate gain ranged from approximately 2.2–7.6% in the loamy sand soil and from 3.9–20.3% in the sandy clay loam soil. In contrast, mulch application resulted in negative nitrate balance values in both soils, indicating net nitrate depletion relative to the initial nitrate content. Under the 2 cm mulch treatment, nitrate loss ranged from approximately 4.3–5.9% in the loamy sand soil and from 4.8–6.5% in the sandy clay loam soil. Increasing mulch depth to 4 cm further increased nitrate depletion, with losses ranging from approximately 4.7–7.0% and 5.4–6.8% in the loamy sand and sandy clay loam soils, respectively. Although mulch application resulted in moderate nitrate depletion, the overall nitrate losses remained relatively limited and generally did not exceed approximately 10% of the initial nitrate content. These results indicate that sawdust mulching substantially improved soil water conservation while causing only limited reductions in nitrate retention within the soil profile. The positive nitrate balance observed under non-mulched conditions can be related to nitrate redistribution and concentration processes occurring during prolonged evaporation, where upward and downward water fluxes influenced nitrate accumulation within the sampled soil profile. In contrast, the negative balance under mulched conditions reflects the modified water movement and reduced evaporative concentration effects associated with surface mulch. The observed balance differences therefore represent changes in nitrate distribution within the soil system rather than simple nitrate gain or loss alone.

In general, the results demonstrate that mulch application altered nitrate balance during evaporation by reducing nitrate accumulation and promoting moderate nitrate depletion, with the magnitude of change depending on both mulch depth and soil texture. Statistical analysis revealed that mulch application significantly affected nitrate balance after evaporation (p ≤ 0.01), whereas soil texture also exerted a significant influence on nitrate retention and depletion patterns within the soil profile. The interaction between mulch depth and soil texture was significant, indicating that the magnitude of nitrate depletion under mulched conditions differed between the loamy sand and sandy clay loam soils. Non-mulched treatments exhibited significantly greater nitrate accumulation within the soil profile compared with the mulched treatments, while increasing mulch depth significantly reduced nitrate accumulation and promoted more stable nitrate distribution during evaporation.

3.7. Composite Sustainability Index (CSI)

A Composite Sustainability Index (CSI) was developed to provide an integrated evaluation of the combined effects of mulch application and soil texture on the major hydrological and chemical processes controlling soil sustainability under arid conditions. While the individual analyses presented in previous sections described the responses of soil water storage, evaporation, salinity, and nitrate dynamics separately, practical soil management decisions often require simultaneous consideration of all these factors. Therefore, a composite index was constructed to integrate multiple sustainability indicators into a single quantitative metric that reflects the overall performance of each treatment.

The CSI incorporated four principal indicators representing the dominant processes affected by mulch application: soil water conservation, evaporation suppression, salinity control, and nitrate control. Each indicator was normalized to a common scale ranging from 0 to 1, where higher values represented more favorable environmental performance. The normalized indicators were then combined to generate a single sustainability score for each treatment. The resulting CSI values are presented in

Table 6. Composite sustainability index (CSI) integrating soil water conservation, evaporation suppression, salinity control, and nitrate control under different sawdust mulch treatments.

Soil Type	Mulch Depth	Soil Water Conservation	Evaporation Suppression	Salinity Control	Nitrate Control	CSI
Loamy sand	0 cm	0.12	0.18	0.21	0.26	0.19
Loamy sand	2 cm	0.86	0.88	0.79	0.80	0.83
Loamy sand	4 cm	0.92	0.94	0.88	0.78	0.88
Sandy clay loam	0 cm	0.15	0.14	0.18	0.20	0.17
Sandy clay loam	2 cm	0.89	0.85	0.86	0.87	0.87
Sandy clay loam	4 cm	0.94	0.92	0.91	0.84	0.90

.

As shown in Table 6, substantial differences in sustainability performance were observed among mulch treatments and between soil textures. In both soils, the non-mulched treatment (M0) consistently produced the lowest CSI values, with scores of 0.19 and 0.17 for the loamy sand and sandy clay loam soils, respectively. These low values reflected the combined effects of poor soil water conservation, weak evaporation suppression, greater salinity accumulation, and unfavorable nitrate redistribution during the evaporation period. The low CSI values obtained under non-mulched conditions demonstrate the vulnerability of uncovered soils to evaporation-driven degradation processes under arid environments. Application of sawdust mulch substantially improved all sustainability indicators and consequently increased CSI values in both soils. According to Table 6, the 2 cm mulch treatment (M1) increased CSI from 0.19 to 0.83 in the loamy sand soil and from 0.17 to 0.87 in the sandy clay loam soil. These marked improvements indicate that even a moderate mulch depth effectively enhanced overall soil sustainability through simultaneous improvements in soil water conservation, evaporation suppression, salinity control, and nitrate management.

Table 6 further shows that the 4 cm mulch treatment (M2) produced the highest CSI values in both soils, reaching 0.88 in the loamy sand soil and 0.90 in the sandy clay loam soil. The higher scores obtained under M2 indicate that the thicker mulch layer provided the best overall performance when all sustainability indicators were considered together. However, the improvement achieved by increasing mulch depth from 2 to 4 cm was relatively modest compared with the substantial gains obtained by the initial application of mulch. This trend agrees with the results presented previously for soil water storage (Table 3), where both mulch treatments markedly improved water conservation relative to the control, whereas differences between the 2 and 4 cm mulch depths were comparatively small. The CSI results are also consistent with the evaporation, salinity, and nitrate analyses presented in earlier sections. Treatments exhibiting higher soil water storage and lower cumulative evaporation generally produced higher CSI values. Similarly, treatments that reduced salt accumulation and moderated nitrate redistribution achieved improved sustainability scores. The nitrate-control component of the CSI was derived from nitrate redistribution patterns observed during the evaporation phase (Table 5), whereas Table 4 describes nitrate recovery and mass balance during the free-drainage stage. Consequently, the CSI integrates information from both the hydrological and chemical responses of the soil system while avoiding reliance on a single indicator.

To evaluate the sensitivity of the CSI to weighting assumptions, treatment rankings were recalculated using four alternative weighting schemes emphasizing soil water conservation, evaporation suppression, salinity control, or nitrate control. The resulting treatment rankings are presented in

Table 7. Sensitivity analysis of CSI treatment rankings under alternative weighting schemes.

Treatment	Equal Weighting	Water Priority	Evaporation Priority	Salinity Priority	Nitrate Priority
Loamy sand, 0 cm	5	5	5	5	5
Loamy sand, 2 cm	4	4	4	4	4
Loamy sand, 4 cm	2	2	2	2	3
Sandy clay loam, 0 cm	6	6	6	6	6
Sandy clay loam, 2 cm	3	3	3	3	2
Sandy clay loam, 4 cm	1	1	1	1	1

. As shown in Table 7, the rankings remained highly stable across all tested scenarios. The sandy clay loam soil with 4 cm mulch (SCL-M2) consistently achieved the highest ranking, whereas the non-mulched sandy clay loam treatment (SCL-M0) remained the lowest-ranked treatment under all weighting schemes. Only a minor interchange between LS-M2 and SCL-M1 was observed when nitrate control received greater weighting, reflecting the slightly superior nitrate-control performance of SCL-M1. Overall, the sensitivity analysis demonstrated that the CSI conclusions were robust and not materially influenced by the weighting assumptions adopted for index construction.

An important advantage of the CSI is its ability to reveal overall treatment performance that may not be fully apparent when individual variables are evaluated separately. For example, some treatments may perform well with respect to water conservation but provide smaller improvements in nitrate control, whereas other treatments may produce more balanced responses across several indicators. By integrating all indicators into a single metric, the CSI facilitates direct comparison among treatments and provides a comprehensive assessment of sustainability performance.

Differences between soil textures were also evident in the CSI values. The sandy clay loam soil consistently achieved slightly higher CSI values than the loamy sand soil under comparable mulch treatments. This behavior reflects the greater water-holding capacity and lower susceptibility to rapid moisture loss characteristic of finer-textured soils. Nevertheless, the relative improvement resulting from mulch application was particularly pronounced in the loamy sand soil, where evaporation losses and moisture depletion were most severe under non-mulched conditions.

The consistency between Table 6 and the individual results presented in Table 3, Table 4 and Table 5 confirms the robustness of the composite index. Treatments that achieved superior performance in water conservation, evaporation suppression, salinity control, and nitrate management also received the highest CSI scores. This agreement demonstrates that the CSI successfully captured the integrated response of the soil system to mulch management under arid conditions.

Overall, the CSI provided a practical framework for synthesizing multiple hydrological and chemical indicators into a single sustainability metric. The results demonstrate that sawdust mulching substantially improved overall soil sustainability through its combined effects on conserving soil water, suppressing evaporation, reducing salinity accumulation, and limiting nitrate redistribution. Although the 4 cm mulch treatment produced the highest CSI values, the 2 cm mulch treatment achieved nearly comparable sustainability performance while requiring less mulch material. Therefore, the CSI highlights the potential of moderate sawdust mulch applications to provide efficient and sustainable soil management under arid and semi-arid conditions, while also serving as a useful decision-support tool for evaluating alternative soil conservation practices.

4. Discussion

4.1. Influence of Soil Texture on Hydraulic and Water Retention Behavior

Soil texture exerts fundamental control over unsaturated hydraulic behavior through its influence on pore-size hierarchy, connectivity, tortuosity, and capillary continuity. The contrasting responses observed between loamy sand and sandy clay loam are consistent with differences in pore-network architecture that are widely recognized to influence water retention, hydraulic conductivity, and moisture redistribution under drying conditions. These differences arise from the inverse relationship between pore radius and matric potential, where smaller pores generate higher capillary forces that sustain water retention under increasing suction [3,4]. The higher volumetric water content in sandy clay loam may be explained by its greater proportion of micropores and intra-aggregate storage domains, which maintain liquid continuity under elevated matric stress. This dual retention mechanism—capillary and physicochemical adsorption—may contribute to delayed desaturation and prolonged water retention. In contrast, loamy sand is characterized by rapid hydraulic breakdown due to dominance of macropores functioning mainly as transmission pathways. Once air-entry pressure is exceeded, these pores empty rapidly, causing abrupt collapse of conductive water films and exponential reduction in hydraulic conductivity [32]. This sharp decline in coarse soil moisture reflects early disconnection of liquid pathways during evaporation-driven drying. Similar pore-scale transitions have been reported in sandy arid soils where conductive networks fragment rapidly once capillary continuity is disrupted [33]. In contrast, the observed behavior of the sandy clay loam soil is consistent with mechanisms reported in the literature whereby partially connected water films may persist across a wider suction range, allowing more gradual desaturation through capillary film flow and adsorbed water migration along particle surfaces. Recent pore-network studies suggest that micropore systems can preserve hydraulic continuity through stable capillary bridges even at high matric potential [34].

Hysteretic effects may further amplify these contrasts. Coarse soils exhibit rapid loss of conductive domains and early air entrapment, whereas fine soils undergo gradual pore emptying and delayed hydraulic failure. Such hysteretic effects strongly regulate evaporation rates, moisture redistribution, and solute transport under arid conditions [35]. Importantly, results confirm that effective hydraulic storage is governed more by pore accessibility and connectivity than total porosity, as macropores contribute minimally to retention under unsaturated conditions [36]. Soil texture appears to establish the hydraulic baseline influencing subsequent evaporation and solute transport processes. Thus, soil texture appears to play a dominant role in regulating water retention and solute redistribution processes under arid environments.

4.2. Evaporation Regulation by Mulch Cover

Evaporation from unsaturated soils is governed by coupled thermal, hydraulic, and aerodynamic processes operating at the soil–atmosphere interface. Under arid conditions, evaporation initially proceeds through a capillary-supported stage characterized by hydraulic continuity between deeper wet layers and the evaporating surface. As drying progresses, hydraulic conductivity declines and evaporation gradually shifts toward a diffusion-controlled regime dominated by vapor transport through increasingly disconnected pore spaces. The application of sawdust mulch substantially altered this process, likely through mechanisms associated with changes in surface energy exchange and near-surface hydraulic conditions. Organic mulch likely functioned as a porous insulating layer that reduced surface energy exchange and increased resistance to vapor diffusion, thereby suppressing evaporation intensity [7,37]. The observed reduction in evaporation is consistent with a reduced hydraulic connection between surface and subsurface moisture sources resulting from lower evaporative demand and an earlier transition toward the lower-flux diffusion-controlled stage.

The strong reduction in evaporation observed under mulched conditions is consistent with reduced liquid-phase upward flow, although the relative contribution of individual transport mechanisms was not directly measured in the present study. Evaporation in arid soils depends strongly on the maintenance of hydraulic continuity between deeper wet zones and the evaporating surface. Once capillary continuity weakens, vapor diffusion becomes the dominant transport mechanism and evaporation declines sharply. Previous studies have shown that organic mulches reduce evaporation primarily by shortening the duration of capillary-supported evaporation rather than by directly blocking vapor movement [38]. The nonlinear response to increasing mulch thickness may indicate the existence of a hydraulic threshold beyond which additional resistance yields progressively smaller reductions in evaporation [9]. Similar threshold behavior has been reported in dryland mulch systems where moderate mulch depths achieved most of the attainable evaporation suppression while thicker layers produced only incremental improvements [39]. Soil texture also influenced mulch effectiveness, which may be related to differences in pore characteristics and capillary transport behavior reported for contrasting soil textures. Coarse-textured soils exhibited greater relative improvement under mulching, possibly because their weaker capillary continuity makes evaporation more sensitive to surface resistance. In contrast, finer-textured soils may maintain water retention over longer drying periods because of stronger capillary forces associated with smaller pore sizes. Nevertheless, mulch substantially reduced evaporation in both soils because suppression of surface energy exchange directly weakened the hydraulic gradients driving upward water movement.

The insulating properties of sawdust mulch likely contributed to stabilization of near-surface hydrothermal conditions. Organic residues possess low thermal diffusivity and therefore reduce conductive heat transfer into the soil profile, resulting in lower evaporative demand and weaker vapor pressure gradients. Increased humidity within the mulch layer may further reduce atmospheric vapor deficit near the soil surface. These combined thermal and hydraulic effects likely contributed to the more uniform soil moisture distribution observed under mulched conditions.

Finally, mulch application appears to function as an integrated regulator of evaporation by simultaneously modifying surface energy balance, capillary continuity, vapor diffusion resistance, and hydraulic redistribution processes. These mechanisms are consistent with the observed improvements in soil water conservation and reduced evaporative stress under arid conditions. The present findings are consistent with the general effectiveness of organic mulching reported in previous studies [7,39,40] and further suggest that mulch performance depended strongly on the interaction between surface resistance and soil hydraulic properties. Compared with earlier investigations that focused primarily on evaporation reduction, the current results demonstrated substantial reductions in cumulative evaporation under both soil types. Relative to the non-mulched treatment, cumulative evaporation decreased by approximately 70.6% and 69.8% under the 2 cm mulch treatment in the loamy sand and sandy clay loam soils, respectively, whereas the corresponding reductions under the 4 cm mulch treatment reached 77.9% and 79.1%. Thus, increasing mulch depth from 2 to 4 cm provided only an additional reduction of approximately 7.3 and 9.3 percentage points in the loamy sand and sandy clay loam soils, respectively. These results indicate that most of the evaporation suppression was achieved with the initial mulch application, while further increases in mulch thickness produced comparatively smaller gains. This behavior suggests that hydraulic threshold effects may play a more important role than mulch quantity alone in controlling evaporation suppression under arid conditions.

4.3. Mechanisms of Salt Redistribution Under Evaporation and Mulching

Salt redistribution within unsaturated soil profiles is closely linked to water movement driven by hydraulic gradients and evaporation. Under arid conditions, evaporation promotes upward transport of saline soil solution toward the soil surface, where water removal concentrates dissolved salts and increases salinity in near-surface layers [41]. The pronounced salt accumulation observed under non-mulched conditions indicates that evaporation-driven upward transport was the dominant process governing salinity redistribution during the experimental period.

The greater salinity accumulation observed in the sandy clay loam soil can be attributed to its higher water retention and stronger ability to sustain upward movement of dissolved salts for longer periods during drying. In contrast, the loamy sand soil exhibited weaker retention and more rapid disruption of upward water flow, limiting prolonged salt accumulation near the soil surface. These results demonstrate that soil texture played an important role in controlling the intensity and distribution of salinity development during evaporation [42].

Mulch application substantially reduced salinity accumulation by decreasing evaporative demand and limiting upward water fluxes. Reduced evaporation lowered the transport of dissolved salts toward the soil surface and resulted in a more uniform salinity distribution throughout the soil profile [10,11]. The weaker salinity gradients observed under mulched treatments indicate that suppression of evaporation was the primary mechanism responsible for reducing salt redistribution and mitigating surface salinization [43]. The use of potassium nitrate as the nitrogen source also introduced potassium ions into the soil solution, contributing to the overall ionic composition during redistribution. Consequently, electrical conductivity patterns reflected the combined movement and concentration of dissolved ions under drainage and evaporation processes. The observed salinity dynamics therefore represent the integrated response of soluble ions to changes in water fluxes and evaporative concentration.

Recent hydrochemical investigations have emphasized that evaporation-induced salinity accumulation may be strongly influenced by the persistence of liquid-phase conditions within soil pores [44]. Once dominant flow pathways weaken, solute redistribution shifts increasingly toward diffusion-controlled transport processes. Mulch likely reduced the intensity of evaporation-driven hydraulic gradients, thereby limiting sustained upward solute transport. Thus, the findings support the interpretation that surface salinization under arid conditions emerges from coupled interactions between evaporation intensity, soil hydraulic properties, and advective solute transport.

4.4. Nitrate Mobility and Leaching Vulnerability

Nitrate transport within unsaturated soil is governed predominantly by advective water movement because nitrate ions exhibit minimal adsorption to negatively charged mineral surfaces and remain highly soluble within the soil solution. Consequently, nitrate mobility depends strongly on hydraulic conductivity, pore connectivity, residence time, and drainage intensity [12,27]. The contrasting nitrate transport behavior observed between the two soils reflects fundamental differences in hydraulic architecture and vadose-zone flow dynamics. The substantially greater nitrate leaching observed in the loamy sand soil resulted from rapid preferential transport through highly conductive macropore domains. Coarse-textured soils possess larger and better-connected transmission pores that facilitate rapid percolation and reduce solute residence time within the soil matrix. Under near-saturated conditions, nitrate displacement becomes closely coupled with gravitational water flow, resulting in efficient downward transport beyond the root zone. Similar nitrate transport responses have recently been documented in sandy agricultural soils subjected to high irrigation and fertilizer loading under arid environments [45].

The near-linear increase in nitrate leaching with increasing fertilizer application reflects concentration-dependent enhancement of convective mass transport. Greater nitrate availability within the soil solution increased the quantity of dissolved ions transported during drainage events. This relationship illustrates the strong coupling between solute concentration and advective water flux under high hydraulic loading [14]. Recent reactive transport simulations have similarly demonstrated that nitrate losses increase disproportionately once nitrogen application exceeds the hydraulic retention capacity of the vadose zone [46]. Fine-textured soils reduced nitrate displacement primarily through hydraulic rather than chemical retention mechanisms. Slower water movement and prolonged residence time increased opportunities for diffusion, dispersion, microbial immobilization, and potential denitrification processes [47]. The use of sawdust mulch may also contribute to nitrate retention by modifying the near-surface soil environment and reducing the intensity of water-driven nitrate redistribution. The lower nitrate movement observed under mulched conditions was mainly associated with reduced evaporation-induced upward fluxes and improved soil water conservation, which limited the transport pathways responsible for nitrate accumulation and displacement. Although nitrate adsorption remained limited, reduced pore-scale water velocity substantially decreased nitrate transport efficiency during drainage. The accumulation of nitrate within lower profile layers in the sandy clay loam soil indicates partial hydraulic retention within the vadose zone rather than complete leaching loss. Reduced permeability delayed downward transport and promoted nitrate storage within deeper unsaturated layers. In contrast, the loamy sand soil facilitated more direct displacement into the leachate because rapid percolation bypassed extended retention within the soil matrix.

Under evaporative conditions, nitrate redistribution shifted from downward convective transport toward upward capillary transport. Strong evaporative gradients induced upward movement of nitrate-rich soil solution toward the soil surface, producing accumulation within upper layers. This behavior confirms that nitrate redistribution under unsaturated conditions is governed largely by coupled hydraulic flux pathways rather than by sorption-controlled processes. Similar evaporation-driven nitrate stratification has recently been reported in irrigated dryland systems experiencing prolonged surface drying and capillary rise [48].

Mulch application substantially weakened upward nitrate transport by suppressing evaporation and reducing capillary flow intensity. Lower evaporative demand preserved more uniform moisture distribution and decreased upward convective nitrate displacement. Consequently, nitrate accumulation near the surface declined markedly under mulched conditions. These findings demonstrate that nitrate transport vulnerability under arid conditions depends strongly on the interaction between hydraulic conductivity, evaporation intensity, and pore-network continuity. Coarse-textured soils exhibited greater susceptibility to leaching under drainage conditions, whereas evaporation-induced redistribution became dominant under prolonged drying. Mulching effectively moderates both processes through stabilization of vadose-zone hydraulic conditions.

While previous studies have consistently identified coarse-textured soils as highly vulnerable to nitrate leaching [12,13,45], the present results further demonstrate how evaporation and mulch management modify this vulnerability through changes in hydraulic flux pathways. The observed differences between the two soils indicate that nitrate losses cannot be explained solely by fertilizer application rate but must also be interpreted in relation to soil hydraulic architecture and water redistribution processes. This integrated perspective extends the conventional assessment of nitrate leaching by explicitly linking nutrient transport to coupled soil water dynamics.

In this context, complete recovery of nitrate from the soil–leachate system is not always achievable due to the dynamic nature of solute redistribution and inherent variability associated with soil–water interactions. Minor deviations in nitrate mass balance may also reflect spatial heterogeneity within the soil profile and analytical resolution limits during sampling. Although sawdust properties were described based on established literature for wood-derived residues, inherent variability in material characteristics such as particle size distribution, carbon-to-nitrogen ratio, electrical conductivity, and water-holding capacity may still influence local hydraulic and solute transport behavior. These variations are expected under practical applications of organic mulches and do not alter the overall mechanistic interpretation of nitrate transport, which remains primarily governed by water flux dynamics.

Under controlled laboratory conditions, significant gaseous nitrogen losses are considered unlikely; therefore, observed deviations in nitrate balance are attributed mainly to redistribution processes within the soil profile and measurement-related uncertainty rather than true nitrogen loss from the system.

4.5. Coupled Transport of Water, Salts, and Nitrate

Water flow constitutes the primary driving force governing the redistribution of dissolved salts and nitrate within unsaturated soils. Consequently, changes in soil moisture distribution strongly influence the spatial movement and accumulation of solutes throughout the soil profile. These coupled processes are commonly described by Richards’ equation for unsaturated water flow and convection–dispersion equations for solute transport [49]. Within this theoretical framework, changes in soil water content are expected to modify hydraulic conductivity, matric gradients, and solute fluxes, thereby influencing the redistribution of both salts and nitrate throughout the soil profile. The correspondence between moisture redistribution patterns and solute accumulation observed in the present study is consistent with the coupled flow and transport behavior described by these governing relationships.

The experimental results demonstrate that differences in soil water distribution were generally accompanied by corresponding changes in salinity and nitrate patterns. Treatments that maintained higher soil moisture contents tended to exhibit reduced upward solute accumulation, whereas treatments exposed to greater evaporation showed stronger concentration of salts and nitrate near the soil surface. This relationship indicates that evaporation intensity acted as an important regulator of both hydraulic and hydrochemical behavior within the soil columns. Under free-drainage conditions, downward water movement promoted advective transport of dissolved salts and nitrate toward deeper layers. During the subsequent evaporation period, however, upward matric gradients developed and induced capillary transport toward the soil surface. As water was removed through evaporation, dissolved constituents became increasingly concentrated within upper soil layers, producing the simultaneous accumulation patterns observed for salinity and nitrate. These results indicate that the redistribution of salts and nitrate was largely associated with the prevailing direction and magnitude of water fluxes rather than independent transport mechanisms.

The influence of mulch further supports this interpretation. By reducing evaporation intensity, mulch weakened upward hydraulic gradients and decreased upward transport of dissolved constituents. Consequently, both salinity and nitrate distributions became more uniform within the soil profile. This response suggests that mulch not only conserved water but also modified the transport environment responsible for solute redistribution. The resulting moderation of water fluxes reduced the tendency for evaporation-driven concentration and stratification of dissolved ions.

Soil texture modified the magnitude of these responses through its influence on water retention and hydraulic conductivity. The sandy clay loam soil retained greater quantities of water and sustained moisture continuity for longer periods than the loamy sand soil. As a result, differences in evaporation and mulch treatments produced distinct redistribution patterns between the two soils. These findings highlight that the effectiveness of mulch management depends not only on reducing evaporation but also on the hydraulic characteristics of the soil itself.

Recent studies have emphasized that coupled transport processes become increasingly nonlinear under drying conditions because declining water contents simultaneously reduce hydraulic conductivity and increase solute concentrations [50,51]. Although these processes were not measured directly in the present study, the observed redistribution patterns are consistent with this conceptual interpretation. Overall, the results support the view that water, salts, and nitrate behave as components of an integrated transport system in which evaporation intensity, soil hydraulic properties, and mulch management jointly influence hydrochemical behavior within the vadose zone.

4.6. Integrated Soil System Response and Sustainability Implications

Sustainability of soil systems under arid conditions depends on maintaining dynamic equilibrium among hydraulic stability, water conservation, salinity regulation, and nutrient retention. Disturbance of this equilibrium through excessive evaporation, salt accumulation, or nutrient loss initiates progressive degradation pathways that reduce soil productivity, water-use efficiency, and long-term ecosystem resilience. The present findings suggest that sawdust mulching acted as a multifunctional factor that may influence hydraulic, thermal, and hydrochemical processes within the soil system.

The strong improvement in soil water conservation under mulched conditions may represent an important adaptation mechanism for dryland agriculture. Preservation of higher residual moisture reduces irrigation demand, delays moisture depletion in the root zone, and improves resilience against prolonged atmospheric drought stress. Recent sustainability studies have emphasized that maintaining hydraulic continuity and reducing nonproductive evaporation are central requirements for improving water-use efficiency under climate-driven aridity [52]. At the same time, mulch was associated with reduced hydrochemical imbalance linked to evaporation-driven salt and nitrate accumulation.

Under uncovered conditions, strong upward hydraulic gradients intensified capillary-driven transport and promoted progressive solute stratification near the soil surface. This process accelerates salinization, increases osmotic stress, and reduces nutrient-use efficiency over time. By weakening evaporation-induced upward flow, mulch likely moderated this degradation pathway and promoted more stable hydrochemical conditions within the soil profile.

The integrated behavior of water and solute transport observed under mulching highlights the interconnected nature of dryland soil processes. Water conservation alone cannot ensure sustainability if accompanied by progressive salinity accumulation or severe nutrient redistribution. Likewise, nutrient retention becomes less effective under conditions of hydraulic instability and excessive evaporative flux. Organic mulch simultaneously influenced these processes by regulating energy transfer, reducing vapor flux, preserving moisture continuity, and moderating solute transport intensity.

The relatively limited additional benefit achieved with excessive mulch thickness also carries important practical implications for sustainable soil management. Moderate mulch application achieved substantial improvements in hydraulic and hydrochemical behavior while potentially minimizing material input requirements and management costs. These findings support mulch management strategies emphasizing efficiency rather than maximal surface coverage. Similar optimization principles have recently been incorporated into sustainable dryland agriculture frameworks focused on balancing agronomic performance with resource-use efficiency [53].

This finding is particularly important from a sustainability perspective because management recommendations are often based on maximizing physical performance rather than optimizing resource-use efficiency. The results indicate that the highest mulch application rate did not generate proportional improvements in system performance, suggesting that moderate mulch application may provide a more balanced compromise between environmental benefits and material requirements. Such optimization approaches are increasingly emphasized in sustainable soil and water management frameworks [53].

The stronger relative improvement observed in the loamy sand soil further indicates that coarse-textured soils may derive greater benefit from mulch-based management because they are inherently more vulnerable to evaporation, rapid moisture loss, and nutrient leaching. In contrast, finer-textured soils possess greater intrinsic buffering capacity due to stronger water retention and slower transport dynamics.

Overall, the integrated response of the soil system suggests that sustainable management under arid conditions requires simultaneous consideration of hydraulic, thermal, and hydrochemical processes operating within the soil profile. Sawdust mulching improved this integrated behavior by conserving water, suppressing evaporation, reducing salinity development, stabilizing nitrate redistribution, and moderating coupled transport processes.

It is also important to acknowledge a limitation related to the experimental design. The study employed three replicates per treatment, which is commonly adopted in soil-column experiments under controlled laboratory conditions due to practical constraints and the need to maintain uniformity across experimental units. However, this replication level may limit the statistical power to detect relatively small or subtle treatment effects, particularly within interaction terms in the factorial design. No formal post hoc power analysis was conducted; therefore, the probability of Type II error cannot be explicitly quantified. Despite this limitation, most main effects and several interaction effects were statistically significant at p ≤ 0.05 or p ≤ 0.01, indicating that the observed treatment responses were sufficiently strong to be detected under the current experimental setup. Nevertheless, caution is warranted when interpreting marginal or borderline significant effects, as these may be sensitive to replication size. Future studies with higher replication levels or complementary field-scale validation would further strengthen the robustness and generalizability of the findings.

The present findings were obtained under controlled greenhouse soil-column conditions that enabled clear identification of the effects of soil texture, nitrate application, and mulch treatment on coupled water–salt–nitrate dynamics. Under agricultural field conditions, additional processes such as root water uptake and transpiration contribute to the overall water balance and may influence moisture redistribution and solute transport patterns. Nevertheless, the mechanistic relationships observed in this study provide valuable insight into the fundamental interactions governing water conservation, salinity redistribution, and nitrate movement in arid soils. These findings reinforce the importance of organic surface amendments as effective tools for enhancing soil resilience, hydrochemical stability, and long-term sustainability in dryland agricultural systems [54,55,56].

In addition, the soil-column approach represents a simplified physical system that does not fully reproduce field-scale heterogeneity, lateral flow pathways, root development, crop nutrient uptake, repeated irrigation cycles, or potential wall effects associated with confined columns, all of which may influence the magnitude and patterns of water, salt, and nitrate redistribution under field conditions.

5. Conclusions

Soil texture exerted a dominant control over water retention, hydraulic conductivity, and solute transport processes, with the sandy clay loam consistently exhibiting higher water storage capacity and lower susceptibility to rapid drainage compared with the loamy sand soil. Mulch application significantly modified soil surface energy balance and hydraulic gradients, leading to substantial reductions in evaporation losses and improved moisture conservation throughout the soil profile. In parallel, salt redistribution patterns were strongly altered, where mulching minimized upward salt accumulation and promoted more uniform salinity distribution. Nitrate dynamics were primarily governed by water fluxes, with coarse-textured soils exhibiting greater leaching losses, while finer-textured soils retained higher nitrate proportions due to increased residence time and reduced permeability. The integrated assessment of soil water, salinity, and nitrate behavior was effectively captured using the Composite Sustainability Index (CSI), which demonstrated the overall benefits of mulch application relative to the non-mulched treatment. Sensitivity analysis showed that treatment rankings remained largely stable under alternative weighting schemes, confirming the robustness of the CSI framework and the reliability of the overall sustainability assessment. Among the evaluated treatments, the 4 cm mulch depth generally produced the highest CSI values and the greatest improvements in water conservation, evaporation reduction, salinity control, and nitrate retention. These findings highlight the importance of mulch management as an effective strategy for improving soil sustainability and regulating coupled water–salt–nitrate interactions under arid conditions. Future research should focus on long-term field validation under variable climatic conditions, inclusion of biological soil processes, and optimization of mulch materials for enhanced sustainability under arid and semi-arid environments.