Effects of Long-Term Soil Management Under Alfalfa Cultivation on Soil Fertility and Salinity in Arid Agroecosystems of the Ziban Region, Algeria

Abstract

In arid regions, the soil degradation from salinization, low organic matter content, and compaction severely limits agricultural productivity. Leguminous perennials such as alfalfa (Medicago sativa L.) have the potential to restore soil quality, but their long-term effects remain underexplored in North African drylands. This study aimed to evaluate the impacts of long-term (7–8 years) alfalfa cultivation on soil fertility and salinity in the Ziban region of Algeria. Ninety topsoil samples (0–30 cm) from cultivated and adjacent uncultivated plots were collected and analyzed, determining organic matter (OM), soil organic carbon (SOC), soil nitrogen stock (SNS), electrical conductivity (EC), sodium adsorption ratio (SAR), pH, major cations (Ca²⁺, Mg²⁺, Na⁺), sulfate (SO₄²⁻), bulk density (BD), and texture. Compared with uncultivated soils, alfalfa cultivation increased OM by 82.26%, SOC by 78.38%, and SNS by 102.99%, while reducing EC by 40.36%, SAR by 28.94% and BD by 6.16% (p < 0.05), indicating significant improvements in fertility, structure and reductions in sodicity. PCA revealed distinct gradients separating fertility–salinity parameters from compaction–sodicity in cultivated and uncultivated soils. These results confirm that alfalfa systems enhance nutrient cycling, reduce salt stress, and improve structural stability in arid agroecosystems through reduced bulk density and increased organic matter in arid agroecosystems. Integrating alfalfa into land management strategies could promote sustainable restoration of degraded soils in drylands. Further research should optimize irrigation and organic inputs to maximize these benefits under climate-stress conditions.

1. Introduction

Soil quality can be defined as the potential of soil as a living ecosystem to function within environmental and land use constraints to support plant and animal productivity, to regulate the quality of water and air, and to promote resilience [1]. The need for monitoring soil physicochemical properties, especially in places subjected to unsustainable agricultural practices and land degradation [2], has been proven as a useful tool through many studies. In fact, the challenge in maintaining soil fertility in arid lands is exacerbated due to the availability of little organic inputs, salinization, and desertification of soils and the present accelerating effects of climate change [3,4].

Soil degradation is common in the arid and semi-arid zones, resulting from the unpredictable precipitation pattern, periodic droughts and unwise human land use. These stressors result in shallow rooting zones, depleted soil organic carbon (SOC) and soil nitrogen (SN), reduced aggregation, lower infiltration rates, and intensified surface runoff and erosion [5,6]. Salinization and sodification are generally common phenomena occurring in drylands. The major driving factors of these are natural—as arid climate, lithology, and topography—and man-made, especially due to irrigation and poor drainage in cultivated lands [7,8]. The accumulation of salts in the upper soil layers deteriorates the physical structure and availability of nutrients, inhibiting vegetation growth. Africa has 37% of the drylands globally, with more than two-thirds of its area classified as arid or semi-arid [9].

The Ziban region in southern Algeria is subjected to such challenges. This region receives low annual rainfall (<200 mm), and the groundwater is saline and is regularly active by aeolian activities as the greatest threats to the health of its soils in terms of waterlogging, salinity, and alkalinity [10,11]. Formed by colluvial and aeolian deposits, soils from this region suffer from low organic matter and structural instability, making their improvement and restoration difficult.

It is widely recognized that soil organic carbon (SOC) and soil nitrogen (SN) are primary indicators of soil fertility and ecosystem functionality [12,13]. SOC has even further qualities concerning the enhancement of aggregate stability and water retention, nutrient cycling as a long-term carbon sink, and originates primarily from plant biomass, undergoing continuous turnover via microbial decomposition and root exudation. SN is, rather, one of the cornerstones of the plant metabolism, being involved in amino acid and chlorophyll synthesis, and is tightly coupled to SOC dynamics through microbially mediated processes [14,15]. It further accounts for poor efficiency on nitrogen use in arid soils, with more than 50% of loss, usually attributed to leaching, volatilization, and denitrification.

A range of approaches is recommended for the restoration of arid soils, but in all cases sustainable actions should be incorporated for the increased organic inputs, improvement of structure, and salinity mitigation. Leguminous perennial species such as alfalfa (Medicago sativa L.) may exhibit considerable potential, compared with other approaches [16]. Alfalfa enriches SOC and SN stocks via root turnover, rhizodeposition, and biological nitrogen fixation [17,18]. It is a perennial, deep-rooted, abiotic stress-tolerant plant, thus making it applicable to degraded arid environments. In addition to improving soils, alfalfa yields quality forage for livestock and participates in ecological improvements [19,20]. Moreover, legumes such as alfalfa benefit from symbiotic association with nitrogen-fixing rhizobia, capable of converting atmospheric N₂ into forms that are available to plants. Consequently, there is reduced reliance on synthetic fertilizers and enhanced microbial activity and nutrient cycling [21]. Thus, the positive effects of alfalfa on SOC have been linked with increased root biomass, microbial biomass carbon, and carbon sequestration, especially in deeper soil layers [16,17,18,19,20,21,22,23].

Several studies assessing the impact of alfalfa cultivation on soil quality have been conducted in semi-arid and temperate regions, demonstrating its ability to enhance soil fertility and structural stability while influencing salinity dynamics. For instance, Li et al. (2023) showed that alfalfa crop rotation in a semi-arid environment significantly increased SOC and SN and reduced soil and nutrient loss through improved vegetation cover and soil stability [24]. Similarly, Wang et al. (2024) reported that in the temperate Loess Plateau of northern China, long-term alfalfa cultivation initially improved soil structure and nutrient status but later led to gradual declines in SOC and SN stocks beyond 8–10 years of monoculture, illustrating the complex long-term balance between fertility improvement and degradation [25].

However, field-based evidence from arid North-African drylands remains scarce, and the combined relationships among soil fertility, salinity, and structural stability under long-term alfalfa cultivation are still poorly understood. Understanding these interactions is essential for designing sustainable management strategies in regions affected by chronic salinity, low organic matter, and limited water availability. By focusing on the arid Ziban region of Algeria, this study provides new insight into how long-term alfalfa cultivation influences the balance between soil fertility and salinity while improving soil structural stability, thereby contributing original data to an understudied agroecosystem.

The main objective of this study was to assess the long-term effects of alfalfa cultivation on soil fertility and salinity in the arid agroecosystems of the Ziban region, Algeria, by comparing cultivated plots with adjacent uncultivated reference soils. This paired-site approach aimed to generate quantitative evidence on fertility–salinity trade-offs under prolonged legume management and to provide practical guidance for soil-quality restoration and sustainable land management in arid agroecosystems.

It was hypothesized that long-term alfalfa cultivation would increase SOC and SN while decreasing EC, SAR, and bulk density (BD) as a result of enhanced microbial activity, improved aggregation, and cation exchange driven Na⁺ displacement.

In a wider Mediterranean and arid-zone perspective, several recent investigations have explored how different soil-management strategies influence the balance between soil fertility and salinity. Novara et al. (2020) demonstrated that cover-crop management in Mediterranean vineyards improved microbial diversity and nitrogen cycling, supporting SOC stabilization under water-limited conditions [26]. Muscarella et al. (2024) further highlighted that soil salinity and nutrient dynamics are strongly influenced by irrigation quality, underlining the importance of managing ionic balance to sustain soil fertility in arid environments [27]. Likewise, Paliaga et al. (2025) showed that the use of enriched biochar and zeolite under saline irrigation improved soil fertility and microbial biomass while mitigating salinity stress [28]. These Mediterranean findings highlight complementary mechanisms and reinforce the importance of assessing fertility–salinity trade-offs under long-term legume cultivation in North-African arid systems.

2. Materials and Methods

2.1. Study Area

The study was conducted in April 2024 in the Ziban region of southeastern Algeria. This agroecological zone lies between the Saharan Atlas Mountains to the North and the Saharan plains to the South, forming a transition between semi-arid and arid climates. The region extends between 33°30′ and 35°15′ N latitude and 4°15′ to 6°45′ E longitude, with elevations ranging from 124 m to over 1930 m above sea level [29]. The geographic location of the study area is shown in Figure 1.

Figure 1. Geographic location of the study area in the region of Ziban, Algeria.

According to the Köppen classification, the climate is hot desert (BWh), with average annual precipitation under 150 mm and potential evapotranspiration exceeding 2500 mm. Temperatures range from 11 °C in January to 35 °C in July [8].

Topographically, Ziban includes flat plains, gently rolling hills, and surrounding mountain ranges, resting on Quaternary geological formations conducive to groundwater storage [8]. Pedologically, the soils are dominated by Gypsisols, Solonchaks, and Cambisols [30], according to the World Reference Base for Soil Resources [31]. Irrigation water originates from local groundwater wells, with electrical conductivity (ECw) ranging from 1.46 to 5.32 dS m⁻¹ [29]. The Ziban region was selected for its agricultural importance in Algeria, combining oasis-based irrigated systems and significant livestock potentially dependent on the alfalfa crop [29]. Alfalfa is the principal forage crop sustaining dairy and meat production, thereby providing an appropriate context for assessing its influence on soil quality in arid environments.

2.2. Soil Sampling and Laboratory Analyses

The fieldwork was conducted in April 2024 across the Ziban region. A total of 90 topsoil samples (0–30 cm) were collected from 45 paired plots: comprising 45 alfalfa cultivated plots and 45 adjacent non-cultivated plots. Each cultivated plot had been continuously cropped for 7–8 years, while the corresponding uncultivated soils were bare and undisturbed for several decades. Before alfalfa establishment, most fields had been used for cereal crops or left fallow.

Within each plot, three soil cores were taken randomly from a 20 m × 20 m area using a hand auger and composited to form one representative sample per site. The paired plots were located less than 200 m apart and shared comparable soil type, slope, and hydrological conditions. All samples were georeferenced using the Universal Transverse Mercator (UTM) system. Each soil sample was air-dried at room temperature and sieved through a 2 mm mesh prior to laboratory analysis [32].

2.3. Soil Physicochemical Analysis

Soil texture was determined using the hydrometer method by Bouyoucos, based on the procedure of Gee and Or [30]. Bulk density (BD) was determined using the core sampling method [33].

Soil pH and electrical conductivity (EC) were measured in a 1:5 soil–distilled water extract at 25 °C using pH and conductivity meters, respectively, following the guidelines of the U.S. Salinity Laboratory Staff [34]. Total equivalent calcium carbonate (CaCO₃) was measured volumetrically using the Bernard calcimeter method [35].

Organic matter (OM) content was determined by the Walkley–Black dichromate oxidation method [36], and adjusted with a correction factor of 1.742 [37]. Soil Nitrogen (SN) was measured using the Kjeldahl digestion method [38].

Soluble cations (Ca²⁺, Mg²⁺, Na⁺, K⁺) and sulfate (SO₄²⁻) were extracted using a 1:5 soil–distilled water ratio. Calcium (Ca²⁺) and magnesium (Mg²⁺) were determined via complexometric titration with EDTA using murexide as an indicator; Mg²⁺ was calculated by difference. Sodium (Na⁺) and potassium (K⁺) were measured using flame photometry. Sulfate (SO₄²⁻) was determined gravimetrically by precipitation with barium chloride in an acidified medium [39].

The Sodium Adsorption Ratio (SAR) was calculated using this formula [34]:

SAR = Na⁺/√[(Ca²⁺ + Mg²⁺)/2]

Soil organic carbon stock (SOC) and soil nitrogen stocks (SNS) were calculated using the equations proposed by Pearson [37]:

SOC stock (t·ha⁻¹) = BD × D × SOC

(1)

SNS (t·ha⁻¹) = BD × D × SN

(2)

where

BD = bulk density (g·cm⁻³);

D = sampling depth (cm);

SOC = Soil Organic Carbon concentration (%);

SN = soil nitrogen concentration (%).

All ion concentrations were expressed in milliequivalents per liter (meq·L⁻¹).

2.4. Statistical Analysis

Descriptive statistics (mean, minimum, maximum, standard deviation) were calculated separately for cultivated and uncultivated soils. The Shapiro–Wilk test was used to assess data normality. As most variables deviated from normality, non-parametric methods were used.

The Mann–Whitney U test was used to compare soil properties between land uses. Spearman correlation coefficients were used to evaluate relationships among variables. Principal Component Analysis (PCA) was performed on standardized variables using the correlation matrix to reduce dimensionality and identify dominant fertility–salinity–structure patterns.

All statistical analyses were conducted using XLSTAT v2019.2.2. and IBM SPSS Statistics v26.0. Statistical significance was set at α = 0.05.

3. Results

3.1. Descriptive Statistics of Soil Fertility Indicators

Table 1 shows that organic matter (OM) content was significantly higher in cultivated soils (mean = 1.13%) compared to uncultivated soils (0.62%; p < 0.001). Similarly, soil organic carbon (SOC) and soil nitrogen (SN) concentrations were higher in cultivated plots (0.66% and 0.13%, respectively) compared to uncultivated ones (0.37% and 0.06%; p < 0.001), indicating increased biological activity and organic input with alfalfa cultivation.

Table 1. Descriptive statistics of soil fertility-related indicators by land use type.

Land Use Type		OM (%)	SOC (%)	SN (%)	pH	EC (dS/m)	CaCO3 (%)
Cultivated	Min	0.37	0.22	0.1	5.67	0.51	13.76
	Max	1.95	1.13	0.2	7.92	4.77	52.32
	Mean	1.13	0.66	0.13	7.1	1.98	28.78
	SD	0.36	0.21	0.03	0.51	1.18	9.56
Uncultivated	Min	0.26	0.15	0.03	6.03	0.42	17.65
	Max	1.2	0.7	0.15	8.44	12.84	49.91
	Mean	0.62	0.37	0.06	7.48	3.32	27.71
	SD	0.18	0.11	0.03	0.4	3.17	6.96
(Mann–Whitney U)	p-value	<0.001 **	<0.001 **	<0.001 **	<0.001 **	0.049 *	0.781 ns

Note: * p < 0.05; ** p < 0.01; ns = not significant.

Both group of soils, cultivated and uncultivated, were alkaline, with pH values of 7.1 in cultivated soils and 7.5 in uncultivated soils (p < 0.001). Electrical conductivity (EC) was higher in uncultivated soils (3.32 dS·m⁻¹) than it was in the cultivated soils (1.98 dS·m⁻¹; p = 0.049), suggesting salt accumulation under unmanaged conditions due to capillary rise and limited leaching. The total equivalent calcium carbonate (CaCO₃) content was moderately high for both land uses, confirming the calcareous nature of these soils and showing no significant difference (p = 0.781).

3.2. Soil Texture and Bulk Density

Granulometric analysis confirmed a sandy loam texture in both groups of soils (Table 2), with similar sand content (59.02%). Minor differences were observed in clay (9.5%) and silt content (31.47% in cultivated and 30.77% in uncultivated), suggesting that alfalfa cultivation had minimal effect on inherent texture class. These properties remained stable due to their geogenic origin. Bulk density (BD) was slightly lower in cultivated soils (1.37 g·cm⁻³) than in uncultivated plots (1.46 g·cm⁻³; p < 0.001), reflecting the improved structure and porosity under alfalfa cultivation.

Table 2. Descriptive statistics of soil texture and structural indicators.

Land Use Type		Sand (%)	Clay (%)	Silt (%)	BD (g/cm3)
Cultivated	Min	38.19	1.98	10.57	1.28
	Max	85.16	18.4	51.37	1.44
	Mean	59.02	9.5	31.47	1.37
	SD	13.02	4.12	10.36	0.05
Uncultivated	Min	35.13	1.96	8.0	1.35
	Max	86.85	19.27	47.59	1.56
	Mean	59.02	9.5	31.47	1.46
	SD	13.33	4.0	10.44	0.06
Mann–Whitney U	p-value	0.997 ns	0.875 ns	0.900 ns	<0.001 **

Note: ** p < 0.01; ns = not significant.

3.3. Salinity and Sodicity Parameters

Among soluble ions, mean concentrations of Ca²⁺, Mg²⁺, Na⁺, and SO₄²⁻ were higher in uncultivated soils (Table 3). In particular, Na⁺ averaged 9.99 meq·L⁻¹ in uncultivated plots compared to 2.54 meq·L⁻¹ in cultivated soils (p < 0.001). SAR was higher in uncultivated soils (8.81) than in cultivated ones (6.26), suggesting that alfalfa cultivation reduced sodicity—likely through enhanced root uptake and leaching processes. Significant differences were also observed for Ca²⁺, Mg²⁺, and K⁺ (p < 0.01), while SO₄²⁻ and SAR showed no significant differences (p > 0.05).

Table 3. Descriptive statistics of salinity and sodicity-related indicators.

Land Use Type		Ca2+ (meq/L)	Mg2+ (meq/L)	Na+ (meq/L)	K+ (meq/L)	SO42− (meq/L)	SAR
Cultivated	Min	2.13	0.17	0.53	0.14	8.45	1.92
	Max	13.65	21.03	8.45	0.77	95.57	19.49
	Mean	5.26	7.02	2.54	0.37	37.49	6.26
	SD	3.38	5.9	2.14	0.14	27.11	5.05
Uncultivated	Min	2.12	1.13	0.38	0.06	9.81	1.21
	Max	15.78	60.11	34.91	3.18	92.5	29.53
	Mean	7.69	19.06	9.99	0.96	40.23	8.81
	SD	4.3	19.8	11.41	0.82	25.23	8.29
Mann–Whitney U	p-value	0.007 **	0.009 **	<0.001 **	<0.001 **	0.373 ns	0.311 ns

Note: ** p < 0.01; ns = not significant.

3.4. Soil Organic Carbon and Nitrogen Stocks Under Different Land Use Systems

Table 4 presents the average stocks of soil organic carbon (SOC) and nitrogen (SN), as well as the carbon-to-nitrogen (C/N) ratio in the topsoil (0–30 cm) by land use types.

Table 4. Soil Organic Carbon and Nitrogen Stocks under different land use types.

Land Use	C/N	SOC Stock t ha−1	SN Stock t ha−1
Cultivated soil	5.1867	26.7333	5.3562
Uncultivated soil	7.142	16.0835	2.6382
p-value	<0.001 **	<0.001 **	<0.001 **

Note: ** p < 0.01.

Soil organic carbon (SOC) and soil nitrogen (SN) stocks varied notoriously between both land use types (Table 4). Cultivated alfalfa plots exhibited a lower C/N ratio (5.19) compared to uncultivated soils (7.14; p < 0.001), indicating a faster decomposition rate of organic matter in the cultivated system. SOC stock in cultivated soils averaged 26.73 t ha⁻¹, representing a 66.2% increase relative to uncultivated soils (16.08 t ha⁻¹; p < 0.001). Similarly, SN stock was substantially higher in cultivated soils (5.36 t ha⁻¹) than in uncultivated soils (2.64 t ha⁻¹; p < 0.001), corresponding to an increase of 103.1%. This confirms the positive impact of alfalfa cultivation on organic matter accumulation and nutrient enrichment.

3.5. Correlation Analysis

Due to deviations from normality confirmed by the Shapiro–Wilk test, Spearman’s correlation coefficients were used to assess relationships among soil variables.

Spearman correlation coefficients between soil properties from cultivated (alfalfa) and uncultivated areas are presented in Table 4 and Table 5, respectively, while the corresponding correlation heatmaps (Figure 2) visually showed the relative strength and direction of relationships. Such analyses reveal the effect of alfalfa cultivation on the interaction of the fertility, salinity, and structure indicators.

Table 5. Spearman’s correlation coefficients among soil variables under cultivated soils.

	SOC (%)	SN (%)	pH	EC (dS/m)	CaCO3 (%)	BD (g/cm3)	Ca2+ (meq/L)	Mg2+ (meq/L)	Na+ (meq/L)	SO42− (meq/L)	Sand (%)	Clay (%)	Silt (%)	K+ (meq/L)
SN (%)	0.635
pH	0.069	−0.258
EC (dS/m)	−0.03	−0.246	0.290
CaCO3 (%)	0.249	0.239	0.437	0.285
BD (g/cm3)	−0.29	−0.152	−0.6	−0.252	−0.637
Ca2+ (meq/L)	0.117	−0.090	0.147	0.856	0.212	−0.030
Mg2+ (meq/L)	−0.23	−0.170	0.106	0.832	0.211	−0.119	0.723
Na+ (meq/L)	0.024	−0.180	0.498	0.733	0.469	−0.456	0.576	0.500
SO42− (meq/L)	0.233	−0.072	0.562	0.736	0.602	−0.558	0.685	0.598	0.718
Sand (%)	−0.3	−0.145	−0.55	−0.370	−0.624	0.890	−0.153	−0.279	−0.430	−0.660
Clay (%)	0.460	0.418	0.336	0.195	0.572	−0.731	0.167	0.062	0.183	0.444	−0.72
Silt (%)	0.185	0.079	0.537	0.370	0.565	−0.813	0.157	0.348	0.446	0.638	−0.95	0.526
K+ (meq/L)	−0.03	−0.145	−0.001	0.671	−0.004	−0.117	0.575	0.663	0.155	0.383	−0.26	0.186	0.254
SAR	0.064	−0.118	0.467	0.462	0.570	−0.443	0.310	0.191	0.899	0.548	−0.34	0.195	0.310	−0.140

Figure 2. Spearman correlation heatmaps among soil physicochemical properties for (a) cultivated and (b) uncultivated soils. Blue and red represent negative and positive correlations, respectively; darker shades indicate stronger coefficients (p ≤ 0.05 and p ≤ 0.01 levels indicated).

In the cultivated soils, the high correlation between SOC and SN (r = 0.635) exhibits simultaneous accumulation of carbon and nitrogen under alfalfa management.

Cationic relationships were particularly clear: Ca²⁺ and Mg²⁺ had a significant association with each other (r = 0.723) and both were positively related to electrical conductivity (EC) (r = 0.856 and 0.832, respectively). Hence, salinity in cultivated soil is determined mainly by such divalent cations. Furthermore, Na⁺ was found to have a strong positive correlation with EC (r= 0.733), while SAR had a good association with Na⁺ as well, hence supporting the judgment in sodicity risk assessment.

The BD negatively correlated with SOC (r = −0.291), indicating that organic matter promoted a good soil pore space and less compaction in the respective soils. These relationships further point to the increased benefits of alfalfa for soil structure improvement.

As expected, soil texture variables exhibit inverse relationships: sand is negatively correlated with both clay and silt (r = −0.723 and r = −0.953, respectively), whereas clay and silt display moderate positive correlations with SOC and nutrient-related parameters. Thus, these findings suggest finer particles’ contribution to organic matter stabilization and nutrients under retention under cultivation (Table 4, Figure 2a).

On the contrary, the correlation structure of uncultivated soils changed notably. SO C statistically insignificant correlations with SN (r = −0.003), which basically showed that the carbon and nitrogen cycles decoupled from each other under uncultivated conditions.

Ca²⁺ exhibited positive associations with Mg²⁺ (r = 0.341) and Na⁺ (r = 0.513), but a weak negative correlation with EC (r = −0.103). By contrast, SO₄²⁻ (r = 0.693) and Na⁺ (r = 0.355) were more influential on EC, which indicates a natural accumulation of salts without contribution of enrichment from management practices. While SAR continued to have very strong positive results with Na⁺ and EC, further supporting the role in identifying sodicity risks.

A distinct pattern emerged with SOC, which now showed negative correlations with Na⁺, SAR, Ca²⁺, and Mg²⁺, indicating that increased salinity and sodicity restrict microbial activity and root biomass, thus limiting organic matter stabilization.

Structurally, BD was strongly positively correlated with sand (r = 0.955) and negatively with clay (r = −0.742) and silt (r = −0.930). This confirms the trend whose majority deposits comprise coarse-textured, compacted and low-aggregated soils under unmanaged systems (Table 6, Figure 2b).

Table 6. Spearman’s correlation coefficients among soil variables under uncultivated soils.

	SOC (%)	SN (%)	pH	EC (dS/m)	CaCO3 (%)	BD (g/cm3)	Ca2+ (meq/L)	Mg2+ (meq/L)	Na+ (meq/L)	K+ (meq/L)	SO42− (meq/L)	SAR	Sand (%)	Clay (%)
SN (%)	−0.003
pH	0.035	0.115
EC (dS/m)	−0.147	0.430	0.080
CaCO3 (%)	−0.248	0.320	−0.003	0.442
BD(g/cm3)	−0.465	−0.215	−0.162	−0.284	−0.084
Ca2+ (meq/L)	−0.493	−0.127	−0.136	−0.103	−0.056	0.624
Mg2+ (meq/L)	−0.323	−0.014	−0.190	0.220	0.309	0.200	0.341
Na+ (meq/L)	−0.285	0.282	−0.365	0.355	0.269	0.190	0.513	0.496
K+ (meq/L)	0.000	0.248	0.015	0.389	−0.348	−0.018	−0.031	0.263	0.197
SO42− (meq/L)	−0.286	0.216	0.127	0.693	0.500	−0.140	−0.125	−0.059	0.025	−0.140
SAR	−0.241	0.060	−0.208	−0.268	0.072	0.554	0.527	−0.280	0.360	−0.321	−0.093
Sand (%)	−0.431	−0.202	−0.189	−0.227	−0.125	0.955	0.604	0.283	0.282	0.084	−0.152	0.484
Clay (%)	0.380	0.072	0.094	0.122	0.211	−0.742	−0.631	−0.065	−0.260	−0.186	0.114	−0.578	−0.757
Silt (%)	0.439	0.197	0.191	0.233	0.086	−0.930	−0.561	−0.300	−0.252	−0.057	0.142	−0.435	−0.973	0.630

3.6. Principal Component Analysis of the Soil Properties

Principal Component Analysis (PCA) was performed on standardized data sets for both types of soils-cultivated and uncultivated-to identify the common soil variables that shape soil quality in arid conditions. Only two principal components (PC1 and PC2) that had eigenvalues greater than 1 were retained for both land use types. These axes explained 70.09% of the variance associated with cultivated soils and 79.22% related to uncultivated soil, and the corresponding PCA biplots (Figure 3 and Figure 4) illustrate the loading vectors and directional contribution of each variable, while loadings are detailed in Table 6 and Table 7. In cultivated soils, PC1 included 50.54% of the total variance and primarily presented a fertility–salinity gradient (Table 6). It was strongly and positively loaded by soluble salts, including EC (r = 0.8761), SO₄²⁻ (r = 0.9197), Na⁺ (r = 0.5239), K+ (r = 0.612), Ca²⁺ (r = 0.8238) and Mg²⁺ (r = 0.7203). Such variables most likely reflect a chemical enrichment from the effects of irrigation inputs and fertilization practices in alfalfa systems. Their co-occurrence in the presence of Ca²⁺ and Mg²⁺ highlights their role in improving soil aggregation and buffering sodicity under managed conditions.

Figure 3. PCA biplot for cultivated soils.

Figure 4. PCA biplot for uncultivated soils.

Table 7. PCA loadings for cultivated soils.

Soil Factors	PC1	PC2
EC	0.8761	0.3949
BD	−0.6404	0.5592
Ca2+	0.8238	0.2902
Mg2+	0.7203	0.2139
Na+	0.5239	0.6239
SO42−	0.9197	−0.0058
Sand %	−0.7601	0.5666
Silt %	0.7484	−0.5375
K+	0.612	0.0032
SAR	0.215	0.5974
Eigenvalue	5.054	1.9546
% of variance	50.5404	19.5457
Cumulative variance (%)	50.5404	70.086

Conversely, BD and sand had strong negative loadings (−0.6404 and −0.7601, respectively), suggesting that improved chemical fertility is found in finer-textured, less-compact soils. This also fits an extensive scenario in which organic matter inputs from alfalfa biomass improve structure, reduce compaction, and promote nutrient retention.

PC2 explains 19.55% of the variance represented by a sodicity–texture axis, with positive loadings from SAR (r = 0.5974), Na⁺, BD, and sand, and a strong negative loading from silt (r = −0.5375). Such a contrast suggests that sodicity and structure degradation risks are associated with coarse textures, while higher silt content enhances water retention and resilience of soils.

In uncultivated soils, PC1 accounted for 46.44% of variance and represented texture-structure variables, with bulk density (BD) (r = 0.9557) and sand (r = 0.8879) and Ca²⁺ (0.6283) being the dominant positive contributors, while silt was strongly negatively loaded (r = −0.8647). The axis also highlighted the predominance of compacted, coarse-textured soils on unmanaged land, PC2, which explains 32.78% of the variation represented a salinity-cation accumulation gradient, with high loading of Na⁺ (r = 0.7785), K⁺ (r = 0.7163), Mg²⁺ (r = 0.7262), EC (r = 0.6608), and Ca²⁺ (0.398). Such trends are symptomatic of the natural accumulation of salts in arid soils as a result of evaporation, concentration, and restricted leaching. The PCA loadings for uncultivated soils are presented in Table 8.

Table 8. PCA loadings for uncultivated soils.

Soil Factors	PC1	PC2
EC	−0.7079	0.6608
Ca2+	0.6283	0.398
Mg2+	−0.1392	0.7262
Na+	−0.3147	0.7785
K+	−0.5018	0.7163
BD	0.9557	0.2171
Sand %	0.8879	0.4081
Silt %	−0.8647	−0.4087
Eigenvalue	3.7155	2.6224
% of variance	46.4442	32.7799
Cumulative variance (%)	46.4442	79.2241

4. Discussion

4.1. Organic Matter, Carbon, Nitrogen, and Nutrient Accumulation Under Changing Land Use

In our study, organic matter (OM), soil organic carbon (SOC), and soil nitrogen (SN) were significantly higher in alfalfa-cultivated soils than in uncultivated soils, demonstrating that long-term alfalfa management improves soil fertility even under arid conditions. This enrichment results from continuous organic inputs through litter deposition, rhizodeposition, and root turnover, which stimulate microbial activity and enhance nutrient cycling. The observed increases in soil organic carbon (SOC) and soil nitrogen (SN) further confirm the strong role of perennial legumes in sustaining organic and nutrient pools in degraded drylands.

These findings are consistent with previous studies showing that integrating alfalfa (Medicago sativa L.) into cropping systems significantly enhances OM and nutrient status through biological nitrogen fixation and organic matter inputs. Alfalfa, a deep-rooted perennial legume, contributes substantial below- and above-ground residues that promote carbon sequestration and microbial growth [40,41,42,43,44]. The rapid turnover of fine roots, closely linked to the stabilization of SOC and SN, has also been observed in similar agroecosystems [22,45]. Zhang et al. [46] emphasized that microbial necromass plays a critical role in SOC formation under livestock–alfalfa systems. Similarly, Song et al. [16] reported that long-term alfalfa cultivation increases microbial biomass and OM content, while Ma et al. [47] found that alfalfa powder application under drought enhances nutrient content and dry matter accumulation. These improvements align with the rhizosphere priming effect (RPE), where root exudates stimulate microbial enzymatic activity that promotes OM mineralization and nutrient release [43,44].

Alfalfa systems also improve nitrogen dynamics through symbiosis with diazotrophic bacteria, converting atmospheric N₂ into plant-available forms [16]. Studies across arid and semi-arid regions have reported similar increases in nitrate (NO₃⁻), soil-available nitrogen (SAN), phosphorus, and trace elements under long-term alfalfa cultivation [45,47,48]. Recent studies across arid and semi-arid regions confirm that converting degraded land or fallow plots to alfalfa pastures increases total and available nutrient pools [23,46].

In contrast, long-term uncultivated or fallowed soils lacking vegetative cover and organic inputs typically exhibit poor OM and nutrient content. These soils are vulnerable to erosion and nutrient loss. Lower levels of OM, N, P, and K were reported in fallows compared to alfalfa rotations [24]. Nutrient imbalances, especially in nitrogen and phosphorus, limit microbial and plant productivity [49]. While limited fallowing may partially restore OM [50], our findings demonstrate that continuous organic inputs from perennial legumes like alfalfa are essential to maintain nutrient cycling and soil functionality in arid agroecosystems. These microbial and biochemical processes collectively explain the observed increase in SOC and SN under alfalfa cultivation.

4.2. Salinity and Sodicity Dynamics Under Contrasting Land Use

Salinity and sodicity levels (EC and SAR) were significantly higher in uncultivated soils than in alfalfa-cultivated soils, confirming that unmanaged arid lands are more vulnerable to salt accumulation. Conversely, long-term alfalfa cultivation markedly reduced EC and SAR, demonstrating its capacity to improve ionic balance and mitigate salt stress in arid agroecosystems. The reduction in salinity under cultivation likely results from enhanced leaching and cation exchange processes driven by alfalfa’s deep root system, which improves infiltration and displaces Na⁺ with Ca²⁺ and Mg²⁺ on the exchange complex.

Similar patterns have been reported in other arid and semi-arid regions, where unmanaged soils show progressive salt accumulation due to low rainfall, high evapotranspiration, shallow saline groundwater, and capillary rise [51,52]. Fine-textured and poorly drained soils further exacerbate these conditions, leading to high EC and SAR values [7,53].

Sodicity develops when Na⁺ replaces Ca²⁺ and Mg²⁺ on exchange sites, causing clay dispersion, reduced porosity, and poor structure [54,55]. These effects reduce water movement and hinder root penetration. According to Stavi et al. [7], such sodic soils exhibit higher bulk densities and weaker aggregation, increasing compaction and salt buildup conditions consistent with our uncultivated soils. The lower EC and SAR values observed under alfalfa cultivation align with studies showing that deep-rooted legumes enhance infiltration, water flux, and nutrient balance. Alfalfa promotes downward water movement and cation exchange, increasing Ca²⁺ availability and flushing Na⁺ from the root zone [56,57]. Long-term alfalfa management has also been shown to reduce salinity while enhancing nitrogen and phosphorus availability [57]. These mechanisms support the observed improvement in soil chemical quality under cultivation. Effective irrigation strategies can further amplify these benefits. Yang et al. [58] found that precision irrigation during critical growth stages in alfalfa significantly reduced EC and SAR while maintaining water-use efficiency and productivity. Organic amendments, such as compost and manure, also support soil desalination by enhancing porosity, leaching capacity, and water retention [59,60,61]. When combined with gypsum, these inputs boost Ca²⁺ exchange and accelerate sodicity recovery [60].

Conservation tillage and surface mulching are additional tools for mitigating salinity. These methods reduce evaporation losses and improve infiltration, especially in compacted sandy loam soils. Research shows that no-till and minimum tillage systems sustain lower EC and improved aggregate stability and hydraulic conductivity relative to conventional tillage [62,63].

Altogether, these findings highlight the importance of integrating alfalfa cultivation with precise irrigation, organic amendments, and conservation tillage to rehabilitate degraded arid lands.

4.3. Soil Structural Quality and Bulk Density

Bulk density (BD) was significantly higher in uncultivated soils than in alfalfa-cultivated soils, indicating greater compaction and weaker structure in unmanaged plots. Conversely, the lower BD observed under alfalfa cultivation reflects improved soil aggregation, porosity, and structural stability. These findings demonstrate that long-term alfalfa cultivation enhances soil physical quality through continuous organic inputs and root-driven structural development.

This result agrees with earlier reports showing that deep-rooted legumes improve porosity, aeration, and nutrient retention capacity by reducing compaction and increasing macroaggregate formation [64,65,66,67].

Lower BD under alfalfa cultivation aligns with prior studies showing that deep-rooted legumes improve porosity and nutrient retention capacity [64]. Su et al. [65] found that converting annual crops to alfalfa increased SOC and SN stocks by 20.2% and 18.5%, respectively. These improvements are primarily attributed to ongoing organic inputs from root biomass and microbial activity, which promote macroaggregate formation and cohesion [66,67,68].

Soil aggregation in arid environments is greatly influenced by interactions between SOC, polyvalent cations (Ca²⁺, Mg²⁺), and microbial exudates [52,66]. In our cultivated soils, where SOC and OM were notably higher, these interactions likely promoted aggregate stability and pore connectivity. In addition, alfalfa’s extensive root system and the secretion of exudates contribute to the formation of stable biopores, which enhance soil aeration, water infiltration, and microbial habitat connectivity, further improving soil structure. Root entanglement and the secretion of polysaccharides and other organic compounds by alfalfa roots also enhance flocculation and soil cohesion [68]. Furthermore, secondary carbonates and soil inorganic carbon (SIC) can reinforce aggregation and structure in arid conditions with limited rainfall [53]. In contrast, the higher BD of uncultivated soils suggests structural degradation due to the absence of organic inputs and vegetation cover. Excess Na⁺ in unmanaged soils disrupts aggregation, leading to increased compaction and restricted infiltration. These findings correspond with previous observations that sodic and OM-depleted soils exhibit poor structural integrity and lower infiltration capacity [7,62].

4.4. Soil Organic Carbon and Nitrogen Stocks and C/N Ratio

Cultivated soils exhibited markedly higher SOC and SN stocks compared to uncultivated plots, demonstrating the role of alfalfa in enhancing nutrient sequestration. The pronounced increase in SOC and SN reflects the long-term accumulation of organic residues and enhanced nitrogen fixation under alfalfa systems. The lower C/N ratio observed in cultivated soils relative to uncultivated ones further indicates more efficient nitrogen cycling, rapid organic matter decomposition, and higher microbial activity in managed soils.

These results are consistent with previous findings that perennial legumes increase SOC and SN through rhizodeposition, root turnover, and biological N₂ fixation [16,23,69].

In degraded soils, alfalfa enhances nutrient storage largely by reducing bulk density and increasing root depth and activity [62]. For instance, Song et al. [16] demonstrated that root biomass density in the upper 60 cm strongly influences SOC, SN, and microbial biomass nitrogen (MBN), while Guan et al. [22] reported substantial SOC gains across a 2 m profile after seven years of alfalfa cultivation. Wang et al. [70] observed deep-layer SOC and SN sequestration during early- to mid-growth stages in alfalfa fields on the Loess Plateau.

Brahim et al. [17] found that intercropping barley with alfalfa on saline, gypsiferous desert soils increased SOC and SN stocks by 126% and 179%, respectively, within 21 months. The increases found in our study align with these results, reinforcing the capacity of alfalfa systems to restore fertility and sequester carbon even under saline and nutrient-poor conditions. The lower C/N ratios in cultivated soils observed here confirm a faster turnover of organic matter and efficient microbial nitrogen use. This finding agrees with earlier studies showing that soils receiving leguminous residues or manure exhibit narrower C/N ratios than those receiving non-legume residues [71]. Reduced C/N ratios are linked to faster decomposition and improved nutrient availability [72,73].

In contrast, higher C/N ratios in fallowed or undisturbed soils suggest limited SN availability and slower organic matter mineralization [74,75]. Benslama et al. [74] and Hao et al. [75] both noted that microbial activity and residue accumulation decline above a C/N threshold of approximately 9.6, leading to lower biological turnover. Thus, the C/N ratio reflects both the stability of SOM and microbial-driven nutrient turnover [76].

4.5. Limitations and Practical Recommendations

The paired plot study investigated topographic and pedological aspects, yet restrictions might still exist. Salinity and nutrient variability may have been influenced by spatial heterogeneity, seasonal monitoring, and insufficient irrigation water data. This study was limited to the surface horizon (0–30 cm) and a certain stand age (7–8 years); younger or older alfalfa stands could demonstrate variation in accumulation rates and structural responses.

Further studies would be enhanced by multi-season and multi-depth sampling at various growth stages of alfalfa, assessment of applied irrigation-water chemistry, and a comprehensive use of soil quality indicators or sensor-based tools to monitor long-term soil restoration. Optical sensing, specifically Fiber Bragg Grating (FBG) sensors, has recently emerged as a promising technology for precisely monitoring soil salinity and moisture dynamics with distinct temporal resolution. FBG sensors demonstrate remarkable stability and increased sensitivity in detecting humidity and water content; hence, they will improve traditional soil testing methods by allowing continuous field data collection for adaptive irrigation management [77].

5. Conclusions

This study evaluated long-term impacts of alfalfa (Medicago sativa L.) cultivation on soil fertility and salinity in the dry Ziban region of Algeria through a paired-site approach. Long-term management of alfalfa significantly enhanced soil organic carbon (SOC) and soil nitrogen (SN), while decreasing bulk density (BD), electrical conductivity (EC), and sodium adsorption ratio (SAR), indicating improved fertility, structure, and ionic equilibrium.

The findings confirm that alfalfa cultivation is an effective approach for rehabilitating degraded dry soils by augmenting nutrient cycling, promoting soil aggregation, and reducing salinity and sodicity. Incorporating alfalfa into dryland agroecosystems can thereby enhance sustainable land management, soil restoration, and regional climate change mitigation.

Future studies should incorporate multi-seasonal and multi-depth monitoring alongside integrated indices such as the Soil Quality Index (SQI) and Minimum Dataset (MDS) to evaluate long-term soil-ecosystem dynamics.