Effects of Plastic Film and Gravel-Sand Mulching on Soil Moisture and Yield of Wolfberry Under Ridge-Furrow Planting in an Arid Desert Region of China’s Loess Plateau

Abstract

In arid areas, the combined use of plastic sheeting under gravel-sand mulch on ridge-furrow planting systems is an emerging practice to minimize soil water evaporation and micro-plastic pollution. In this study, we conducted a two-year field experiment near Gobi-Tengger Desert in Ningxia, China, to evaluate the effects of a plastic film underneath a layer of pure sand (MS1), pure gravel (MS2) and mixed gravel-and-sand (MS3) mulch on the soil hydrothermal properties, water use efficiency, yield, and fruit quality of wolfberry, compared to bare soil (CK). The results showed that mulching significantly increased soil temperature and water content in the 0–20 cm surface layer, though the effects varied with soil depth and water availability between a supplemental irrigated year (2022) and a rain-fed year (2023). Mulching markedly altered soil water dynamics, enhancing the capture and retention of light-to-heavy rainfall events. Consequently, all mulches significantly increased seasonal water consumption (ET) and water use efficiency (WUE) compared to CK. The MS1 treatment consistently achieved the highest yield and WUE, and the highest accumulation of beneficial fruit compounds like polysaccharides and flavonoids. However, this treatment also resulted in elevated soil salinity. Our findings demonstrate that combined mulching, especially MS1, is a highly effective strategy for optimizing soil conditions, water productivity, and fruit quality in wolfberry cultivation, although long-term salinity management requires attention.

1. Introduction

The ridge-furrow (RF) planting system is a strategic agricultural practice widely used in the world across diverse environments in Asia, Africa, Southern Europe, and North America. It is particularly beneficial in regions with limited rainfall or poorly drained soil. The system utilizes raised ridges to capture rainfall, improve drainage and reduce waterlogging, and furrows to store runoff from the ridges for root absorption, promoting stable soil moisture conditions. In temperate zones, RF also protects crops from frost damage by elevating plants above frost pockets that can form in lower areas. In arid areas on Loess Plateau of northwest China, crops are often planted in the furrows instead to maximize water use and reduce evaporation. Thus, the RF system is adaptable to both traditional small-holder farms and large-scale mechanized production. The clearly defined rows and channels facilitate the use of machinery for planting, irrigation, and harvesting and make it popular in modern agricultural. These advantages explain the widespread use of the system in many parts of the world and show that it can be adapted to both traditional and modern farming practices.

In recent decades, mulching with plastic film (PFM) has been widely used in the world. PFM in RF systems can conserve more moisture, warm the soil, and reduce weed pressure [1,2,3,4]. This often leads to accelerated early crop growth and increased yields [5]. It also affects the soil microenvironment by improving soil hydrothermal conditions and minimizing salinization [6]. Color, thickness, and other characteristics of the mulch affect its usability and durability, and the differences in absorption and reflectance between different colors of plastic mulch affect soil hydrothermal conditions [7,8]. Transparent and black plastic film mulches were tested for temperature control and water use efficiency in arid regions [9]. The results showed that the black plastic film was better in increasing soil temperature and preventing weed growth compared to transparent plastic film. It has been shown that mulching with plastic film in the ridge-furrow system significantly increases water use efficiency and crop yield in arid and semi-arid regions [10,11]. However, mulching plastic films in direct sunlight inevitably leads to fragmentation of the film and contamination of the soil by microplastics [12,13].

Gravel–sand mulching (GSM) in combination with PFM can further reduce evaporation and improve soil water retention [14]. GSM is a practical option in arid environments due to its durability and benefits in reducing water loss and suppressing weeds. GSM has been confirmed by numerous researchers as a promising and effective method for increasing infiltration, reducing evaporation and erosion, and preserving heat in the arid and cold regions [15,16,17]. There are differences in evapotranspiration under GSM due to different particle sizes and their combinations [18,19]. Xie et al. [20] studied the influence of particle size of the mulch layer on soil temperature, evapotranspiration and water use efficiency. The results indicate that a lower proportion of gravel particles with a diameter of 20–60 mm improves water use efficiency. Another experiment showed that water use efficiency was significantly higher with a mulch layer of gravel with a diameter of 2–5 mm than with a layer of gravel with a diameter of 20–60 mm [19]. Compared with bare soil, GSM can significantly improve crop productivity by increasing soil moisture and nutrient efficiency [13,21,22], although Zhao et al. [23] showed that GSM performed better in the first three years, and from the fifth year onwards, water use efficiency rapidly decreases due to the inevitable soil mixing with the gravel-sand mulch.

However, most of existing studies have been focused on the effects of using PFM or GSM alone on RF system performance in semi-arid regions [2,3,24,25,26,27,28]. In arid regions, it is a practical choice to combine the use of PFM and GSM to further enhance rain-harvesting and soil water storage efficiencies, and to prevent plastic fragmentation and soil pollution. But there has been a lack of field-based study to quantify the various soil moisture and yield benefits to a particular crop. There are fewer studies that also consider the effects of dewfall events on soil water content in the arid-desert areas under the RF and mulching conditions. Therefore, field studies are needed to monitor the changes in soil moisture, temperature and salinity with different particle sizes of the mulch material, and how they may affect the performance of a crop such as wolfberry in the arid and desert environment. This work supports the selection of suitable cultivation modes for wolfberry production in arid and semi-arid regions.

The aims of this study were: (1) to evaluate the effects of plastic film mulching (PFM) combined with different gravel-sand mulch types (pure sand, pure gravel, and mixed gravel-sand) on soil hydrothermal properties (temperature and moisture) and salinity in a ridge-furrow (RF) system in an arid region; (2) to evaluate the effects of varying rainfall intensities on soil water dynamics under different mulching practices; (3) to assess the impact of mulching on water consumption (ET), water use efficiency (WUE), yield, and fruit quality of wolfberry.

2. Materials and Methods

2.1. Study Site, Climate and Soil Properties

The experiments were conducted in 2022 and 2023 at a field site (36°59′52′′ N, 105°13′25′′ E, elevation 1740 m a.s.l.) in Yindong Natural Village, Xingren Town, Zhongwei City of Ningxia Province, China, near the Gobi-Tengger- Desert, as shown in Figure 1. The annual average precipitation is 180–250 mm, and the long-term average annual potential evapotranspiration is 2100–2400 mm. As evapotranspiration is approximately 10 times more than precipitation, this region is arid [29]. About 70% of the precipitation is concentrated in the three months from July to September. The average annual air temperature is 6.8 °C (maximum 39 °C and minimum −24 °C) with a frost-free period of 155 days, and the average air temperature during the growing season from April to August is 19.3 °C. Long-term precipitation and air temperature data (2015–2023) were obtained from the China Meteorological Administration (CMA) National Meteorological Information Center (Station ID: GHCN-Daily [53707], 16 km from the experimental site).

The soil was classified as Orthic Sierozem with a sandy loam texture. The soil is slightly saline (but not sodic) according to US Salinity Lab [30]; certain plants can be sensitive to its low salinity. The popular crops grown on this soil are wolfberries and watermelons. Physically, the soil particle sizes include 8% gravel (>2 mm diameter), 66% sand (0.05–2 mm), 13% silt (0.002–0.05 mm), and 8% clay (<0.002 mm). The dry bulk density was 1.42 g/cm³, and the organic matter content (OMC) was 0.78%. The saturated water content was 46%, the field capacity (FC) was 22.7%, and the permanent wilting point (PWP) was 7%. Chemically, the soil was alkaline with a pH of 8.21. The electric conductivity (EC) was 2.89 dS/m, and the sodium adsorption ratio (SAR) was 10.73. The nutrient availability was characterized by an alkali-hydrolyzable nitrogen content of 2.8 meq/L, while available phosphorus (AP) and available potassium (AK) were 0.16 meq/L and 3.8 meq/L, respectively.

2.2. Experimental Design

2.2.1. Surface Mulching Experimental Design

A flat field within an existing wolfberry plantation was selected for the study. The experiment utilized 5-year-old wolfberry ‘Ningqi No. 5′ plants. The wolfberry plants were spaced with 250 cm between rows and 100 cm within rows. Using the tree rows as the centerline, soil was manually excavated symmetrically on both sides to form a furrow. The furrow was leveled and maintained at a width of 60 cm. The loosened soil was piled on both sides to create ridges. The ridge surfaces were subsequently compacted and flattened, resulting in a total ridge width of 190 cm, which includes a 30 cm-wide operation path on top of the ridge, and a ridge height of 20 cm. This configuration established a ridge-furrow (RF) micro-catchment system (Figure 2a). A black plastic film was then placed on the ridge, covering about 1/3 of the furrow width. The black plastic film had a thickness of 0.014 mm. Finally, both the plastic-covered ridge and the exposed furrow were covered with a layer of mulch, which consisted of gravel, sand, or a mixture of gravel and sand, depending on the specific treatment (Figure 2a). The mulch materials were applied uniformly by hand. The mulch layer was laid to a thickness of 2 cm on the ridges and 6 to 8 cm in the furrows. The path surface on the ridge was covered with PFM and a 2 cm thick layer of soil. The experiments were designed so that the soil processes and the reactions of the plants could be observed under the following four conditions:

(1)

MS1: RF surface was covered by PFM with a layer of coarse sand over it (size range 1–2 mm, Figure 2c);

(2)

MS2: RF surface was covered by PFM with a layer of gravel over it (5–20 mm, Figure 2d);

(3)

MS3: RF surface was covered by PFM with a layer of mixed sand and gravel over it, with 40% sand and 60% gravel (Figure 2e);

(4)

CK (control): Bare RF surfaces exposed to the air.

Each treatment was conducted with three replicates in the field experiment. For each replicate plot, 6 wolfberry trees were selected as the experimental subjects, ensuring that each treatment had a consistent and sufficient number of individuals for data collection. In total, across the three replicate groups, the experiment included 18 wolfberry trees, which provided a solid basis for measuring yield and fruit quality. The key growth stages of wolfberry were divided into four distinct phenological phases: the spring shoot growth stage (late April to mid-May), the flowering stage (late May to mid-June), the fruit ripening stage (late June to mid-August), and the leaf fall stage (late August to early September). Irrigation and fertilization were applied simultaneously throughout the entire growth period. In 2022, a total of three irrigation events were conducted, each with a quota of 187.5 m³/ha, accompanied by fertilizer application at a rate of 175 g per plant. In contrast, no irrigation or fertilization was implemented during the 2023 growing season.

2.2.2. Subsurface Soil Monitoring System

As shown in Figure 2a,f, a soil moisture, temperature and electrical conductivity sensor (TEROS 11) was horizontally inserted on a vertical wall of a dug hole (about 10 cm diameter) at 2, 10 and 20 cm depths below the furrow surface. The dug hole was then refiled with the same soil. Horizontally, the sensors were installed at the center of each treatment plot, and at the midpoint of the furrow and between the two middle wolfberry trees. This layout ensured that the sensor readings were not affected by edge effects of the plot or individual tree-specific microenvironments. One set of the three same sensors was installed for each treatment as shown in Figure 2b. One TEROS11 sensor was installed at each of the 2 cm, 10 cm, and 20 cm depths in every treatment plot. Data collection was conducted at a frequency of once per half hour using these sensors. All sensors are field calibrated prior to installation. TEROS 11 sensor (METER Group, Pullman, WA, USA) with a resolution of 2% for volumetric water content and 0.1 °C for temperature.

2.2.3. Analysis of Soil Physicochemical Properties

Soil Analysis Methods

Soil samples were collected on 5 May 2022 before the experimental installation started. Field capacity (FC) and saturated water content by volume (SWC) were determined using core sampling methods with cutting rings (100 cm³). Soil dry bulk density (BD) was measured by oven-drying core samples at 105 °C to constant weight, Permanent wilting point (PWP) by the centrifuge method at −1.5 MPa. Soil organic matter (SOM) content was measured using the potassium dichromate volumetric method (Walkley-Black oxidation). Soil pH was measured in a 1:2.5 soil: water suspension by potentiometry. Electrical conductivity (EC) was determined via electrode method in saturated paste extract [31].

Cation analysis was performed as follows:

(1)

Na⁺ Flame photometry

(2)

Ca²⁺ and Mg²⁺: Atomic absorption spectrophotometry (AAS)

(3)

Sodium adsorption ratio (SAR) was calculated as

$$SAR=\left[{Na}^{+}\right]/\sqrt{\left[{Ca}^{2+}\right]+\left[{Mg}^{2+}\right]/2} \text{(concentrations in meq/L).}$$

Nutrient availability:

(1)

Alkali-hydrolyzable nitrogen: Alkali diffusion method.

(2)

Available phosphorus: Spectrophotometric molybdenum-blue method.

(3)

Available potassium: Flame photometric determination after extraction with neutral 1 M ammonium acetate.

2.2.4. Determination of Yield Indicators

The yield indicators consisted of dried fruit yield, 100-berry weight of dried fruits, fresh-to-dry weight ratio (FDR), and granularity index. During the fruit ripening period, all mature wolfberry fruits from each plot were harvested. Fresh fruit yield and the weight of 100 berries of fresh fruits were determined on-site using an electronic balance (precision: 0.01 g). Following fruit drying, dried fruit yield and the 100-berry weight of dried fruits were determined by weighing. A 50 g sample of dried fruits was randomly weighed from each plot to determine the granularity index. The fresh-to-dry weight ratio was calculated based on the dried fruit yield and fresh fruit yield.

2.2.5. Determination of Fruit Quality Indicators

The quality indicators assessed included total sugars, polysaccharides, flavonoids, betaine, and β-carotene [32,33]. A 100 g fresh sample of wolfberry fruits was randomly selected from each plot for quality analysis. Total sugar content was determined using the acid hydrolysis phenol-sulfuric acid colorimetric method. Polysaccharide content was determined using the phenol-sulfuric acid method. Flavonoid content was determined using spectrophotometry. Betaine content was determined using the Reinecke salt precipitation method. β-Carotene content was determined using the ethanol- petroleum ether extraction direct colorimetric method.

2.2.6. Wolfberry Water Consumption

The water consumption (evapotranspiration, ET) of wolfberry (mm) was calculated using the following formula [34]:

$$ET=P_r+U+I-D-R-∆W$$

(1)

where $P_r$ is the effective rainfall during the growth period (mm), $U$ is the groundwater recharge (mm), $I$ is the irrigation amount (mm), $D$ is the deep percolation (mm), $R$ is the surface runoff (mm), ∆W is the change in soil water storage in the soil profile between the beginning and end of the experiment (mm). Given that the groundwater table in the experimental area was deep and the drip irrigation amount was limited with minimal infiltration, the terms $U,D$ and $\mathrm{R}$ were considered negligible and thus omitted from the calculation. The effective rainfall ($P_r$) was determined as: $P_r=aP$.

Here, the rainfall event was <5 mm, $a=0$; the rainfall event ranged between 5 and 50 mm, $a=1.0~0.8$; the rainfall event was >50 mm, $a=0.7~0.8$ [35].

Therefore, the formula was simplified to:

$$ET=P_r+I-∆W$$

(2)

2.2.7. Water Use Efficiency of Wolfberry

Water Use Efficiency (WUE) of wolfberry was calculated as:

$$WUE=Y/ET$$

(3)

where $Y$ is the dry fruit yield of wolfberry (kg/ha).

2.2.8. Environmental Monitoring System

A leaf wetness sensor (LWS, METER Group, USA) with a sensitivity of 0.02 mm was installed 10 cm above the flat soil surface, an air temperature and humidity sensor (ATMOS 14, METER Group, USA) with resolutions of 0.1 °C and 0.1% RH was installed at a height of 100 cm, and a rain gauge (ECRN-100, METER Group, USA) with a resolution of 0.2 mm was mounted at 160 cm above the ground. Sensors were calibrated on-site prior to installation. Rainfall intensity standard was based on China’s NMCC [5].

2.2.9. Data Acquisition

All the surface and subsurface sensors were plugged into input ports of 3 ZL6 Data Loggers (METER Group, USA) to simultaneously record the soil and meteorological data at 0.5 h interval for the entire monitoring period from May to December in 2022 and 2023. All the measured data were automatically recorded in the dataloggers. Data was then downloaded in Excel format.

2.3. Data Processing

Typical rainfall intensities: According to China’s national standard for precipitation grade (see [5]), rainfall of 0.1–9.9 mm in 24 h is categorized as light rain, 10.0–24.9 mm is moderate rain, and 25.0–49.9 mm is heavy rain. To delineate the dynamic relationships between rainfall and SWC, we analyzed the changes in SWC under 2 typical rainfall intensities in 2022 and 3 typical rainfall intensities in 2023: (a) a light rainfall of 5 mm/day on 27th of July (27 July 2022); (b) a moderate rainfall of 19.4 mm/day on 11th of July (11 July 2022); (c) a light rainfall of 5.4 mm/day on 10th of July (10 July 2023); (d) a moderate rainfall of 15 mm/day on 25th of August (25 August 2023); and (e) a heavy rainfall of 47 mm/day on 10th of August (10 August 2023). There were no heavy rain events (larger than 25 mm per day) during the monitoring period in 2022.

The monthly precipitation, monthly dew amount and soil moisture and temperature data were calculated and analyzed using Microsoft Excel 2022 (removing nulls and outliers based on ± 4SD exclusion) and plotted using Origin 2022 software. A two-way analysis of variance (ANOVA) was employed to assess the effects of different mulch treatments and soil depths on soil water content, soil temperature, and electrical conductivity, followed by LSD post hoc tests for multiple comparisons. The dataset contained a missing value for the CK treatment at the 20 cm depth for soil temperature, water content, and electrical conductivity due to a sensor failure. This incomplete case was excluded from the two-way ANOVA, which was performed on the remaining data (complete case analysis). A one-way ANOVA was conducted to evaluate the differences in wolfberry water consumption, yield, and quality parameters among the different treatments, with LSD tests used for subsequent multiple comparisons. Statistical analyses were performed using SPSS 27.0 (SPSS for Windows, Version 27.0, Chicago, IL, USA). Data underwent strict quality control (missing value interpolation via IDW, outlier removal per WMO standards) before analysis.

3. Results

3.1. Soil Temperature Variations

As shown in Figure 3, temperature variability decreased with increasing depth. A general rising trend was observed until late July, followed by a decline. Statistical analysis (

Table 1. Soil temperature (mean ± standard deviation, °C) under different mulch treatments at various soil depths during 2022 and 2023.

Treatment	Soil Temperature
	2022			2023
	2 cm	10 cm	20 cm	2 cm	10 cm	20 cm
MS1	17.63 ± 10.28aB	17.84 ± 9.76aB	18.45 ± 8.91aA	16.5 ± 10.37bA	16.59 ± 10.17bA	16.44 ± 9.59bA
MS2	17.24 ± 10.56aB	18 ± 9.76aA	18.11 ± 8.84aA	16.66 ± 10.47bB	17.28 ± 10.19aA	17.3 ± 9.62aA
MS3	17.63 ± 10.77aB	17.73 ± 10.07aB	18.13 ± 9.11aA	17.42 ± 10.35aA	17.32 ± 10.13aA	17.5 ± 9.62aA
CK	16.34 ± 11.49b	16.72 ± 10.94b		16.01 ± 11.06c	16.41 ± 10.37b
Treatment (A)	**			*
Soil depth (B)	**			**
A × B	ns			ns

Note: Different lowercase letters in the same column indicate significant differences in soil temperature among treatments according to Duncan’s multiple range test (p < 0.05); different uppercase letters in the same row indicate significant differences in soil temperature across soil depths (p < 0.05). ** denotes highly significant differences (p < 0.01), * denotes significant differences (p < 0.05), and ns indicates no significant effect (p > 0.05).

) confirmed the significance of these variations (p < 0.001). In 2022, MS1 resulted in the highest temperatures, increasing the top-layer temperature by 1.30 °C compared to CK, followed by MS3 (+1.29 °C) and MS2 (+0.90 °C). The pattern shifted in 2023, where MS3 showed the most consistent warming effect across all depths, recording the highest temperature at 2 cm (17.42 °C). At 10 cm, both MS2 and MS3 maintained significantly higher temperatures (17.28 °C and 17.32 °C) than MS1 and CK. These findings underscore that plastic mulching effectively increases soil warmth, albeit with considerable diurnal and seasonal fluctuations reflected in the standard deviations.

3.2. The Pattern of Rainfall and Dewfall

Figure 4 shows the dynamic changes in rainfall and dewfall amount (depth of water) during the 2022 and 2023 monitoring periods, revealing distinct seasonal patterns and inter-annual variability. In 2022, daily fluctuations showed rainfall ranging from 0 to 19.4 mm (mean: 0.45 mm) and dewfall from 0 to 0.98 mm (mean: 0.18 mm). Total accumulated precipitation reached 116.8 mm, peaking in July with 53 mm rainfall and 4.7 mm dewfall. Dewfall contributed only 9.61 mm (9% of total rainfall). In 2023, August recorded the highest rainfall (62.60 mm), contributing to a higher annual total of 139.26 mm. Both years exhibited concentrated summer precipitation (July–August), consistent with monsoonal influences, underscoring significant seasonal and inter-annual variations critical for agro-hydrological processes.

3.3. Effects of Combined Plastic Film and Gravel-Sand Mulching on Soil Water Content

Figure 5 and Figure 6 summarize the dynamic and average values of soil water content (SWC) under different mulching treatments (MS1, MS2, MS3) and the control (CK) across 2022 and 2023, demonstrating significant effects of treatment, depth, and year. In 2022, MS1 consistently retained the highest moisture at 2 cm (24.6%, above field capacity), followed by MS3 (18.4%) and MS2 (17.9%), all markedly higher than CK (9.1%, near permanent wilting point). A similar trend occurred at 10 cm, with higher SWC than at 2 cm. At 20 cm, MS3 (17.9%) outperformed MS2 (16.6%) and MS1 (15.5%), though all values were below field capacity. In 2023, the overall SWC decreased, but MS1 again showed superior surface retention, while at 10 cm, MS2 (18.4%) and MS3 (17.6%) provided more stable moisture than CK. Statistical analysis (

Table 2. Results of the two-way ANOVA for soil water content (SWC) as affected by treatment and soil depth during 2022 and 2023.

	Soil Water Content (%)
	2022	2023
Treatment (A)	**	**
Soil depth (B)	**	**
A × B	**	**

Note: ** denotes highly significant differences (p < 0.01), according to Duncan’s multiple range test.

) confirmed highly significant effects of treatment, depth, and their interaction (p < 0.01) for both years. Supplemental Figures S1 and S2 further revealed that under varying rainfall intensities, mulching (especially MS1 and MS3) enhanced infiltration and retention—most notably improving topsoil (0–10 cm) moisture after light rain and increasing water penetration down to 20 cm following heavy events. Overall, MS1 and MS3 were most effective in storing soil moisture across depths and years, highlighting their practical potential for improving water conservation in rain-fed agricultural systems [36,37,38,39,40].

3.4. Effects of Different Mulch Treatments on Water Consumption and Water Use Efficiency by Wolfberry Across Two Distinct Hydrological Years

Table 3. Water consumption (mm) of Wolfberry at different phenological stages under different mulch treatments and water use efficiency (kg/m³) of Wolfberry during 2022 and 2023.

	Treatment	MS1	MS2	MS3	CK
Growth Stages
Spring Shoot Stage	2022	23.99 ± 0.02a	24.05 ± 0.21a	23.95 ± 0.04a	23.46 ± 0.29b
	2023	2.05 ± 0.03a	0.42 ± 0.05c	1.16 ± 0.04b	−0.59 ± 0.07d
Flowering Stage	2022	42.43 ± 0.02d	42.65 ± 0.05b	42.94 ± 0.03a	42.48 ± 0.1c
	2023	7.31 ± 0.07b	7.37 ± 0.17b	7.82 ± 0.06a	6.99 ± 0.2c
Fruit Ripening Stage	2022	45.43 ± 0.05b	44.86 ± 0.11d	46.03 ± 0.05a	45.09 ± 0.14c
	2023	65.69 ± 0.12d	67.44 ± 0.19a	66.43 ± 0.1c	67.24 ± 0.19b
Leaf Fall Stage	2022	0.39 ± 0.08b	0.52 ± 0.11a	0.54 ± 0.07a	0.38 ± 0.11b
	2023	16.6 ± 0.082a	15.58 ± 0.06d	16.18 ± 0.05c	16.34 ± 0.12b
Entire growth Season	2022	112.24 ± 0.14b	112.08 ± 0.09c	113.46 ± 0.09a	111.4 ± 0.41d
	2023	91.64 ± 0.03a	90.81 ± 0.15b	91.59 ± 0.08a	89.98 ± 0.16c
WUE	2022	2.05 ± 0.003a	2.02 ± 0.002c	2.03 ± 0.002b	1.67 ± 0.008d
	2023	1.87 ± 0.001a	1.77 ± 0.003c	1.83 ± 0.002b	1.41 ± 0.002d

Note: Data are presented as mean ± standard deviation (SD). Values within a row followed by different letters are significantly different at p < 0.05 according to Duncan’s multiple range test.

summarizes the water consumption patterns of wolfberry under different mulch treatments during two hydrologically distinct years, highlighting significant interannual variability and treatment-specific efficacy shaped by irrigation and rainfall availability. In the irrigated year (2022), all mulching treatments enhanced water consumption across key phenological stages compared to CK, with MS3 exhibiting the highest total seasonal water use (113.46 mm). Conversely, in the non-irrigated year (2023), water consumption trends shifted considerably. CK showed a net water gain during the Spring Shoot Stage (−0.59 mm), whereas both CK and MS2 demonstrated superior rainwater utilization during the Fruit Ripening Stage (67.24 mm and 67.44 mm, respectively). Seasonally, MS1 resulted in the highest total consumption (91.64 mm) in 2023, though differences among treatments were marginal. Statistical significances (p < 0.05) between treatments within each year and stage are clearly indicated. More importantly, mulch treatments consistently improved water use efficiency (WUE) in both years. In 2022, MS1 achieved the highest WUE (2.05 ± 0.003 kg/m³), significantly surpassing MS2, MS3, and CK (1.67 ± 0.008 kg/m³). This advantage persisted in 2023, with MS1 again recording the highest WUE (1.87 ± 0.001 kg/m³), followed by MS3 and MS2, all markedly higher than CK (1.41 ± 0.002 kg/m³). The results underscore that mulching–particularly MS1- can significantly enhance water productivity in wolfberry cultivation under both supplemental irrigation and rain-fed conditions, providing valuable strategies for efficient water management in arid and semi-arid regions. Based on the ANOVA results (Tables S1 and S2), annual variation had a highly significant impact (p < 0.001) on seasonal water consumption across all growth stages of wolfberry. The distribution of water use shifted markedly between years, with substantially lower consumption during the spring shoot and flowering stages in 2023, but higher consumption during the fruit ripening and leaf fall stages. In contrast, water use efficiency (WUE) remained statistically consistent between the two years (p = 0.161).

3.5. Dynamics of Soil Salinity

As illustrated in Figure 7 and

Table 4. Electrical conductivity (EC) (dS/m) under different mulch treatments at various soil depths during 2022 and 2023.

Treatment	Electrical Conductivity (EC) (dS/m)
	2022			2023
	2 cm	10 cm	20 cm	2 cm	10 cm	20 cm
MS1	3.2 ± 0.9aA	2.7 ± 0.8bB	3.4 ± 0.8aA	2.1 ± 0.9aB	1.8 ± 0.9aC	3.1 ± 1.2aA
MS2	2 ± 1cB	2.9 ± 0.5bA	0.03 ± 0.02bC	2.1 ± 1.4aA	1.9 ± 1.1aAB	1.7 ± 1bB
MS3	2.5 ± 0.9bB	3.3 ± 1.1aA	3.2 ± 0.58aA	0.6 ± 0.3cC	1.4 ± 0.9bB	2.8 ± 0.9aA
CK	2.1 ± 0.9c	2 ± 1.1c		0.9 ± 0.6b	0.7 ± 0.5c
Treatment (A)	**			**
Soil depth (B)	**			**
A × B	**			**

Note: Data are presented as mean ± standard deviation (SD). Different lowercase letters in the same column indicate significant differences in soil electrical conductivity among treatments (p < 0.05); different uppercase letters in the same row indicate significant differences in soil electrical conductivity across soil depths (p < 0.05). ** denotes highly significant differences (p < 0.01).

, mulching treatments significantly influenced soil electrical conductivity (EC) during the monitoring periods in 2022 and 2023, with pronounced interannual variations and treatment-specific effects. Rainfall initially raised soil moisture and EC, but prolonged moisture led to salt leaching, reducing near-surface EC, while nitrogen and phosphorus deficiencies further modulated ion mobility and accumulation. In both years, MS1 consistently resulted in the highest EC across soil depths, reaching 3.1 dS/m in 2022 and 2.3 dS/m in 2023, significantly exceeding other treatments and the control (CK). MS3 and MS2 generally followed, and both outperforming CK. Statistical analysis confirmed that treatment, depth, and their interaction all exerted highly significant effects on EC (p < 0.01). Notably, MS1 promoted deeper salt accumulation (10–20 cm), maintaining stable but elevated EC levels, which could exceed the 3 dS/m threshold detrimental to root growth and crop performance in sensitive species. These findings underscore the critical role of mulching type in managing soil salinity, with MS1 enhancing water and salt retention but necessitating caution in salt-sensitive arid agroecosystems.

3.6. Wolfberry Yield and Contents of Nutritional Compounds

As shown in Figure 8 and

Table 5. Effect of different mulch treatments on the yield of dried wolfberries.

Treatment		Fresh-to-Dry Weight Ratio (FDR)	Weight of 100 Berries g	Berry Count per 50 g
2022	MS1	4.16 ± 0.03d	19.64 ± 0.09a	254.67 ± 1.15d
	MS2	4.36 ± 0.02b	18.59 ± 0.07c	269 ± 1b
	MS3	4.28 ± 0.03c	19.13 ± 0.09b	261.33 ± 1.15c
	CK	4.51 ± 0.03a	17.71 ± 0.03d	282.33 ± 0.58a
2023	MS1	4.24 ± 0.01d	19.11 ± 0.05a	261.67 ± 0.58d
	MS2	4.34 ± 0.01b	18.45 ± 0.07c	271 ± 1b
	MS3	4.29 ± 0.01c	18.71 ± 0.04b	267.33 ± 0.58c
	CK	4.46 ± 0.03a	17.44 ± 0.03d	286.67 ± 0.58a

Note: Different lower-case letters indicate significant differences among mulch treatments according to Duncan’s multiple range test (p < 0.05).

, mulching treatments significantly enhanced the yield of wolfberry over the two-year study. In 2022, yields under MS3 and MS1 were highest (2307.9 and 2300.8 kg/ha, respectively), representing an increase of approximately 24% compared to CK (1861.33 kg/ha). Although overall yields decreased in 2023, the same treatment ranking persisted: MS1 (1714.3 kg/ha) > MS3 (1679.08 kg/ha) > MS2 (1607.44 kg/ha) > CK (1276.67 kg/ha), with all differences statistically significant (p < 0.05). Fruit dry matter ratio (FDR) was lowest under MS1 (4.16 in 2022; 4.24 in 2023), indicating superior dry matter accumulation, while CK consistently showed the highest FDR, reflecting poorer fruit quality. The 100-berry weight was greatest in MS1 (19.64 g in 2022; 19.11 g in 2023), following the trend MS1 > MS3 > MS2 > CK, corroborated by lower berry counts per 50 g under mulching. These results demonstrate that mulching, particularly MS1 and MS3, effectively promotes yield, improves fruit size, and enhances dry matter content, offering practical strategies for elevating wolfberry production in arid regions. As shown in Tables S3 and S4, annual variation significantly reduced dried berry yield in 2023 (p = 0.006), while other yield-related traits, including fresh-to-dry ratio, hundred-berry weight, and berry count, remained stable between years.

As shown in

Table 6. Effect of different mulch treatments on contents of nutritional compounds in wolfberry.

Treatment		β-Carotene Content μg/g	Polysaccharides Content g/100 g	Total Soluble Sugars Content g/100 g	Total Flavonoids Content g/100 g	Betaine Content g/100 g
2022	MS1	1558.78 ± 1.95a	5.73 ± 0.04a	52.25 ± 0.57a	0.84 ± 0.03a	0.44 ± 0.08c
	MS2	1412.43 ± 2.47c	5.2 ± 0.06c	50.77 ± 0.46b	0.73 ± 0.03bc	0.67 ± 0.04a
	MS3	1646.61 ± 1.55b	5.37 ± 0.08b	51.7 ± 0.2a	0.78 ± 0.02b	0.54 ± 0.04b
	CK	1374.71 ± 2.49d	4.2 ± 0.14d	49.38 ± 0.43c	0.55 ± 0.01d	0.76 ± 0.02a
2023	MS1	1486.98 ± 2.5a	5.56 ± 0.03a	51.35 ± 0.09a	0.73 ± 0.03a	0.45 ± 0.03d
	MS2	1312.43 ± 2.29c	5.26 ± 0.03c	50.24 ± 0.06c	0.64 ± 0.02c	0.60 ± 0.01b
	MS3	1446.57 ± 1.5b	5.35 ± 0.04b	50.7 ± 0.2b	0.69 ± 0.02b	0.53 ± 0.02c
	CK	1274.55 ± 2.24d	4.04 ± 0.03d	49.05 ± 0.07d	0.51 ± 0.01d	0.73 ± 0.01a

Note: Data are presented as mean ± standard deviation (SD). Values within a column followed by different letters are significantly different at p < 0.05 according to Duncan’s multiple range test.

, mulching treatments significantly improved the nutritional quality of wolfberry, with consistent advantages observed across key functional compounds over both years. MS1 and MS3 markedly enhanced β-carotene content, reaching 1646.61 μg/g and 1558 μg/g, respectively, in 2022, significantly higher than MS2 and CK (1374.71 μg/g). Polysaccharides, total soluble sugars, and total flavonoids were also highest under MS1, demonstrating its superior effect in promoting the accumulation of these beneficial components. In contrast, betaine accumulation exhibited an inverse trend, with the highest values under CK, suggesting that moderate water stress promotes betaine synthesis. Overall, MS1 proved most effective for enhancing comprehensive fruit quality, highlighting its potential for optimizing nutrient-rich wolfberry production in water-limited environments. As shown in Tables S5 and S6, none of the measured nutritional compounds in wolfberry were significantly influenced by inter-annual variability (p > 0.05 for all parameters).

3.7. Response of Wolfberry Yield to Water Consumption

Figure 9 shows the quadratic relationships between water consumption (evapotranspiration, ET) and dried berry yield of wolfberry. In 2022, the yield–ET model is $Y=-263.15{ET}^2+53467ET-3\times {10}^6$ ($R^2=0.9973$), indicating an extremely strong fit. Yield first increased with ET, peaking at an optimal ET (around 101 mm), then slightly declined. For 2023, the model $Y=-295.92{ET}^2+53972ET-2\times {10}^6$ ($R^2=0.9402$) shows a similar trend: yield rose with ET up to a threshold (near 91.5 mm) before decreasing. These results suggest there exists an optimal ET range for maximizing wolfberry yield in both years. Mulching treatments (MS1-MS3) likely regulated ET (as shown in prior data) to approach these optima, influencing yield. The observed decline in the stability of the yield-ET relationship in 2023 can be attributed to the absence of irrigation. This is in sharp contrast to 2022, where consistent irrigation and fertilization practices maintained a tighter and more stable coupling, buffering yield against ET variability. Overall, managing ET (via mulching or irrigation) to align with the optimal range is critical for maximizing wolfberry productivity.

4. Discussion

4.1. The Importance of Rainfall and Dewfall for SWC

Rainfall is the main source of soil moisture in the arid region, and dewfall is an important supplementary source of water. Dewfall and rainfall occur on different days and are highly complementary in time [41]. The rainy season in this area from May to October with the highest rainfall and dewfall in July or August. Correspondingly, SWC showed an increasing trend before July and August and a decreasing trend afterward. Although the amount of dewfall is less than 10% of rainfall, the duration of dewfall is much longer than that of rainfall, as shown in Figure 4. This is consistent with the study of Zhang et al. [5]. This study shows that dew and vapor absorption by the mulch layer and soil are important sources of SWC when SWC is lower than FC [14], which is crucial for the physiological condition of vegetation and the sustainability of microclimate in arid and semi-arid regions.

4.2. Variability of the Soil Water Content

The combined mulching with plastic film and gravel-sand significantly increased soil water retention, which is consistent with research in other studies [16,42,43,44,45]. The soil water content was higher in the MS1 (pure sand) treatment than in MS2 (pure gravel) and MS3 (mixed gravel and sand). The lowest SWC was found under CK (bare soil). The reason may be related to the physical properties of the different mulch materials and their specific effects on soil water evaporation and infiltration [15]. First, the pure sand mulch in MS1 had the smallest grain size and lower air permeability compared to MS2 and MS3, which effectively reduced the evaporative loss of soil moisture through evaporation. In addition, the pores of MS1 material are more uniform, which can promote water infiltration into a deeper soil layer, thus maintaining a higher moisture content in the soil [16,46]. In contrast, the large gravel size in MS2 exhibited relatively larger air permeability due to larger pore spaces, resulting in higher evaporation rates and making it more difficult to retain water in the soil. The mixed gravel and sand mulch (MS3) had a complicated pore structure due to the uneven distribution of particle sizes, resulting in zig-zagged and/or disconnected paths of the trapped sand between gravels, which resulted in higher amount of capillary water retention in the mulch layer, reducing infiltration into the soil and increasing evaporation from the mulch layer. However, compared to CK, the mulch treatments significantly increased water retention in the soil and reduced evaporation from soil [47,48].

The thermophysical properties of the mulch materials (heat capacity and conductivity, relative humidity and shape, grain size distribution, etc.) also had important effects on the dynamics of soil moisture and temperature dynamics [49]. The 1–2 mm thick sand can absorb and conduct heat more evenly during the day, reducing the extent of temperature fluctuations in the soil and thus indirectly reducing evaporation losses. Zhang et al. [50] indicated that the thermal conductivity increases with increasing particle size for uniform sands, so the larger sized 5–20 mm gravels store more heat in the surface layer and accelerate water evaporation than the sand. On the other hand, the MS3 treatment may result in lower overall water retention and insulation properties of the mulch than the homogeneous coarse sand mulch (MS1) due to the large differences in thermophysical properties of the different particle sizes.

Dewfall is also an important factor influencing SWC. Lower soil temperatures favor the condensation of dew. Bare soil has a lower heat storage capacity than gravel and sand, and bare soil cools faster than gravel, which favors condensation. In addition, the higher thermal conductivity of gravel and sand may be a reason for the lower condensation rate compared to bare soil [51]. In the upward direction, the vaporized soil moisture accumulates directly under the plastic film, which leads to frequent saturation of the topsoil [5]. Due to this effect, it would be better to completely cover the furrow surface with plastic film to retain all the upward vapor flow and heat from the deep soil.

4.3. Variability of the Water Consumption

The results underscore a critical interaction between mulch type and hydrological conditions, challenging the notion of a universally optimal mulch. The superior performance of MS3 in the irrigated year (2022) aligns with the expectation that combined plastic film and gravel-sand mulching creates an effective barrier against soil evaporation while likely facilitating soil temperature moderation, thereby enhancing plant water uptake and transpiration [13]. This treatment optimized the use of available irrigation water.

The anomalous negative water consumption observed for CK during the arid 2023 Spring Shoot Stage is a pivotal finding. It can be robustly attributed to dew deposition and water vapor adsorption [52]. The bare soil, with higher thermal conductivity and no physical barrier, cools more rapidly at night, condensing atmospheric moisture directly into the soil profile. This process represents a crucial non-rainfall water input in arid ecosystems. The mulches, while conserving moisture, also partially insulate the soil and prevent direct vapor adsorption, hence their positive but low consumption values in this stage.

The shift in performance during the 2023 Fruit Ripening Stage highlights the trade-off between water conservation and infiltration. The high permeability of the gravel layer in MS2 and the absence of any barrier in CK allowed for more efficient infiltration and utilization of the substantial rainfall (67.4 mm), leading to peak water consumption. In contrast, the finer textures of MS1 (coarse sand) and particularly MS3 (mixed gravel-sand) may have hindered infiltration, causing more water to be lost as surface runoff or through evaporation from the mulch layer itself.

The analysis revealed a significant shift in seasonal water consumption patterns between the two years, characterized by substantially lower water use during the vegetative growth phases (spring shoot and flowering stages) in 2023, compensated by higher consumption during the fruit ripening and leaf fall stages. Despite this notable redistribution of water use across the growing cycle, the overall water use efficiency (WUE) remained statistically unchanged. This indicates that the wolfberry plants maintained a consistent level of productivity per unit of water consumed, adapting their water uptake pattern in response to inter-annual variations in environmental conditions or management practices without compromising the efficiency of water utilization.

In conclusion, the optimal mulch strategy is highly dependent on the water regime. MS3 is recommended for irrigated systems to maximize water productivity. However, in rainfed or drought-prone environments, MS1 appears superior for its overall moisture conservation capability over a full season. Interestingly, in scenarios where large, discrete rainfall events are anticipated, the natural infiltration capacity of bare soil (CK) can be nearly as effective as certain mulched treatments.

4.4. Variability of the Electrical Conductivity in the Soil

In arid saline areas, plastic film mulching on ridge and furrow has an indirect effect on electrical conductivity (EC) by regulating soil hydrothermal conditions and activating soil nutrients [53,54]. In this study, the soil pH is 8.21 and the soil EC is between 1.5 and 4 dS/m, so the soil is slightly saline [55] and sensitive to certain crops. In this study area, the lack of nitrogen and phosphorus in the soil can inhibit plant growth, which leads to a reduced capacity of the roots to absorb water and salt ions [56]. After precipitation events, unabsorbed salts can accumulate in the soil, which can lead to an increase in electrical conductivity (EC) values, especially during the period of heavy evaporation from June to August.

There are differences in evapotranspiration under gravel-sand mulch due to different particle sizes and their combinations [18,19]. When large-sized gravel is used to cover furrows, it creates a highly porous structure that accelerates vertical water infiltration, promotes downward salt migration, reduces salt accumulation at the surface, and thus lowers the electrical conductivity (EC) of the soil within the furrows. In addition, the strong reflection of sunlight can lower the surface temperature of the soil and reduce the enhancement of salt dissolution caused by high temperatures, indirectly preventing an increase in EC. In contrast, a small-sized sand-gravel cover can form a compact layer that increases the water retention capacity but reducing infiltration rates, resulting in longer water retention. With insufficient irrigation or rainfall, salts accumulate on the surface, leading to an increase in EC. In this study, the average EC at different depths were the highest with MS1 treatment, followed closely by MS3 and MS2, indicating that the MS1 (1–2 mm sand mulching) increased EC values, while the MS2 (5–20 mm gravel) effectively reduced EC values.

4.5. Wolfberry Yield and Quality Response to Plastic and Gravel-Sand Mulching on Ridge-Furrow Planting System

Black plastic film combined with fine gravel-sand mulch can effectively improve the soil microenvironment and promote plant growth and fruit development [57]. MS3 and MS1 were excellent in water retention, maintaining temperature level and optimizing root permeability, resulting in more adequate water and nutrient supply for wolfberries growth, thus significantly increasing yield.

The mulch treatments were able to reduce the moisture content of the fresh fruits and improve the efficiency of dry matter accumulation [58]. The lowest FDR of MS1 (4.16) implied that it had a higher percentage of dry matter in its fruits, which may be attributed to the good water and fertilizer retention capacity of the coarse sand cover, which prompted the plant to convert more photosynthetic products into dry matter during fruit ripening.

In contrast, although MS2 had a higher yield (22.0%) compared with CK, it was weaker than MS1 and MS3 in terms of 100-grain weight, grain size, and fresh-to-dry ratio, which could be related to the relatively poor water-holding capacity of gravel and the limited effect of soil structure optimization [59]. CK was significantly worse than the mulch group in terms of yield and fruit quality due to rapid water evaporation and large fluctuations in soil temperature and moisture, which resulted in limited plant growth and poor fruit development.

Black plastic film in combination with gravel-sand mulch promoted the anabolism of carotenoids by improving soil temperature, moisture and permeability [44,45]. MS3 provided a more suitable growth environment for the plants with benefits in water retention and air permeability, resulting in the highest β-carotene accumulation. The mulch treatment effectively enhanced photosynthesis and carbohydrate synthesis of the plants [60,61]. MS1 ensured the water requirement of the plant with the best water retention during the critical growth period, which promoted the accumulation of polysaccharides, while the mixed material of MS3 promoted the accumulation of sugars with the optimized soil structure and improved nutrient utilization rate [15]. Coarse sand mulch can increase the total amount of MS1 flavonoids by regulating the plants’ antioxidant system and stimulating the synthesis of secondary metabolites; drought stress induced a plant stress response that triggered a large amount of synthesis of betaine as an osmotically regulated substance, while mulch treatments attenuated such a stress response mechanism while improving environmental conditions.

Inter-annual variability significantly influenced dried berry yield (p = 0.006), which decreased notably in 2023, while key physical fruit traits, including fresh-to-dry ratio, hundred-berry weight, and berry count, remained statistically consistent between years. Furthermore, the contents of major nutritional compounds such as polysaccharides, soluble sugars, flavonoids, and betaine showed no significant inter-annual differences, demonstrating stable fruit biochemical quality despite variations in growing conditions. These results suggest that although seasonal environmental factors considerably affect overall productivity, the fundamental physical and nutritional quality of wolfberry fruit remains relatively robust across years.

5. Conclusions

Based on a two-year field study, combined plastic film and gravel-sand mulching effectively improved the soil micro-environment and wolfberry productivity in the arid region. The mulch treatments (MS1, MS2, MS3) consistently enhanced soil temperature and moisture in the upper layers (0–20 cm), leading to better plant growth, higher water use efficiency (WUE), and significant increases in yield and fruit quality—particularly polysaccharides, soluble sugars, and flavonoids. MS1 and MS3 performed better in terms of yield and quality; however, all mulching treatments elevated soil salinity, with MS1 resulting in the highest EC values. A quadratic relationship was identified between water consumption (ET) and yield, indicating an optimal ET range achievable through mulching. While MS1 offered a balanced outcome in yield, WUE, and quality, long-term salinity management is essential for sustainability. These results support the use of mulching as a viable water-saving strategy in arid agroecosystems, provided that salt accumulation is monitored and controlled.