Study on the Effects of Planting Alfalfa (Medicago sativa L.) and Adding Biochar on Soil Fertility in Jujube Orchards

Abstract

Soil fertility has an important impact on orchard yield and quality, and sandy soil limits the economic yield of orchards due to its low water and fertilizer retention capacity. Although biochar and alfalfa planting have been widely utilized separately in soil improvement, few studies have examined the effects of combined alfalfa planting and biochar application on jujube orchard soils. This study investigates the effects of alfalfa planting alone and alfalfa planting combined with different levels of biocarbon addition on soil properties. A field experiment was conducted in a jujube orchard in Yanchuan County, Shaanxi Province, with four treatments: clear tillage control (CK), alfalfa planting only (B1), alfalfa planting + 1.5 kg·m⁻² biocarbon (B2), and alfalfa planting + 3 kg·m⁻² biocarbon (B3). The results show that planting alfalfa significantly increased soil moisture content (SMC) and soil organic matter (SOM) content by 27.79% and 17.65%, respectively, and biochar addition significantly increased soil carbon, nitrogen, and phosphorus content by 8.11–37.7%, enhanced the soil moisture content (SMC) by 98.13–100.22%, promoted the growth of alfalfa, and increased vegetation cover (p < 0.05). The combination of biochar and alfalfa improves soil fertility more effectively than alfalfa alone. It can increase the soil N and P nutrient contents, improve soil available nutrients, promote alfalfa growth in a short period, and provide a feasible solution for soil improvement in the future.

1. Introduction

Driven by the rural revitalization strategy, the Yellow River basin, which is regarded as a crucial ecological barrier and strategic economic region in China, has achieved agricultural transformation through its distinctive fruit industry [1]. Within this context, the jujube industry in the Shaanxi segment fulfills a tripartite functional role: it acts as a vehicle for ecological restoration, a pillar of rural industry, and a demonstration window for policy implementation [2]. This industry holds substantial theoretical and practical value for establishing a developmental paradigm in the Yellow River riparian zones centered on the principle of “promoting industrial ecologization through eco-industrialization” [3]. However, the alluvial soils within these riparian zones, predominantly formed by fluvial deposition processes [4], are typically sandy in texture. These soils exhibit severe nutrient leaching and generally contain less than 8 g·kg⁻¹ of organic matter [5], resulting in inherently weak mineral nutrient retention capacity. To sustain orchard productivity, long-term reliance on high-intensity synthetic fertilization has been necessary. This practice not only leads to diminishing marginal returns in agricultural production, but also poses significant risks of inducing compound ecological hazards, notably secondary salinization [6]. These factors critically constrain the sustainable development of the regional agricultural ecosystem. Consequently, the scientific amelioration of sandy soils has emerged as a pressing imperative. Current soil improvement measures can be categorized into two main types based on the amendments used to enhance soil properties: chemical improvements, which involve the application of organic fertilizers, biochar, and other amendments to increase the soil’s nutrient storage capacity [7]; and physical improvements, which entail the use of water retention agents and soil structure conditioners to enhance the soil’s water-holding capacity and aggregate stability [8]. Additionally, cover crops can form a protective layer on the ground surface by utilizing their roots to bind sand particles together, which is also meaningful for soil improvement [9].

In the northwest region of China, the cultivation of cover crops in orchards is an effective method for enhancing the absorption of nutrients by sandy soils, as it accelerates the reproduction of soil microorganisms [10,11,12]. Cover crops can reduce soil bulk weight, increase soil porosity [11], and enhance the soil water-holding capacity, thus improving the soil’s physicochemical properties. More importantly, the enhancement effect exhibits a significant increase with the increase in the number of years of grazing. Existing cover crop methods are mainly divided into the use of spontaneous cover crops or sown cover crops. The cover crop species utilized are mainly in the Poaceae and Fabaceae families [13], such as ryegrass (Lolium perenne L.), alfalfa (Medicago sativa L.), and clover (Trifolium repens L.). Alfalfa is a perennial plant belonging to the Fabaceae family. It is commonly cultivated in the rural regions of northern China and serves as a significant source of animal feed [14]. Additionally, its remarkable tolerance to drought, salinity, and alkalinity allows for it to thrive in harsh environments [15]. Currently, it is widely employed to enhance degraded sandy soils and mitigate soil salinization [16,17].

Biochar (produced from peanut shells), a solid compound produced by the pyrolysis of biological residues at high temperatures in a partially or completely anoxic environment, has been widely applied to soils [18]. Studies have shown that biochar (produced from maize) application significantly improves degraded soils, enhances soil quality, and increases crop yields [19]. Biochar amendment in saline soils can increase soil porosity and reduce the soil bulk weight, improving the water retention capacity of the soil. In addition, wood biochar can significantly enhance the organic carbon content and other nutrients in saline soils [20,21]. In addition, biochar (produced from maize) can effectively alleviate the negative impacts of sandy land on plant growth [22,23]. Biochar produced from cacao shell plays a role in soil improvement for a long time due to its high stability [24], and some studies have concluded that the effect of biochar (produced from willow stem wood) on the soil structure improvement of sandy soils with relatively poor structures is better than that in other types of soils [25,26].

Existing studies primarily focus on cover crops and biochar addition on the effects of soil health. For instance, Blanco-Canqui et al. (2024) investigated the effects of combining biochar with cover crops on improving soil health in sloping and semi-arid soils [27]. Arif et al. (2021) investigated the use of biochar and leguminous cover crops as an alternative to summer fallowing for managing soil organic carbon and nutrients in a wheat–maize–wheat cropping system under semi-arid climate conditions over 75 days [28]. When these two approaches are combined for soil amelioration, a potentially complementary mechanism may emerge. For example, biochar could enhance the retention of nutrients derived from alfalfa’s nitrogen fixation in the soil, thereby promoting nutrient utilization by plants, including the cover crops themselves. However, studies addressing enhancements in soil fertility through the application of biochar and cover crops in jujube orchards and sandy soil ecosystems remain unclear. This study focuses on the application of biochar and alfalfa in jujube orchards, exhibiting distinctive plant–soil interactions.

Based on the jujube planting base in Yanchuan County, this study investigates the effects of planting alfalfa on the physicochemical properties of sandy soils and the influence of biochar addition on the nutrient contents of sandy soils and the growth of alfalfa to provide a scientific basis for the improvement of sandy soils in the future. The hypotheses of this study are as follows: (1) Although planting alfalfa in sandy soils is beneficial in the long term, it may reduce soil nitrogen and phosphorus levels in the short term. (2) Biochar addition can increase soil nutrient storage, promoting the growth of alfalfa.

2. Materials and Methods

2.1. Overview of the Experimental Area

The experiment was conducted at a jujube orchard located in Fuyihe Village, Yanchuan County, Yan’an City, Shaanxi Province (with a latitude of 36°43′48″ N, a longitude of 110°26′24″ E, and an altitude of 535 m). According to the Yan’an Meteorological Station, this area is located in the hilly and gully terrain of the Loess Plateau, positioned within the transitional zone between a warm–temperate semi-arid climate and a semi-humid climate. Precipitation is concentrated in summer, while winters are cold and dry. The study region is a rain-fed agricultural area, with an average annual temperature of 10.6 °C, an average annual sunshine duration of 2500–2800 h, and an annual precipitation of less than 500 mm. The soil layer has a thickness of 80 cm, with a sandy texture. In the soil layer ranging from 0 to 60 cm, the pH level measured 7.8, while the content of soil organic matter (SOM) stood at 1.80 g·kg⁻¹, the nitrogen total (Nt) concentration was found to be 0.92 g·kg⁻¹, and the phosphorus total (Pt) content was 0.57 g·kg⁻¹. The ammonium nitrogen (NH₄⁺-N) content was 112.85 mg·kg⁻¹, the nitrate nitrogen (NO₃⁻-N) content was 6.42 mg·kg⁻¹, and the available phosphorus (AP) content was 47.55 mg·kg⁻¹. Additionally, the available potassium (AK) content was measured at 140 mg·kg⁻¹, while the available calcium content was recorded at 15.1 mg·kg⁻¹. The specific determination method is shown in Section 2.4. Indicator Determination Method. Overall, the study site is characterized as a high-nitrogen environment.

2.2. Experimental Design

The experiment utilized first-generation imported alfalfa as the original seed variety. The main raw material of the biochar (procured by Tano New Materials Co., Ltd., sourced from Pingdingshan, China) used in the experiment was maize straw. The basic physical and chemical properties of the biochar are as follows: the pH level was measured at 9.4, the SOM content was 598.85 g·kg⁻¹, the Nt concentration was found to be 19.52 g·kg⁻¹, and the Pt content was 8.38 g·kg⁻¹. Additionally, the NH₄⁺-N content was 314.25 mg·kg⁻¹, the NO₃⁻-N content was 13.85 mg·kg⁻¹, and the AP content was 814.71 mg·kg⁻¹. Individual experimental plots measured 50 m² (8 m length × 6.25 m width). Thirty-year-old jujube (Ziziphus jujuba Mill.) trees were arranged in a 3 × 3 matrix configuration within each plot, with a planting density of 2.0 m × 2.0 m (inter-row and intra-row spacing). Four treatments were implemented: CK (Clean Tillage Control, no cover crop or biochar), B1 (Alfalfa cover crop only), B2 (alfalfa cover crop combined with biochar application at a rate of 1.5 kg·m⁻²), and B3 (alfalfa cover crop combined with biochar application at a rate of 3 kg·m⁻²). All treatments received consistent care, except for the variables involved. A uniform basal fertilizer application was made to all plots prior to sowing: nitrogen fertilizer at 0.0634 kg·m⁻² (supplied as urea (CH₄N₂O), 0.1378 kg·m⁻²), phosphorus fertilizer at 0.02 kg·m⁻² (supplied as calcium superphosphate (Ca(H₂PO₄)₂), 0.1 kg·m⁻²), and potassium fertilizer at 0.02 kg·m⁻² (supplied as potassium dihydrogen phosphate (KH₂PO₄), 0.059 kg·m⁻²). Biochar and basal fertilizers were incorporated into the 0–20 cm soil layer using a rotary tiller (model 1WGQ-70, manufactured by Jining Quancheng Machinery Equipment Co., Ltd., sourced from Jining, China) to ensure thorough homogenization with the soil matrix before alfalfa sowing. Alfalfa was manually sown across the entire plot area in May 2023 at a seeding rate of 22.5 kg·hm⁻². No supplemental fertilization was applied after sowing. Pest, disease, and irrigation management in the orchard followed standard local practices. All weeds within the sampling areas were completely removed. Each treatment was replicated three times, resulting in a total of twelve experimental plots encompassing a combined area of 600 m².

2.3. Sample Collection

One year after sowing, one jujube tree was randomly selected from each plot, and one plant sample along with six soil samples were collected, with each treatment including three replicate plots. A five-point sampling method was employed using the selected jujube tree as the center. At a distance of 0.5 m in the four cardinal directions (east, south, west, and north), a 50 × 50 cm sampling frame was utilized to delineate the area. Within this frame, the number of alfalfa plants and their heights were recorded. Subsequently, scissors were used to collect the portions of alfalfa above 5 cm from the ground, which were then cut into pieces and placed into a sample bag. After completing the collection from the four sampling frames, the bag was shaken well and was subsequently placed into a sealed bag. Following the collection of the plant material, a soil drill was utilized to gather soil samples from the 0–60 cm soil layer at the location for collecting the plant sample. The soil was stratified at 10 cm intervals, and the soil from each layer was placed into a tray. After initially removing plant roots and stones with tweezers, the samples were transferred to a dry container, and the collection of the next sampling point was conducted. Once the collection of the four sampling points from the same soil layer was completed, the samples in the dry container were thoroughly mixed to ensure uniform blending of the soil samples from the same layer. Additionally, this experiment does not involve secondary sampling. Finally, the mixed soil samples were stored in sealed plastic bags and air-dried naturally in the laboratory. The air-dried soil was passed through a 100-mesh sieve, and the remaining plant residues and stones were further separated.

2.4. Indicator Determination Method

For the determination of soil moisture content (SMC, %), aluminum boxes containing soil were weighed and oven-dried at 105 °C until they were a constant weight, and the water content was calculated by measuring the mass of the dried aluminum boxes and soil samples as follows:

$$SMC=(A-B)/(B-C)$$

(1)

In the formula, A (g) signifies the total weight prior to drying, B (g) denotes the total weight following drying, and C (g) represents the weight of the aluminum container itself. The formula used to calculate soil water status (SWS, g·cm⁻²) is outlined below:

$$SWS=SMC\times BD\times Sd$$

(2)

In this study, the soil bulk density (BD, g·cm⁻³) was determined to be 1.34, while soil depth (Sd, cm) denotes the soil layer depth, such as 10 cm for the 0–10 cm layer.

Soil organic matter (SOM, g·kg⁻¹), nitrogen total (Nt, g·kg⁻¹), alfalfa nitrogen content (ANC, mg·plant⁻¹), and soil phosphorus total (Pt, g·kg⁻¹) were determined using the dichromate oxidation method, the Kjeldahl method, and the colorimetric method (digestion with sulfuric acid and perchloric acid) [29,30,31], respectively. Soil ammonium nitrogen (NH₄⁺-N, mg·kg⁻¹) and nitrate nitrogen (NO₃⁻-N, mg·kg⁻¹) were quantified using an AA3 continuous flow analyzer (Germany) following extraction with potassium chloride. Soil available phosphorus (AP, mg·kg⁻¹) was determined via the Olsen 0.5M sodium bicarbonate method [32]. Soil pH was measured using a PHS-3C pH meter (manufactured by Shanghai Yidian Scientific Instrument Co., Ltd., sourced from Shanghai, China) at a water-to-soil ratio of 2.5:1 after the formation of a suspension [33]. Soil available nutrients include NO₃⁻-N, NH₄⁺-N, and AP. Alfalfa plant height (Ph, cm) was measured using a tape measure. The plant samples were rinsed with deionized water and were subsequently killed by heating at 105 °C for 0.5 h. Following this, they were dried at 80 °C until a constant weight was achieved. After weighing and recording, the biomass (BM, g), individual plant biomass (IPB, g·plant⁻¹), and plant water content (PWC, %) were determined by dry biomass. Soil stoichiometric ratios, specifically the carbon–nitrogen (C/N) ratios, nitrogen–phosphorus (N/P) ratios, and carbon–phosphorus (C/P) ratios, were calculated using the following formulas:

$$C/N=SOM/Nt$$

(3)

$$N/P=Nt/Pt$$

(4)

$$C/P=SOM/Pt$$

(5)

2.5. Data Analysis

The experimental data were visualized using Excel 2016 to create data charts. Origin 22.0 software was employed to generate graphs illustrating the variations in soil nutrient contents across different soil layers under various treatments. SPSS 26.0 software was utilized for statistical analysis. Data for different treatments were determined by one-way analysis of variance (ANOVA), Duncan’s test was applied for significance analysis, where p < 0.05 was considered statistically significant, and a principal component analysis (PCA) was also conducted. To better evaluate the comprehensive soil improvement effects of different treatments, the comprehensive scores of each treatment need to be calculated [34]. First, SPSS was used to determine the eigenvalues of each principal component and the factor loading matrix. Subsequently, the calculation expressions for the scores of each principal component factor were derived as follows:

$$B={\displaystyle \frac{G}{\sqrt{F}}}$$

(6)

$$D=B_1{Y}_1+B_2{Y}_2+\dots +B_i{Y}_i$$

(7)

In this formula, G is the factor loadings and F is the corresponding eigenvalues of the factor, Y represents the individual indicators for each treatment, D denotes the factor score, and i is the serial number of the indicators involved in the calculation. After obtaining the scores for each principal component, a weighted summation was performed using the variance contribution rates of the principal components as weighting coefficients. This yielded comprehensive scores for different treatment groups, which were employed to reflect the soil improvement effects of each group. Based on the relationships among soil nutrients, plant growth, and the comprehensive scores, partial least squares (PLS) and Mantel test correlation analyses were conducted in R 4.3.1 to investigate the impacts of planting alfalfa and applying biochar on soil fertility in the jujube orchard. The primary R package utilized was PLS-PM [35,36].

3. Results

3.1. Effect of Biochar Addition on Alfalfa Growth

Biochar application had a significant promotional effect on the growth and nutrient uptake of alfalfa. Compared to group B1 (

Table 1. Effects of different treatments on alfalfa growth indexes.

Treatment	Ph (m)	IPB (g·plant−1)	PWC (%)	BM (g·m−2)	ANC (mg·plant−1)
B1	0.52 ± 0.02 c	23 ± 0.95 b	63.58 ± 2.84 a	1361.76 ± 133.34 a	150.48 ± 13.91 b
B2	0.59 ± 0.02 b	21.23 ± 1.19 b	64.58 ± 3.41 a	1441.95 ± 154.34 a	196.9 ± 14.66 a
B3	0.67 ± 0.03 a	26.76 ± 1.82 a	68.37 ± 1.94 a	1602.55 ± 186.02 a	200.79 ± 24.11 a

Note: Alfalfa cover crop only (B1), alfalfa cover crop combined with biochar application at a rate of 1.5 kg·m⁻² (B2), and alfalfa cover crop combined with biochar application at a rate of 3 kg·m⁻² (B3). Plant height (Ph), biomass (BM), individual plant biomass (IPB), plant water content (PWC), and alfalfa nitrogen content (ANC). Different letters indicate significant differences between treatments (p < 0.05).

), the Ph and ANC in group B2 significantly increased by 12.82% and 30.85% (p < 0.05), respectively, while there was no significant change in the IPB. Compared to B1, the Ph, IPB, and ANC in group B3, which had higher biocarbon input, were further enhanced by 28.85%, 16.35%, and 33.43% (p < 0.05), respectively. The Ph and IPB were significantly elevated by 14.20% and 26.02% in group B3 compared to group B2, respectively, and the difference in ANC was not significant (p < 0.05). The PWC and BM did not change significantly in all three groups.

3.2. Effects of Planting Alfalfa and Applying Different Gradients of Biochar on Soil Moisture Content

The impact of different treatments on the SMC is displayed in Figure 1. Under the condition of no carbon addition, planting alfalfa significantly increased the SMC by 27.79% compared to the CK group (p < 0.05), and the enhancement rate of each soil layer ranged from 8.9% to 40.38%. The B1 and CK groups both exhibited a trend of soil moisture decreasing with increases in soil depth. Under the condition of alfalfa planting, the SMC of the B2 and B3 groups with the application of biochar significantly increased by 98.13% and 100.22%, respectively, compared to the B1 group (p < 0.05). The soil moisture contents showed a tendency of decreasing and then increasing with soil depth, and the water contents of the deeper soil (40–60 cm) in the B2 and B3 groups with the addition of biochar significantly increased by 217.01% and 255.98%, respectively, compared to the B1 group (p < 0.05). In terms of overall soil moisture, there was no significant change between the B2 and B3 groups. Overall, the addition of biochar has a significant effect on the deep soil.

3.3. Effect of Planting Alfalfa with Different Gradients of Biochar on Soil Nutrient Status

Analyzing the soil nutrient components (Figure 2) revealed that planting alfalfa increased the SOM by 17.65% and decreased the Nt, Pt, NO₃⁻-N, and NH₄⁺-N contents by 33.05%, 6.66%, 17.34%, and 15.81% (p < 0.05), respectively, under the condition of no biochar application. After planting alfalfa, the SOM, Nt, Pt, NO₃⁻-N, NH₄⁺-N, and AP contents of the biochar-amended B2 group rose by 17.40%, 26.11%, 21.95%, 41.57%, 8.11%, and 28.91% compared to the B1 group (p < 0.05), respectively, while those of the biochar-amended B3 group were elevated by 37.70%, 24.64%, 15.04%, 14.39%, 25.22%, and 11.84% compared to the B1 group (p < 0.05), respectively. In terms of the variation in soil stoichiometric ratios (Figure 3), the CK group had lower C/P ratios and C/N ratios and higher N/P ratios. The cultivation of alfalfa resulted in a reduction in N and P nutrient contents in the soil; however, this condition improved following the addition of biochar.

Analyzing the variations in the distribution of soil nutrients (Figure 2) indicated that the SOM across all groups exhibited a declining trend as soil depth increased, while the content of every soil layer was greatest in group B3, with group B2 the next highest, and group B1 exhibiting higher levels than those found in the CK group. In terms of Nt, the CK group maintained high Nt content at 0–30 cm, then decreased sharply and reached the lowest point at 40–50 cm. The B1 group showed the lowest nitrogen storage capacity throughout the entire profile, and although there was a small rebound at 20–30 cm, it was not significantly different from that of the following soil layer. The Nt content of the B2 group showed a tendency to initially increase and then decrease with depth and was significantly higher compared to the other groups in the 20–40 cm soil layer (p < 0.05). Even in the deepest layer of 50–60 cm, the B2 group maintained a relatively high Nt content, which was significantly higher than in most of the treatment groups (p < 0.05). The Nt content of the B3 group gradually declined with depth, but the overall change was small, and it was significantly lower than that of the B2 and CK groups in the 20–40 cm layer, while at other depths, it was intermediate between CK and B2, with significant differences observed in both groups. (p < 0.05). The Pt content in each soil layer was the highest in group B3, followed by group B2, and was lower in group B1 than in the CK group. The Pt content of the soil showed an overall trend of decreasing with depth.

The NO₃⁻-N and AP levels in various soil layers across each group demonstrated a similar decline as depth increased (Figure 2), with the concentrations of NO₃⁻-N and AP in the B2 and B3 groups being substantially greater than those found in the B1 group within the upper soil layer (0–20 cm) (p < 0.05). The cultivation of alfalfa resulted in a more pronounced variation in the decline of soil NH₄⁺-N with soil depth in the CK group. Compared to group B1, the NH₄⁺-N content of each soil layer significantly increased with the addition of biochar, except for in the 0–10 cm soil layer, where the increase in NH₄⁺-N content at the same soil depth ranged from 8.77% to 38.64% (p < 0.05). With the variation in soil stoichiometry, as the depth of the soil layer increases, the C/N ratios in treatments B2 and B3 exhibit an overall downward trend, whereas treatments CK and B1 demonstrate an upward trend. Conversely, the N/P ratios display the opposite pattern. Additionally, the carbon-to-phosphorus ratio in each treatment group shows a decreasing trend with increasing soil depth (Figure 3).

3.4. Principal Component Analysis, Mantel Test, and Partial Least Squares of Plant Growth and Soil Nutrients

In the PCA of the effect of planting alfalfa on the physicochemical properties of sandy soil (Figure 4a), the variance contribution rates of PC1 and PC2 affecting the comprehensive evaluation results were 68.61% and 17.53%, respectively, and the cumulative variance contribution rate was 86.14%, which reflected the importance of each indicator and their interrelationship. Based on the results of the principal component factor scores (

Table 2. Principal component composite scores of soil fertility of planted alfalfa with no biochar addition.

Treatment	PC1	PC2	Comprehensive Score	Comprehensive Rank
CK	2.63	0.67	2.23	1
B1	2.19	0.56	1.86	2

), the crop of alfalfa is associated with a reduction in soil nutrient content.

To better evaluate the soil fertility improvement of the jujube orchard with the addition of different gradients of biochar under alfalfa planting, 12 indicators of soil carbon, nitrogen, and phosphorus content and alfalfa growth were selected for PCA (Figure 4b). The results show that the variance contributions of the PC1 and PC2 affecting the comprehensive evaluation results were 55.52% and 17.60%, respectively, and that the cumulative variance contribution rate of the two principal components that played a major role in the improvement of sandy soil reached 73.12%, and the main information on soil fertility and the growth of plant aboveground parts was included, which reflected the relative importance of the indices and their interrelationships. The two principal components were designated as the vegetation growth factor and the soil nutrient factor, respectively. Based on the results of the principal component factor scores and the combined scores of the sandy soil improvement level (

Table 3. Principal component composite scores of soil improvement in the B1, B2, and B3 groups under alfalfa planting conditions.

Treatment	PC1	PC2	Comprehensive Score	Comprehensive Rank
B1	1.31	0.61	1.14	3
B2	2.85	3.00	2.89	2
B3	3.82	2.88	3.60	1

), it was concluded that the improvement effect was B3 > B2 > B1.

In the correlation matrix between soil physicochemical properties and plant growth indicators (Figure 5a), the results of the mantel test show that PN was significantly correlated with a wide range of soil nutrient parameters, suggesting that biocarbon addition enhanced the soil nutrient pool and unlocked alfalfa nitrogen fixation from the effects of phosphorus stress (Mantel’s r ≥ 0.25, p < 0.05). PM was significantly correlated with N/P ratios, carbon-to-phosphorus ratio, Nt, and NH₄⁺-N (Mantel’s r ≥ 0.25, p < 0.05). Correlation matrix analysis revealed highly significant synergistic relationships among soil nutrient parameters. Among them, NO₃⁻-N was significantly correlated with AP and Pt, SOM was significantly correlated with NH₄⁺-N, Nt, and Pt, and AP was significantly correlated with NO₃⁻-N, NH₄⁺-N, and Pt (p < 0.05). Significant positive correlations were observed between soil C/N ratios and most of the nutrient indicators, which reflects the key role of carbon and nitrogen balance in regulating soil nutrient cycling. The plant–soil PLS (Figure 5b) demonstrated that biochar addition had a direct path coefficient of 0.326 on soil scores. Biochar addition also indirectly affected soil scores by influencing soil nutrients, with direct and indirect path coefficients of 0.7162 and 0.698, respectively. Additionally, biochar addition indirectly impacted soil scores by promoting plant growth, with an indirect path coefficient of 0.0535.

4. Discussion

4.1. Effects of Biochar Application on Alfalfa Growth

The aboveground part of plants is a key indicator of the growth conditions of plants, with the plant height, aboveground biomass, and other supporting indicators being closely related to the photosynthetic capacity of the aboveground parts of the plant [37]. The application of biochar in this study increased the Ph and ANC. Consistent with the findings of the present study, it has been shown that biochar promotes plant growth by improving the physiological parameters of plant leaves, which subsequently promotes plant growth and significantly enhances the aboveground nitrogen content of alfalfa [38]. In this study, while the Ph of group B2 increased compared to that of group B1, no significant changes were observed in BM and PWC. One possible explanation for this phenomenon is that alfalfa exhibits an upward growth pattern to maximize light acquisition [39]; the experiment was conducted in a jujube orchard. The B3 group, which received a higher carbon input, exhibited a further increase in Ph, with IPB significantly surpassing that of the B1 and B2 groups. We hypothesize that the enhanced biochar input provides adequate nutrients to meet the nutritional requirements of each plant. Additionally, a spatial variation in root distribution between alfalfa and jujube trees was noted, leading to our speculation that the intensity of the competition between the two species was diminished [40].

4.2. Effects of Planting Alfalfa with Different Gradients of Biochar on Soil Water Content

Research indicates that cultivating alfalfa may lower soil bulk density [41], thereby improving the soil’s ability to retain water, which aligns with the present results. In this study, the application of biochar significantly enhanced the SMC. Owing to its lightweight nature, substantial specific surface area, and plentiful oxygen-rich functional groups on the surface, biochar can significantly reduce soil bulk weight while increasing soil porosity and field water-holding capacity [42,43,44,45,46], thus comprehensively enhancing soil water-holding capacity and increasing the water content of each soil layer. In terms of soil moisture distribution, the moisture improvement effect under biochar application was more obvious in deeper soil. This is inconsistent with existing studies that show that biochar mainly acts in the application layer. Several studies have suggested that the incorporation of biochar can enhance biomass accumulation in alfalfa and improve the plant’s capacity for water uptake [27,28]. Alfalfa roots predominantly inhabit the 0–40 cm soil layer, and increased biomass production typically necessitates greater water extraction from this zone [47]. This phenomenon may be the reason for the significant rise in water content observed in the deeper 40–60 cm soil layer, where root density is lower and water extraction by alfalfa is diminished. The present study found no significant difference in SMC between the B2 and B3 groups. A previous study found that applying more than a certain amount of biocarbon (16 t/hm²) [48] did not result in further increases in the crop yield, which was similar to the results of this study. Overall, the addition of biochar significantly increased the SMC and led to elevated soil water retention. In summary, compared to other treatments, we believe that the combined application of moderate amounts of biochar and planting alfalfa is effective to improve soil moisture condition in sandy soils. However, considering that alfalfa planting was only conducted for one year in this study, the improvement effect of combined alfalfa planting and biochar application on the water-holding capacity of sandy soil requires further verification.

4.3. Effects of Planting Alfalfa with Different Gradients of Biochar on Soil Nutrients

Organic matter is crucial for ecosystem cycling, and pasture mulching can increase the number of insects in orchards, while cover crops can be converted into organic matter after natural withering and enhance the SOM content [18], which is consistent with the results of the current study. This work found that the SOM content under alfalfa cover was significantly elevated with the increase in biochar input, while different soil layers exhibited a gradual decrease in SOM content with soil depth, with obvious surface enrichment characteristics. Existing research shows that biochar has a high carbon content and rich pore structure, which is conducive to the fixation and slow release of soil carbon, thus promoting the utilization of carbon sources by plants. In addition, biochar can adsorb low-molecular-weight organic compounds in soil and convert them into organic matter, thereby improving the SOM content [49], which is consistent with the results of the present study. The significant increase in IPB of Group B3 also indirectly confirms this statement. Biochar was applied to the surface layer of soil (0–20 cm), and its nutrient fixation and regulation ability decreased with greater depth, causing the SOM content to exhibit the distribution characteristics of surface layer enrichment.

In this study, alfalfa cultivation resulted in lower soil nitrogen and phosphorus nutrient content. Existing studies have shown that continuous alfalfa cropping reduces soil nitrogen and phosphorus content [11], which is consistent with the present results. This phenomenon can be attributed to the ability of the alfalfa root system to absorb available nutrients from the soil [17], which consequently leads to a decrease in the nitrogen and phosphorus content of the soil. Furthermore, the organic acids secreted by the root system may further accelerate this process [50]. The experimental site, as a long-term fertilizer orchard, exhibited the characteristics of high nitrogen and low phosphorus, with a relatively high overall nitrogen–phosphorus ratio. The lack of effective phosphorus became a limiting factor for alfalfa growth and nitrogen fixation [51], resulting in the alfalfa being more inclined to utilize soil nitrogen to obtain the nitrogen required for growth.

Adding biochar under alfalfa planting conditions improved the soil’s nitrogen and phosphorus content and increased the soil’s available nutrients. There are two reasons for this finding: first, biochar itself is rich in nitrogen, which can be used as a nitrogen source to directly increase the nitrogen content in soil in the short term; and adding biochar can improve the fixation of soil organic nitrogen to NH₄⁺-N, increase the soil NH₄⁺-N content, and promote the mineralization of stable organic nitrogen, thus accelerating the transformation of soil nitrogen and enhancing its biological effectiveness [52]. Some studies have reported that, after adding biochar, the mineralization rate of total soil nitrogen can be increased by about 1.9–2.2 times [53]. Second, biochar application adds a large amount of plant-absorbable phosphorus, with AP content that is 151.3 times higher than that found in sandy soils [52]. Biochar can affect the content of soil phosphorus through promoting the interactions between cations and organic matter in the soil [54], effectively mitigating the effects of phosphorus limitation. Increased phosphorus effectiveness can promote the cascade response of plant nitrogen uptake and nitrogen mineralization [55], ultimately leading to elevated soil NO₃⁻-N levels [56,57], which is consistent with the results of the present study. However, it has also been suggested that a high aromatic carbon content in biochar could potentially reduce soil phosphatase activity [58]. This proposed mechanism involves the porous structure of biochar, which adsorbs soil enzymes and their substrates, potentially leading to decreased enzyme activity [59]. In the present study, the B3 group (with a higher biochar addition) exhibited reduced AP content compared to the B2 group, a pattern consistent with the proposed inhibitory effect described in the literature.

5. Conclusions

In this study, planting alfalfa increased the SOM content and decreased the nitrogen and phosphorus nutrient content of sandy soil. The status of all nutrients was improved after the application of biochar, which increased the soil available nutrients to promote alfalfa growth, and the combined effect of alfalfa planting and biochar application was greater than that of planting alfalfa alone. In terms of the impact on soil nutrients, the treatments with 1.5 kg·m⁻² and 3 kg·m⁻² biochar addition exhibited negligible differences in their effects. Moreover, when considering their combined influences on both soil and plants, the improvement score was highest in group B3, which combined alfalfa and 3 kg·m⁻² of biochar. These findings suggest that the combined application of alfalfa and biochar is advisable for high-nitrogen regions. However, as this experiment was conducted over a duration of only one year, the long-term effects in the jujube orchard habitat require further investigation. In addition, a larger-scale application may face soil nutrient limitations due to the high nitrogen environment in this experiment and the varying climatic conditions across different regions.