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Article

Influence of Composite Amendments on the Characteristics of Sandy Soil

by
Xinrui Sui
1,
Lingyan Wang
1,
Xinyao Lv
1,
Yanan Liu
1,
Yuqi Zhu
1,
Lingyun Fan
1 and
Hanxi Wang
1,2,*
1
Heilongjiang Province Key Laboratory of Geographical Environment Monitoring and Spatial Information Service in Cold Regionsyin, School of Geographical Sciences, Harbin Normal University, Harbin 150025, China
2
Heilongjiang Province Collaborative Innovation Center of Cold Region Ecological Safety, Harbin 150025, China
*
Author to whom correspondence should be addressed.
Sustainability 2025, 17(17), 7619; https://doi.org/10.3390/su17177619
Submission received: 5 July 2025 / Revised: 2 August 2025 / Accepted: 20 August 2025 / Published: 23 August 2025
(This article belongs to the Section Soil Conservation and Sustainability)
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Versions Notes

Abstract

Soil desertification control is a global challenge, and the barrenness of sandy soil limits the growth of plants. To enhance the vegetation growth capacity of sandy soils, the preparation of soil amendments and the experiment of improving desertified soil were conducted. The soil amendment is prepared by mixing polyacrylamide (2.7%), biochar (16.2%), sodium bentonite (16.2%), straw fibers (5.4%), corn straw (2.7%), sheep manure organic fertilizer (54.1%), and composite microbial agents (2.7%). The laboratory experiment was conducted to investigate the effects of varying rates (0, 1.5%, 3%, 4.5%, 6%) of composite soil amendments on the properties of sandy soil and the Lolium perenne L. with a growth period of 30–60 days. The results indicated that the application of composite amendments at different rates maintained the soil pH between 7.0 and 7.5, increased the electrical conductivity, and significantly improved the soil moisture content, soil organic carbon (SOC), total nitrogen (TN), and total phosphorus contents. Under the condition of 3% amendment, the soil TN content increased from 0.74 to 1.83 g·kg−1. The composite amendments remarkably promoted L. perenne growth, as evidenced by increased plant height, dry weight, and nitrogen and phosphorus nutrient content, while the SOC content increased by 1–4 times. The application of composite amendments, prepared by mixing materials such as biochar, organic fertilizer, crop straw, microbial agents, bentonite, and water-retaining agents, enhanced the physicochemical properties of sandy soil and promoted L. perenne growth, and 3% was the most suitable application rate. These findings are expected to advance desertification-controlling technologies and enhance soil carbon sequestration capacity.

1. Introduction

Soil is formed through the combined influence of biotic factors, climate, parent material, topography, and time [1], which results in distinct properties. Soil texture is one of the key factors influencing soil quality and productivity [2]. Sandy soils are characterized by low mineral and organic carbon (OC) content, poor fertility, a scarcity of stable macroaggregates, and limited water retention capacity [3], which negatively impacts vegetation growth. China is one of the countries most seriously affected by desertification, with desertified land covering an area of 173.97 × 104 km2, which accounts for 18.1% of its total land area [4]. At present, the traditional measures of soil improvement in desertified areas primarily focus on sand fixation, which includes mechanical, chemical, and biological methods. These approaches aim to reduce surface wind speed [5], use chemical agents to promote sand particle aggregation [6], and establish vegetation through afforestation and grass planting [7] to stabilize the shifting sand and improve the ecological stability of dune ecosystems. However, these methods fail to address the underlying issue of low soil fertility and the slow and inefficient conversion of nutrients. Population growth has led to an increase in the demand for food, and sandy land used for agricultural fields will become an important source of future food security and economic growth. Improving the quality of sandy soil has become a key focus of current and future research. The use of soil amendments based on biological materials has gained widespread application due to their low cost and effectiveness in improving soil structure and enhancing soil carbon sequestration and fertility.
Unconventional materials have varying effects on soil health indicators, which include physical, chemical, and biological properties. The combination of biochar and aluminum sulfate significantly enhances the saturated water-holding capacity, field capacity, water storage capacity, and saturated hydraulic conductivity of soil in semi-arid grassland regions by 25.74%, 35.45%, 18.47%, and 25.80%, respectively [8]. This leads to notable improvements in both soil hydrological functions and fertility. The mixture of polyacrylamide and sodium polyacrylate has been shown to effectively address the issues of soil particle aggregation, which increases soil porosity and water retention while mitigating soil erosion and desertification [9]. Investigation into the effects of biowaste and biochar on saline soil improvement revealed that soil pH and electrical conductivity (EC) decreased at lower application rates. Higher application rates led to an increase in soil moisture content (SMC) and organic carbon (OC) while reducing the cation exchange capacity, which exerts a positive impact on soil quality [10]. Application of sheep manure and artemisia ordosica biochar enhances the utilization rate of phosphorus, potassium, nitrogen, and microbial biomass carbon in sandy soil, and the composite formulation exhibited performance enhancements of 7.1–23.1%, 4.1–10.9%, 6.2–11.8%, and 12.5–22.6% across key metrics relative to the individual components [11]. The co-application of farmyard manure and bentonite increases both macro- and micronutrient contents in sandy soil, while also improving soil water retention from 70 ± 0.2 to 290 ± 0.3 mm·m−1 [12]. A pot experiment using biochar, bacillus spp., and NPK fertilizer demonstrated that biochar-based microbial fertilizer not only increased total nitrogen (TN) and soil organic matter (SOM) content in the soil but also enhanced the abundance of bacteria associated with carbon and nitrogen metabolism [13].
Previous studies on soil amendments have primarily emphasized single-function enhancements [14]. So, it is necessary to develop a comprehensive organic-inorganic-biological co-improvement system aimed at improving soil carbon sequestration, water and nutrient retention, nutrient supply, soil structure stability, and microbial degradation promotion. Using plant straw to supplement SOM is an effective method to reduce costs. The improvement of sandy soil requires addressing multiple issues simultaneously, including water retention capacity, soil fertility, and microbial community stability. Developing composite soil amendments with long-term stability, multifunctionality, and environmental sustainability is of great significance for the restoration of sandy soils.
The core mechanism of sandy desert soil improvement centers on optimizing soil physical structure, regulating chemical properties, and enhancing biological activity [15]. Soil amendments can fill the macropores between sand particles, promoting the formation of micro-aggregates. This process reduces saturated hydraulic conductivity while improving field water-holding capacity [16]. Bentonite has various mineral elements and good adhesive properties, which can enhance the availability of beneficial nutrients in the soil and promote plant nutrient uptake. It has strong cohesiveness and water absorption capacity, and its long-term application can reduce soil bulk density and increase the stabilization of soil organic carbon (SOC) [17]. Biochar is a carbon-rich product with a large specific surface area (SSA) and rich microporous structure. When added to sandy soils, biochar enhances soil porosity and improves the retention of nutrients such as nitrogen and phosphorus [18]. Plant fiber materials (e.g., straw) are natural biomaterials that can form a dense network structure within the soil, which reduces water evaporation, decreases water flow velocity, limits sand transport, and mitigates soil erosion [19]. Crop straw is composed of tightly bound cellulose, hemicellulose, and lignin, which enhances SOC content and promotes nutrient cycling and root growth when incorporated into the soil [20]. Organic fertilizers are cost-effective alternatives to chemical fertilizers in sustainable agriculture. Long-term use of sheep manure organic fertilizer increases ammonium nitrogen levels while decreasing nitrate nitrogen, thus benefiting crop growth [21]. Compared to other types of animal manure, such as pig and cow manure, sheep manure has a lower salt content. After proper decomposition, it poses a lower risk of soil salinization and alkalization, which makes it more suitable for widespread application across different soil types. Polyacrylamide is commonly used as a water-retaining agent, can reduce the saturated hydraulic conductivity of soils, enhance their water retention capacity, and minimize moisture evaporation, which improves soil water retention [22]. Prolonged application of polyacrylamide can increase the content of large soil aggregates (>0.25 mm), thus improving soil structural stability [23]. However, in arid sandy soils with low microbial activity, polyacrylamide degrades slowly, and long-term use may pose a threat to the environment. Therefore, the application of the formulation is subject to regulatory standards [24]. The increase in microbial species can effectively promote nutrient cycling in soil [25]. Composite microbial agents, which are cultured through fermentation of microorganisms such as bacteria and fungi, can rapidly proliferate in the soil and address issues like soil compaction and monoculture. The application of microbial agents to experimental fields has been shown to reduce soluble salts and pH levels while increasing the number of active microorganisms in the soil [26]. Lolium perenne has a relatively fast growth rate and a well-developed root system, requires relatively low water content during the growth process, and exhibits a certain tolerance to nutrient-poor sandy soil [27,28]. In addition, L. perenne has the characteristic of high economic value. So, L. perenne is a choice for the restoration of sandy soil.
To further improve the growth capacity of sandy soil plants, composite soil amendments were prepared using corn straw biochar, sodium bentonite, composted sheep manure, anionic polyacrylamide, composite microbial agents, rice straw, and corn stover. Focusing on sandy soils from the Hulunbuir region of Inner Mongolia, the study conducted pot experiments with varying application rates across two growing cycles of Lolium perenne planting. The objectives of the study are to (1) investigate the mechanism by which composite amendments affect the physicochemical properties of sandy soils, (2) examine the influence of these amendments on the growth of L. perenne in sandy soils, and (3) evaluate the long-term application effect of composite soil amendments and further optimize the composite soil amendments.
The composite amendment utilized in this study combined multiple functional components to achieve synergistic benefits: straw and sheep manure provided rapid nutrient release, biochar and bentonite ensured sustained fertility, polyacrylamide enhanced soil structural stability and water holding capacity, and microbial inoculants facilitated continuous regulation of the soil microecological environment. This research provides scientific insights and technical guidance for improving soil quality and promoting vegetation restoration in sandy soil regions.

2. Materials and Methods

2.1. Study Site

The sandy soil was collected from the Hulunbeier Sandy Land on the Hulunbeier Plateau in the northeastern part of Inner Mongolia (117°52′48″ E, 48°3′36″ N; https://www.resdc.cn/data.aspx?DATAID=260, accessed on 7 November 2024) (Figure 1a).
The climate of this region was a semi-humid, semi-arid transitional zone with an annual average precipitation ranging from 280 to 400 mm and a temperature range of 0 to 2.5 °C. The effective accumulated temperature was between 1800 and 2200 °C, and the annual sunshine duration was between 2900 and 3200 h, with a frost-free period of 90 to 100 days. The soil in this area was sandy with a high content of medium to fine sand. The basic characteristics of the tested sand soil were determined before the experiment, as shown in Table 1.

2.2. Pot Experiment Design

The study employed a completely randomized design and conducted pot experiments under indoor-simulated desert conditions to examine the effects of composite soil amendments on sandy soil properties and plant growth. Soil samples were collected from sandy soil (0–20 cm) in the Hulunbuir desert region of Inner Mongolia. After removing large stones and non-soil materials, the soil was air-dried and sieved (2 mm mesh) for subsequent experiments. The temperature during the laboratory experiment was 20–30 °C.
The effectiveness of conventional soil amendments on sandy soil has certain limitations. Excessive application of polyacrylamide or bentonite led to soil compaction and reduced microbial activity [11]. Overuse of corn straw or rice straw fibers increased the C/N ratio, which induced short-term nitrogen competition with crops [29]. Excessive biochar applications elevated soil pH [30]. Therefore, the formulation of additives needs to consider the physical and chemical properties of raw materials, economic feasibility, and ecological safety. The proportion of this additive has been optimized based on existing research results. To adjust and optimize the addition rate, five experimental groups were set up in this study, with 3 kg of sandy soil mixed evenly with composite soil amendments at the following application rates: 0 (bare soil control group, CK), 1.5%, 3%, 4.5%, and 6%. The mixtures were placed into pots (inner diameter 21.5 cm, height 21 cm) and labeled as CK, CM1, CM2, CM3, and CM4. Under the same conditions, the same materials and sandy soil were used, and seeds of similar size and plump particles were selected to conduct repeated experiments in pots of the same specification. Each experiment had 3 replicates.
L. perenne possessed a well-developed root system [31], which enhanced the stability of sandy soils. L. perenne had strong growth vigor, enabling rapid ground cover, which reduced SMC evaporation and improved water retention capacity, and demonstrated notable drought resistance. Overall, L. perenne showed considerable promise in desertification control and became an ideal species for desertification prevention and ecological restoration. The experiment was conducted using imported L. perenne seeds. The seeds were surface sterilized by soaking in a 10% H2O2 solution, followed by three rinses with ultrapure water. After sterilization, the seeds were placed in a growth chamber set to 30 °C and a humidity of 70% for germination, which ensured uniformity and minimized experimental error caused by seed quality. Once the seeds germinated, they were transplanted into pots for the pot experiment (Figure 1b). The first planting of L. perenne after the application of the amendment is considered the first cycle. A total of 20 plants were grown per pot, with three replicates per group, for a total of 60 plants. The second planting after harvesting the L. perenne in the first cycle constitutes the second cycle. Based on the plant material usage in the first cycle, the number of plants per pot increased to 50. At the start of each experimental round, soil was sprayed with water to achieve an approximate SMC of 30%, which can be measured using an SMC EC temperature tester. No further watering was applied throughout the experimental period. The experimental duration was 70 days, with plant height measured and recorded every 5 days. At the end of each cycle, the L. perenne plants were harvested from the soil for subsequent measurements of various indicators.

2.3. Preparation of Composite Amendments

According to previous soil improvement studies, the optimal application ratio range of various ameliorant materials per kilogram of dry soil under the optimal improvement conditions can be obtained [32,33]. The raw material ratio of the composite ameliorant was optimized based on these findings to determine the mixing proportions among different materials (Table 2). Based on the designed experimental concentration gradient, the amount of sandy soil, and the number of pot experiments, the required amount of composite soil ameliorant was calculated to be 1.35 kg. The ameliorant was then prepared by uniformly mixing polyacrylamide, corn straw, composite microbial agents, straw fiber, biochar, sodium bentonite, and sheep manure organic fertilizer.

2.4. Soil and Plant Property Analysis

Soil samples were collected from the 0–15 cm cultivation layer of pot experiments, thoroughly homogenized, and then sieved and air-dried prior to analysis. SMC was determined by the drying method, where samples were dried at 105 °C until a constant weight was achieved. pH and EC were determined by the electrode potential method. TN content in the soil was quantified by the Kjeldahl method, which involved digestion, distillation in a Kjeldahl apparatus, and titration with a standard acid solution. Total phosphorus (TP) was determined by the alkaline fusion-molybdenum-antimony spectrophotometric method. SOC content was measured by the combustion oxidation-titration method. For plant measurements, plant height was directly measured. Plant biomass was determined by oven-drying to constant weight. The TN and TP content in plants were analyzed using the Kjeldahl method and the spectrophotometric method, respectively. Plant samples were digested with strong acid, and the resulting phosphate reacted to form phosphomolybdenum blue. Absorbance was measured at 700 nm using a spectrophotometer to determine phosphorus concentration.

2.5. Statistical Analysis

The data was categorized and organized using Excel 2010. Statistical analyses were conducted with IBM SPSS Statistics 26.0. A one-way analysis of variance (one-way ANOVA) tested by data independence, normal distribution, and homogeneity of variance was employed to assess the effects of different application rates of the composite amendments on the basic physicochemical properties of sandy soil and the growth of L. perenne. Graphical representations were created using Origin 2022 (9.9).

3. Results

3.1. Effect of Composite Amendments on Soil Properties

3.1.1. pH, EC, and SMC of Soil

The initial pH of the sandy soil was 8.24, which indicated alkalinity. After the first planting cycle, the soil pH in the CM1, CM2, CM3, and CM4 experimental groups was significantly lower compared to the CK experimental group (p < 0.05) (Figure 2a). The overall pH ranged between 7.2 and 7.5, with the CM1 experimental group showing a soil pH of 7.24. At the end of the second planting cycle, the pH of the CK experimental group decreased to 6.82, while the other experimental groups showed minimal change compared to the first cycle, and the pH range was between 7.0 and 7.5. Soil pH slightly increased with the increased application of composite amendments. Therefore, the composite amendments stabilized the soil pH to a near-neutral range.
The addition of compost amendments to the sandy soil generally resulted in an increasing trend of soil EC (Figure 2b). During the first planting cycle, as the application rate of the amendments increased from 1.5% to 6%, the soil EC rose from 343 to 1340 μS·cm−1. This indicated a significant effect of the composite amendments on soil EC (p < 0.05). In the second cycle, the soil EC in all experimental groups decreased by nearly threefold compared to the first cycle. With increasing amendment application rates, the soil EC still showed an upward trend.
Compared to the CK experimental group, the application of composite amendments led to significant changes in SMC during both planting cycles (p < 0.05), and the SMC increased with the application rate of the amendments (Figure 2c). The change in SMC showed a rapid decrease (from 30% to below 12% in the first 10 days), which was followed by a slow decrease in moisture content (from the 11th to the 25th day). The SMC remains almost unchanged during the final stage of plant growth. There are significant differences in SMC among different experimental groups. Among all experimental groups, the CM3 and CM4 experimental groups showed the most pronounced effects. In the first planting cycle, the SMC in these two groups was 5.59% and 5.66%, respectively. It increased to 7.95% and 6.87% in the second cycle, with no significant difference between the two groups. In the CM1 and CM2, the SMC in the first cycle was 2.25% and 4.6%, respectively, which represented increases of 134% and 379% compared to the CK experimental group. In the second planting cycle, the SMC in the CM1 and CM2 experimental groups decreased to 1.84% and 4.03%, respectively.

3.1.2. SOC, N, and P Content of Soil

After the experiment, the SOC content of each experimental group was measured, and the analysis revealed that the trends were generally consistent (Figure 3a). In both planting cycles, the CK experimental group exhibited the lowest SOC content, with values of 5.66 and 6.54 g·kg−1, respectively. Under the application of the composite soil amendments, the SOC content increased steadily, and the SOC content in the CM1, CM2, and CM3 experimental groups ranged from 10 to 30 g·kg−1 with significant differences compared to the CK experimental group (p < 0.05). The CM4 experimental group showed the highest OC content, nearly four times higher than that of the CK experimental group.
As the application rate of the composite amendments rises, the TP content in the soil generally shows an upward trend (Figure 3b). In the first planting cycle, the TP content in the CM4 experimental group was 0.63 g·kg−1, which was a 39.2% increase compared to the CK experimental group (0.45 g·kg−1). There were no significant differences in TP content among the other experimental groups, with values of 0.47, 0.52, and 0.54 g·kg−1, respectively. In the second planting cycle, the TP content and its variation trend were like those in the first cycle, with changes ranging from 0.01 to 0.05 g·kg−1 across the experimental groups. The smaller differences suggest that the composite amendments had a consistent effect on the TP content in the soil.
The effect of the composite amendments on the TN in soil is shown in Figure 3c. In the first planting cycle, the TN content in the CK and CM1 experimental groups was 0.12 and 0.37 g·kg−1, respectively, with no significant difference observed. When the application rate increased from 3% to 6%, the TN content rose from 0.74 to 1.58 g·kg−1, which showed a significant increase compared to the CK (p < 0.05). In the second planting cycle, the TN content in the CM2 and CM3 experimental groups increased significantly to 1.84 and 1.93 g·kg−1, respectively. The TN content in the CM4 experimental group decreased compared to the first cycle, and no significant difference was observed among the three experimental groups. Significant differences were still present when compared to the CK experimental group.

3.2. Effect of Composite Amendments on L. perenne Growth

3.2.1. L. perenne Biomass

The dry biomass of L. perenne was recorded at the end of the planting period (Figure 4). During the initial stage of planting, with sufficient moisture provided, the germination rates of L. perenne were similar across all experimental groups. Growth differences emerged in the later stages due to variations in nutrient elements and soil characteristics. In the first planting cycle, the dry biomass of L. perenne in the CK experimental group was 0.14 g. The dry biomass in the CM1, CM2, and CM3 experimental groups showed an increase compared to the CK experimental group, while the CM4 experimental group exhibited inhibited growth with lower dry biomass. No significant differences in dry biomass were observed between the experimental groups, which was possibly due to the lower seed sowing density. In the second planting cycle, the seed density was increased to 50 seeds per pot. At the end of the planting period, the dry biomass of the CK experimental group was 0.38 g, while the CM4 experimental group exhibited the highest dry biomass at 0.82 g, an increase of 115.7% compared to the CK. So, the growth of L. perenne in the CM4 was no longer suppressed in the second round. The dry biomass of black L. perenne in the CM1, CM2, and CM3 experimental groups was 0.63, 0.60, and 0.41 g, respectively, and all were higher than in the CK experimental group.

3.2.2. L. perenne Height

The continuous changes in the plant height of L. perenne during the first and second planting cycles are shown in Figure 5. During both planting cycles, the CK experimental group exhibited wilting symptoms around day 20 with final plant heights of 17.4 and 22.4 cm, respectively. The CM1 and CM2 experimental groups consistently promoted the growth of black L. perenne with final plant heights of 21.38 cm and 21.54 cm in the first cycle and 31.11 cm and 30.41 cm in the second cycle. The CM3 and CM4 experimental groups, which involved higher rates of composite amendments, had varying degrees of inhibitory effects on L. perenne growth during the first cycle. The CM4 experimental group significantly reduced plant height by 33.4% compared to the CK experimental group. During the second cycle, the plant heights in the CM3 and CM4 experimental groups reached 28.12 and 29.29 cm, respectively, which represented increases of 25.5% and 30.8% compared to the CK experimental group at the end of the cycle.

3.2.3. N and P Content of L. perenne

The TN and TP content in L. perenne were determined after harvest (Figure 6). Following the application of the composite soil amendments, the TN content of plants was 0.2–0.45 g·kg−1. In both planting cycles, no significant differences in TN content were observed between the CM2, CM3, and CM4 experimental groups. The TN content in the CM1 experimental group was 0.31 g·kg−1 in the first planting cycle, which was like that of the CK experimental group. The nitrogen content decreased to 0.2 g·kg−1 in the second cycle, which was significantly lower than the CK experimental group (0.32 g·kg−1).
The TP content in L. perenne increased with the application of composite amendments. Significant differences in TP content were observed between all experimental groups and the CK experimental group during both planting cycles (p < 0.05), with the differences being more pronounced in the second cycle. The CK experimental group exhibited a smaller change in TP content. In the CM3 experimental group, the TP content in L. perenne increased from 48.32 g·kg−1 (first cycle) to 85.34 g·kg−1 (second cycle), which represented the highest increase. The CM2 experimental group followed with an increase of 76.95%.

4. Discussion

4.1. Characteristics of Soil pH and EC and SMC

Soil pH is an indicator of soil acidity and alkalinity, which can be adjusted by controlling the chemical reactions between different nutrients to regulate the effectiveness of plant nutrients [34]. The addition of different rates of amendments significantly reduced the soil pH, and long-term application led to a stabilization of the pH at a neutral level. The pH of sandy soil was 8.24, which was alkaline. The pH continuously decreased over two cycles in the CK group; this was likely due to soil acidification under the interference of vegetation, which reduced the alkalinity. Plants selectively absorbed base cations, reduced soil base content, and increased the concentration of exchangeable acidic anions, which subsequently lowered soil pH [35]. Plant roots excrete organic acids during growth that release H+, thereby reducing pH. In this research, the soil pH significantly decreased in the first cycle. This was due to the decomposition of organic materials, including corn straw, organic fertilizers, and biochar, which reduced the concentration of alkaline cations and increased the concentration of H+ [36]. Organic acids produced by microbial metabolism and the acidic organic colloids in organic fertilizers neutralized alkaline substances in the soil, which further lowered soil pH. The minerals in bentonite released H+ or promoted the exchange of other cations during hydrolysis, which led to a pH decrease. In the second experimental cycle, pH in the amendment groups slightly increased but stabilized, and this was likely because of the buffering effect of soil microorganisms on the organic acids over time. Some organic acids were further degraded or complexed with metal ions, which reduced their direct impact on pH. The ash in biochar contained exchangeable alkaline cations and carbonates, which exhibited strong alkalinity and can neutralize acidity, thereby reducing the extent of pH decrease [37].
Soil EC reflected the soluble salt content in the soil, which affected plant growth. The increase in EC was primarily associated with factors such as salt accumulation, ion migration, and SMC dynamics. In this study, the rate of amendments was significantly positively correlated with EC. The amendments contained soluble salts rapidly dissolving under the influence of SMC, which increased the ion concentration in the soil solution, thus enhancing the availability of salts and leading to an increase in EC [38]. This study simulated a desert-like arid environment, where the transpiration of L. perenne roots and accelerated evaporation due to drought conditions led to soil solution concentration and an increase in ion concentration, which further raised the EC. Meanwhile, leaching facilitates the downward migration of base ions with percolating water, leading to the depletion of exchangeable base cations in the upper planting layer. Concurrently, the reduction in total ionic charge within the soil solution results in decreased EC [39]. The EC declined in response to changes in the total ion charge of the solution. Plant nutrient absorption, soil adsorption, and SOC decomposition also contributed to the reduction in free ion activity, which led to a decrease in EC.
The determination of SMC was crucial for understanding the water consumption needs of vegetation and provided valuable guidance for agricultural production. The increase in SMC relied on water replenishment, improvements in soil structure, and the addition of moisture-retaining substances. In this experiment, the amendments containing bentonite and polyacrylamide significantly enhanced soil water retention. Bentonite was a non-metallic mineral composed mainly of montmorillonite, which had a high clay content and strong hygroscopicity. When bentonite was applied to soil, it rapidly absorbed water and released water slowly, which enhanced the soil’s water retention and storage capacity [40]. A field experiment conducted in the sandy soil region of the semi-arid area in northern China investigated the effects of bentonite amendment rates on SMC, and bentonite amendments increased the SMC and water retention [41]. Polyacrylamide, which possesses strong water absorption and cohesion properties, has a notable effect on SMC retention [42]. As the application rate of polyacrylamide increased, its impact on the viscosity of percolating water in sandy soil and its ability to impede water infiltration became more pronounced, which improved SMC [43]. An increase in SOC promoted improvements in soil physical properties, thereby further influencing SMC. Biochar with a large surface area and high porosity obviously altered the soil’s water infiltration patterns, retention time, and flow paths. The application of biochar to sandy soil reduced its hydraulic conductivity by 11 times compared to untreated soil [44], which demonstrated its potential to enhance the water retention capacity of sandy soils.
Both soil pH and EC were jointly reflected in the degree of soil salinization and alkalization. The decrease in EC during the second cycle helped reduce the osmotic stress caused by high salt content, which facilitated normal water uptake and nutrient absorption by crops. Lowering EC contributed to the restoration of aggregate structure, which improved soil permeability and aeration, reduced the risk of secondary salinization, and promoted soil health. The proportion and quantity of each component in the composite amendments should be carefully considered during practical applications to avoid excessive salt content in the soil, which may interfere with plant growth. Selecting salt-tolerant plant species can help mitigate the detrimental effects of salinity and alkalinity on both soil and vegetation. In the two planting cycles, the SMC increased after the application of composite amendment to the sandy soil, which was administered to enhance the cohesion between soil particles, improve the stability of aggregates, improve soil aeration and water retention, reduce soil compaction, and minimize temperature fluctuations. These changes promoted microbial activity [45] and plant growth. The increase in moisture facilitated the dissolution and diffusion of nutrients (N, P, and K) in the soil, enhancing nutrient availability to plants, which benefited both soil health and crop yield. When the rate increased from 4.5% to 6%, the SMC changed slightly, so the composite amendment had a certain limit in improving SMC.

4.2. Changes in SOC Content

SOC referred to a range of carbon-containing organic compounds in the soil, primarily in their oxidized forms. SOC was closely linked to soil fertility and played a crucial role in the global carbon cycle [46]. The organic fertilizers, straw, biochar, and other substances in composite soil amendments were considered exogenous OC, which directly increased the soil’s OC content when applied to sandy soil. Biochar had a stable carbon structure with high aromaticity, which allowed it to remain in the soil for a long period. When incorporated into the soil, some biochar was decomposed by soil microorganisms into stable SOM, which had a suppressive effect on the decomposition of native SOC, thereby reducing carbon losses from the soil [47]. This resulted in an increase in SOC content, which was aligned with the findings of this study. Biochar, which contained about 60–80% carbon, increased SOC content by closing the nutrient cycles and enhanced carbon sequestration [48]. Straw and organic fertilizers typically had a high SOC content, and their application enhanced SOC levels, which promoted carbon sequestration and stabilized it within the soil [49]. The incorporation of granular straw into soil significantly increased the SOC content in the surface layer (0–20 cm) of maize dryland soils [50]. After three years of applying organic fertilizer to loam soil, SOC content increased by 19.5% [51]. The addition of organic fertilizers resulted in significant increases in both SOC content and its components [52]. Analysis of experimental data revealed that increasing the application rate of the composite ameliorant from 1.5% to 6% significantly boosted the SOC content from approximately 10 to 40 g·kg−1, so the use of composite ameliorants was effective for enhancing SOC levels and achieving carbon sequestration in sandy soils.

4.3. Changes in Soil Nutrient Content

The input of exogenous nitrogen and phosphorus directly increased the content of TN and TP in the soil. Organic fertilizers and crop straws were rich in organic nitrogen and phosphorus, which were decomposed into inorganic nitrogen forms (such as ammonium and nitrate) and available phosphorus and gradually released by microbiology, which increased soil TN and TP. Microbial inoculants can significantly improve soil structure by promoting organic matter accumulation and forming bio-cements, which enhance water and nutrient retention capacity. Additionally, these inoculants demonstrate multiple functional benefits, including phosphorus solubilization, potassium mobilization, and nitrogen fixation, which indicates substantial potential for improving soil nutrient availability. The results of this experiment indicate that with the increasing addition of composite amendments, the levels of nitrogen and phosphorus in the soil generally showed an increasing trend. TN decreased in the CM4 experimental group, which was likely due to the inherently low nutrient content of sandy soil. The input and enhanced fixation of exogenous nitrogen and phosphorus prevented rapid nutrient loss. Since the CM4 group applied the highest proportion of amendments, which also contained the most nutrients. Nutrient loss was greatest in this group, which led to a decrease in TN content during the second experimental period. The animal manure organic fertilizers contained high nutrient levels, with most nitrogen primarily present in mineral forms, which made it more accessible to plants [53]. Compared to synthetic fertilizers, organic fertilizers effectively increased the phosphorus utilization efficiency at the same application rate [54]. The application of organic fertilizers has been shown to notably increase the carbon, nitrogen, and phosphorus nutrients and their availability in rice paddy soils, which also improved the balance of nitrogen and phosphorus [55].
Biochar had the potential to regulate the nitrogen cycle and increase the content of nitrogen and phosphorus in soils [56,57]. Biochar possessed a porous structure and high SSA. The functional groups formed complex reactions with phosphate ions, which enhanced the fixation and adsorption of nitrogen and phosphorus nutrients in the soil. Biochar promoted nutrient utilization by plants, which reduced nutrient loss in the soil system [58]. The addition of biochar to the soil significantly enhanced nitrification, raised soil nitrogen levels, and increased the availability of nitrogen to plant roots [59]. The application results of pistachio shell-based biochar in sustainable soil improvement showed that pH decreased by 1%, EC reduced by 4–14%, OC increased by 100–200%, and TN increased by 35% compared to untreated soil [45]. Under field conditions, the use of wheat straw biochar as a soil amendment led to an increase in TN, available nitrogen, and available phosphorus [60]. Crop residues such as straw from rice and maize contained lignin, cellulose, and hemicellulose, which altered the composition and stability of soil aggregates when added to the soil, effectively increasing SOC [61]. The increase in SOC significantly improved soil aggregate structure, enhanced soil aeration, and increased the availability of nutrients (N and P). The addition of composite amendments played a significant role in promoting the enrichment of nitrogen and phosphorus nutrients in sandy soil.
The enhancement of soil N and P levels plays a crucial role in maintaining the ecological balance of soil ecosystems and in soil management in desert regions. It promoted the diversity and activity of soil microbial communities [62], enhanced the biogeochemical cycling functions of the soil, accelerated SOC decomposition and nutrient release, and provided sustained nutrients for plants. The higher nutrient content improved the absorption and utilization of plants, stimulated root development, strengthened the root system’s ability to stabilize the soil, and reduced soil erosion. This contributed to desertification control and strengthened the stability of the soil ecosystem. Adequate nutrient supply enhanced plant resistance to drought, cold, and pest diseases, which improved the ecosystem’s resilience to extreme environmental events. Improving the levels of TN and TP can enhance soil quality in desert ecological restoration and soil management.

4.4. Changes in L. perenne Growth Indicators

Sandy soils were often infertile and had low water retention, with vegetation growth typically subjected to drought stress and nutrient deficiencies. The results of this study showed that applying different amounts of composite amendments to sandy soils affected various growth parameters of L. perenne (plant height, dry weight, and nutrient content). There were significant differences in the dry weight and plant height of L. perenne between the first and second planting cycles. In the early stages of both planting cycles, experimental groups received higher amounts of composite amendment (4.5%, 6%), which exhibited shorter plant heights compared to CK. This was because of the high rate of the amendment, which contained substances such as bentonite and polyacrylamide that had strong adsorption and bonding properties. These substances likely reduced the soil’s permeability and aeration, while elevated salt levels had inhibited plant biomass accumulation, thus hindering vegetative growth [63]. When the bentonite content was higher, it competed with the plant roots for SMC, which resulted in an insufficient water supply for plant growth and slowed down development [13]. In the later stages, when no further irrigation was provided to the L. perenne, the CK experimental group exhibited poor water retention, which led to gradual moisture loss. L. perenne showed wilting around day 25 after planting with slow plant height growth, while the other experimental groups experienced wilting around day 35. Apart from SMC, soil salinity was also a significant threat to plant growth and development [64]. In the first planting cycle, the soil EC in the CM3 and CM4 experimental groups exceeded 1000 μS·cm−1, far above the optimal salinity range for L. perenne growth, which restricted plant development. Under prolonged influence, the addition of composite amendments increased the dry biomass of L. perenne. The synergistic effects of materials like bentonite, organic fertilizers, biochar, and crop straw improved the soil’s physical structure and water retention capacity and provided essential macro- and micronutrients for plant growth, enhancing water, nutrients, air, and thermal conditions for L. perenne, which promoted its growth.
The distribution and metabolism of nitrogen and phosphorus in plants were key determinants of growth and productivity [9]. Nitrogen was an important element in basic plant structures such as chloroplasts, and its availability triggered various responses. When nitrogen was abundant, plants synthesized more proteins, which promoted cell division and growth [65]. Phosphorus promoted root development and growth, enhanced plant adaptation to environmental conditions, and improved resistance to diseases and cold stress. In this experiment, the nitrogen and phosphorus content in plants increased with the higher application rates of the amendments, and the effect became more pronounced with longer amendment periods. The addition of biochar and composite microbial agents accelerated the nitrogen cycling process and facilitated a higher abundance of nitrogen-related microbial genes and enzyme activity, which in turn improved nitrogen utilization and plant productivity [66]. The addition of organic fertilizers increased the SOM content, enhanced the nutrient supply capacity, and promoted the release of nitrogen and phosphorus [67], thereby improving nutrient uptake by the plants. The improved soil environment enhanced the plants’ resilience and allowed L. perenne to effectively absorb N and P, which promoted growth and increased yield.
Considering the overall effect of soil improvement and the growth of L. perenne, the use of soil amendments on a larger scale had the potential to positively impact the environment. In pot experiments, soil amendments primarily improved soil structure, but their effect on hydrological processes was limited. When applied on a larger scale, amendments enhanced soil aggregate stability and increased water retention, which reduced runoff from rainfall or irrigation and mitigated soil erosion [17]. This had a positive effect on preventing soil erosion and reducing the spread of desertification. The large-scale use of organic amendments helped to increase SOC stocks, enhanced soil carbon sequestration, lowered CO2 concentrations, and slowed down climate change. After applying soil amendments on a larger scale, soil structure was improved, and nutrient status and moisture retention supported more stable plant growth, which reduced the reliance on continuous human interventions such as irrigation and fertilization.

4.5. Prospects

This study still has some limitations. The conditions of the pot experiments were idealized and did not fully simulate the complex environmental factors in actual desert ecosystems, such as uneven precipitation, wind erosion, and diurnal temperature fluctuations. These factors may affect the stability of the amendments and their role in promoting plant growth. This study primarily evaluated the effects of composite amendments from the perspectives of soil physicochemical properties and plant growth. Exploration of their mechanisms was limited. The widespread application of composite amendments depended not only on their effectiveness but also on factors such as cost, the sustainability of raw material procurement, and environmental impact. This study has yet to conduct a thorough analysis of the economic feasibility and the scalability of the amendments. So, future research will focus on the following areas:
(1)
Field trials will be conducted to further validate the adaptability of the amendments and their effects under various climatic conditions.
(2)
Molecular biology, microbiome analysis, and isotope tracing techniques will be used to better understand the mechanisms by which composite amendments influence soil quality and vegetation restoration.
(3)
Life cycle assessment methods will be applied to evaluate the cost-effectiveness of the amendments and explore more cost-effective, sustainable formulations for broader agricultural applications.

5. Conclusions

This study simulated desert conditions in a controlled environment to investigate the actions of composite amendments on sand soil and plants. Two cycles of pot experiments with L. perenne were conducted, and the main findings are as follows:
(1)
Composite amendments improved soil pH, increased EC, and enhanced SMC. The pH of amended sandy soil remained consistently within a neutral range of 7.0–7.5, which was more favorable for plant growth. The addition of composite amendments led to an increase in soil EC, which further decreased with the cultivation of L. perenne. After two cycles of planting, SMC increased from 5.59% and 5.65% to 7.95% and 6.86% (p < 0.05) in the treatments with 4.5% and 6% amendment additions, respectively.
(2)
The composite amendments significantly enhanced SOC and nutrient content. Under L. perenne cultivation, the OC content in amended sandy soil increased by approximately 1–4 times compared to the unamended soil. In the treatment with 3% amendment, the increases in TN and TP were the highest, with TN rising from 0.74 to 1.83 g·kg−1 (p < 0.05) and TP increasing from 0.52 to 0.58 g·kg−1.
(3)
Appropriate application of composite amendments effectively improved L. perenne plant height, dry weight, and nitrogen and phosphorus content. Under different amendment rates, L. perenne height in the second planting cycle was approximately 10 cm higher than in the first cycle. At the 3% amendment level, L. perenne dry weight was consistently high across both planting cycles, with nitrogen and phosphorus content showing minimal differences compared to the higher 4.5% and 6% amendment treatment groups.
(4)
Considering the changes in pH, EC, SMC, SOC, TN, and TP, the 3% amendment level was identified as the optimal rate for improving sandy soil and supporting L. perenne growth. To further enhance soil improvement effects, it is recommended to reduce the salt content in the biochar used in the amendments.

Author Contributions

Conceptualization, H.W.; methodology, X.S., L.W., L.F., Y.Z., X.L., Y.L. and H.W.; software, X.S., L.F., Y.Z., X.L., Y.L. and H.W.; validation, X.S., L.W., X.L. and Y.Z.; formal analysis, X.S., X.L. and L.F.; investigation, X.S., L.F., X.L., Y.L. and Y.Z.; resources, X.S., L.W. and Y.L.; data curation, X.S.; writing—original draft preparation, X.S.; writing—review and editing, L.W. and H.W.; visualization, X.S.; supervision, X.S., Y.Z., Y.L. and H.W.; project administration, H.W.; funding acquisition, H.W. All authors have read and agreed to the published version of the manuscript.

Funding

The present study was gratefully supported by the High-level Talent Foundation Project of Harbin Normal University (No. 1305124219).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Data used in this research is available upon request from the corresponding author.

Acknowledgments

The author appreciates the efforts made by the editor and the reviewers for this article.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
SOCSoil organic carbon
ECElectrical conductivity
SMCSoil moisture content
SOMSoil organic matter
OCOrganic carbon
TNTotal nitrogen
SSASpecific surface area
TPTotal phosphorus

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Figure 1. The distribution map of sandy soil in Hulun Buir City and the schematic diagram of the indoor pot experiment design. (a) The distribution map of sandy soil in Hulun Buir City; (b) the schematic diagram of the indoor pot experiment design. This figure was based on spatial distribution data of soil texture provided by the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, combined with the soil type map and soil profile data from the Second National Soil Census.
Figure 1. The distribution map of sandy soil in Hulun Buir City and the schematic diagram of the indoor pot experiment design. (a) The distribution map of sandy soil in Hulun Buir City; (b) the schematic diagram of the indoor pot experiment design. This figure was based on spatial distribution data of soil texture provided by the Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, combined with the soil type map and soil profile data from the Second National Soil Census.
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Figure 2. Changes in soil pH, EC, and SMC in different experimental groups. (a) Change in pH of two cycles in different experimental groups; (b) change in EC of two cycles in different experimental groups; (c) change in SMC of two cycles in different experimental groups [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d, e) indicate significant difference between treatments].
Figure 2. Changes in soil pH, EC, and SMC in different experimental groups. (a) Change in pH of two cycles in different experimental groups; (b) change in EC of two cycles in different experimental groups; (c) change in SMC of two cycles in different experimental groups [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d, e) indicate significant difference between treatments].
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Figure 3. Changes in SOC, TN and TP contents in different experimental groups. (a) Change in SOC of two cycles in different experimental groups; (b) change in TP of two cycles in different experimental groups; (c) change in TN of two cycles in different experimental groups [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
Figure 3. Changes in SOC, TN and TP contents in different experimental groups. (a) Change in SOC of two cycles in different experimental groups; (b) change in TP of two cycles in different experimental groups; (c) change in TN of two cycles in different experimental groups [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
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Figure 4. Changes in dry weight of L. perenne in different experimental groups in two planting cycles [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
Figure 4. Changes in dry weight of L. perenne in different experimental groups in two planting cycles [Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
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Figure 5. Changes in L. perenne height in different experimental groups in two planting cycles. (a) The first cycle; (b) the second cycle.
Figure 5. Changes in L. perenne height in different experimental groups in two planting cycles. (a) The first cycle; (b) the second cycle.
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Figure 6. Changes in N and P contents of L. perenne in different experimental groups in two planting cycles [(a) Change in TN of two cycles in different experimental groups; (b) change in TP of two cycles in different experimental groups; Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
Figure 6. Changes in N and P contents of L. perenne in different experimental groups in two planting cycles [(a) Change in TN of two cycles in different experimental groups; (b) change in TP of two cycles in different experimental groups; Letters indicate statistical significance at p ≤ 0.05 level where small letters above column (a, b, c, d) indicate significant difference between treatments].
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Table 1. The basic characteristics of sand soil before the experiment.
Table 1. The basic characteristics of sand soil before the experiment.
ItempHEC (μs·cm−1)SOC (g·kg−1)TN (g·kg−1)TP (g·kg−1)
Quantity8.24 ± 0.1651 ± 3.366.92 ± 0.280.11 ± 0.070.24 ± 0.01
Table 2. The formulation of composite soil amendment materials.
Table 2. The formulation of composite soil amendment materials.
Raw MaterialRate (%)Material Properties
Polyacrylamide2.7Solid content > 90%, molecules number = 1.22 × 107
Biochar16.2Corn straw (500 °C/2 h), pH = 9.46, OC = 42.21%, TN 8.24%, TP = 2.31%, TK = 16.12%
Sodium bentonite16.2Montmorillonite content > 85%
Straw fibers5.4Rice straw (10–20 mm)
Corn straw2.7Particle size < 1 mm
Sheep manure organic fertilizer54.1OC = 50.3%, Total nutrients (N, P, K) = 5.0%, pH = 5.5
Composite microbial agents2.7Bacillus subtilis (3.03 × 1010 CFU/g), Bacillus amyloliquefaciens (2.07 × 1010 CFU/g), Gelatinous Bacillus subtilis (2.02 × 108 CFU/g), Bacillus licheniformis (1.01 × 108 CFU/g).
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MDPI and ACS Style

Sui, X.; Wang, L.; Lv, X.; Liu, Y.; Zhu, Y.; Fan, L.; Wang, H. Influence of Composite Amendments on the Characteristics of Sandy Soil. Sustainability 2025, 17, 7619. https://doi.org/10.3390/su17177619

AMA Style

Sui X, Wang L, Lv X, Liu Y, Zhu Y, Fan L, Wang H. Influence of Composite Amendments on the Characteristics of Sandy Soil. Sustainability. 2025; 17(17):7619. https://doi.org/10.3390/su17177619

Chicago/Turabian Style

Sui, Xinrui, Lingyan Wang, Xinyao Lv, Yanan Liu, Yuqi Zhu, Lingyun Fan, and Hanxi Wang. 2025. "Influence of Composite Amendments on the Characteristics of Sandy Soil" Sustainability 17, no. 17: 7619. https://doi.org/10.3390/su17177619

APA Style

Sui, X., Wang, L., Lv, X., Liu, Y., Zhu, Y., Fan, L., & Wang, H. (2025). Influence of Composite Amendments on the Characteristics of Sandy Soil. Sustainability, 17(17), 7619. https://doi.org/10.3390/su17177619

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