Experimental Study on Wind Erosion Resistance and Plant Growth Performance of Coastal Sand Stabilized by Soybean Hull-Enzyme-Induced Carbonate Precipitation and Seawater

Abstract

To combat coastal wind erosion and develop sustainable stabilization technologies, a resource-efficient technique was developed based on the Enzyme-Induced Carbonate Precipitation (EICP) principle in the coastal regions of China. Utilizing seawater as a multi-ion source and discarded soybean hulls (Glycine max (L.) Merr.) as a crude urease source, this method is synergized with vegetation to form an environmentally friendly anti-erosion strategy. This study first explored the feasibility of soybean hull-derived urease, then analyzed the impacts of urease activity, reaction liquid volume, and seawater concentration on the germination and growth of Kalimeris indica. The results show that the biochemical mineralization process effectively sequesters soluble Ca²⁺ and Mg²⁺ from seawater into stable mineral phases, thereby mitigating salt-induced osmotic stress. Optimal plant growth was achieved at a seawater concentration of 0.2 mol·L⁻¹ and a liquid volume of 200 mL. Furthermore, the biocementation provided robust protection for initial plant growth, achieving an approximately 92.3% reduction in soil loss. Despite the presence of nitrogenous byproducts, the synergistic effect of EICP crusts and developing root systems ensures long-term wind erosion resistance and ecological integrity. This study highlights a functional transition from artificial mineralization to biological anchoring for sustainable coastal restoration.

1. Introduction

Coastal aeolian sand phenomena are extensively distributed across sandy coastlines worldwide, particularly in regions with abundant sand sources and favorable wind conditions [1]. Currently, approximately 200,000 km² of coastal landforms globally exhibit aeolian sand characteristics [2]. In the eastern coastal provinces of China, various types of coastal aeolian sand landforms cover a total area of about 3000 km², accounting for approximately 0.03% of the country’s total land area [3]. Due to their insufficient cohesion and relatively low stability, these coastal aeolian sand landforms are highly susceptible to wind erosion [4,5]. The erosion of coastal aeolian sand presents a significant threat to the environment, ecosystems, and human living space. Current methods for controlling sand erosion by wind include mechanical sand control, chemical sand control, and vegetation sand control [6,7,8]. However, mechanical sand control not only incurs high costs [9], but also faces limitations due to the susceptibility of sand stabilization facilities to burial [10], which constrains their long-term effectiveness in erosion control. Chemical sand control, while effective, is expensive, and the chemicals used are often toxic or difficult to degrade, posing potential environmental pollution issues [11]. Vegetation sand control primarily relies on planting vegetation to stabilize sand dunes [12,13,14], but this method is slow and requires a long time to show results. During the plant growth period, wind erosion may cause seedlings to be displaced from the soil or buried by drifting sand [15], leading to their death. Therefore, it is imperative to implement effective measures to protect seedlings and mitigate the impact of wind erosion during their early developmental stages [16].

In addition to traditional methods, many researchers have investigated the use of Biologically Induced Carbonate Precipitation (BICP) technology—comprising Microbially Induced Carbonate Precipitation (MICP) and Enzyme-Induced Carbonate Precipitation (EICP)—for sand stabilization and dust suppression, leading to substantial interest in this novel soil reinforcement approach across various fields over the past decade [17,18,19,20,21]. Studies have consistently shown that BICP technology can provide substantial resistance to wind erosion and effectively mitigate the impacts of sand erosion by wind [22,23,24]. BICP technology involves the use of urease to hydrolyze urea into NH₄⁺ and CO₃²⁻. The CO₃²⁻ then reacts with Ca²⁺ to form CaCO₃ precipitates. The cementing effect of calcium carbonate binds the soil particles together, thereby increasing soil strength [25,26,27].

Urease, serving as the essential catalyst for the EICP reaction [28], is predominantly extracted from leguminous seeds, such as Jack bean, watermelon seeds, and soybean [29]. Compared to commercially purified urease, crude enzyme extracts derived directly from plant seeds offer significant advantages in geotechnical applications. These crude extracts contain natural organic matrices that function as protective colloids, significantly enhancing the enzyme’s thermal stability and longevity in complex soil environments compared to their purified counterparts [30,31,32]. As one of the most widely processed legumes globally, soybean generates a massive volume of soybean hulls during oil refining and food production [33,34,35]. While these hulls are occasionally repurposed as animal feed, their low bulk density leads to prohibitive transportation costs, resulting in substantial underutilization. In China’s coastal regions, the generation of discarded soybean hulls—a low-value agricultural byproduct with an estimated annual production at a million-ton scale—poses significant environmental challenges and storage pressures [36]. Notably, soybean hulls retain residual proteins and polysaccharides that can be effectively harnessed for EICP mineralization [37]. By utilizing soybean hulls as a sustainable alternative for urease extraction, this approach promotes the waste-to-treasure valorization of agricultural residues. Consequently, this provides a highly promising and eco-friendly strategy for effective wind erosion management in coastal environments.

Traditionally, industrial-grade calcium sources like calcium chloride (CaCl₂) are widely used for EICP/MICP [38]. In recent years, numerous studies have focused on exploring the use of recycled resources as alternatives to traditional calcium salts [39,40], predominantly in freshwater environments. However, their application in remote coastal engineering faces significant challenges, including prohibitive transportation costs, the risk of secondary chloride pollution, and a substantial carbon footprint associated with industrial production. Utilizing seawater as a natural, in situ calcium source not only circumvents these limitations but also aligns with the principles of sustainable and green construction. Seawater contains abundant calcium and magnesium ions [41,42], which can serve as a calcium source for BICP technology. Ou [43] and Mortensen [44] found that MICP techniques can still be effectively applied in high-salinity seawater environments. Additionally, Cheng [45] and Yang [46] demonstrated that using seawater as a calcium source for MICP in sand stabilization can still achieve satisfactory strength. While existing studies have primarily focused on the feasibility of seawater as a calcium source for MICP, the integration of seawater with EICP—particularly using crude urease extracted from soybean hulls, a low-cost agro-industrial byproduct—remains underexplored. Consequently, a cost-effective and sustainable strategy for coastal soil stabilization using these dual-waste resources has yet to be established.

Vegetative sand control techniques are known for their long-term effectiveness in combating desertification, but their short-term efficacy is relatively limited. In contrast, the BICP technique can rapidly form a hardened layer on the surface of sand, thereby achieving effective control of wind erosion within a short time frame. However, there is a paucity of research exploring the interaction between BICP-based sand stabilization and plant growth [47], indicating that further experimental studies are needed to assess the capabilities of biological soil stabilization for vegetation growth and ecological restoration. For instance, Li [48] observed that the germination rate of plants in the solidified layer decreases with the enhancement of its mechanical properties. Wu [49] discovered that EICP sand stabilization enhances plant growth. Meng [50] found that excessively high concentrations of treatment solutions could inhibit seed germination and the growth of local sand-dwelling plants. Kong [51] demonstrated that at a treatment solution concentration of 0.15 mol·L⁻¹, the combination of MICP and vegetation further enhanced sand stabilization effectiveness.

This study explores the utilization of discarded soybean hulls as an alternative source for urease, replacing conventional soybeans, and employs seawater as the calcium source for EICP. This approach achieves waste utilization, reduces costs associated with urease extraction, utilizes locally available resources to lower calcium source costs, and avoids pollution typically associated with the production of synthetic calcium compounds. Building upon this foundation, the study integrates EICP technology with vegetation protection, aiming to propose a greener, more resource-efficient, and highly efficient technology for wind erosion control and sand fixation. To establish a clear scientific logic for this integrated framework, our research consideration centers on a stage-based functional transition: the transient biocemented crust formed by seawater-based EICP serves as an immediate protective window against wind erosion. Over time, as the crust adapts to the environment, the developing canopy of Kalimeris indica theoretically attenuates near-surface wind velocity to prevent erosion, while its root system provides crucial mechanical anchoring to stabilize the plant substrate. To evaluate the feasibility of this multi-stage mechanism, the crude urease activity present in the discarded hulls is initially characterized. Subsequently, seed germination and plant growth experiments are conducted to analyze the effects of factors such as urease activity, reaction liquid volume, and seawater concentration on plant development. Finally, wind tunnel erosion tests are performed to quantify the macroscopic stabilization capacity of the integrated EICP–vegetation system.

2. Materials and Methods

2.1. Sandy Soil

This study conducts reinforcement experiments employing sand, with the particle size distribution illustrated in Figure 1a. The particle sizes are concentrated between 0.1 and 10 mm, with D₁₀ = 0.11, D₃₀ = 0.15, D₆₀ = 0.22, Cu = 2, and Cc = 0.9. The sand is poorly graded with a uniform particle size distribution.

2.2. Preparation of Soybean Hull Urease and Activity Testing

The materials were obtained as a byproduct from an oil refinery in Zhangjiagang (Jiangsu, China), which processes yellow soybeans imported from Brazil. After drying in a 60 °C oven for two hours, the soybean hulls, as shown in Figure 1b, were ruptured using a grinder, followed by sieving through a 100-mesh sieve to obtain soybean hull powder. Subsequently, the soybean hull powder was mixed with deionized water and stirred for 30 min to dissolve. After filtration to remove insoluble matter, the soybean hull powder solution was obtained. The resulting solution was centrifuged at 4 °C and 4000 rpm for 15 min to obtain the supernatant as the desired soybean hull urease solution.

Given the inherent challenges in directly quantifying urease activity, this study employed an indirect method to assess its activity. Initially, a 27 mL solution of 1.11 mol·L⁻¹ urea was introduced into a test tube. Subsequently, 3 mL of the soybean hull urease solution under investigation was swiftly added to the test tube and thoroughly mixed at room temperature. Subsequently, the conductivity of the mixed solution was measured three times using a conductivity meter (Shanghai scientific instrument Co., Ltd., Shanghai, China), with each measurement separated by an interval of 5 min, to calculate the average change in conductivity over a 15 min period. The electrical conductivity meter was rigorously calibrated using standard solutions prior to the testing. In accordance with the findings of Whiffin [52], this change in conductivity serves as a viable indicator of the rate of urea hydrolysis by soybean hull urease per minute.

2.3. Preparation of Reaction Solution

In this study, the reaction solution consisted of two main components: the calcium source solution and the soybean hull urease solution, in a volumetric ratio of 1:1. The calcium source solution was prepared by mixing urea solution and seawater in a 1:1 volumetric ratio. The major ionic composition of seawater is detailed in

Table 1. Main ion components in seawater (mg·L⁻¹).

Ca2+	Mg2+	Na+	K+	Cl−
426.53	1219.86	11,078.58	140.16	18,690.10

.

2.4. Experimental Design and Procedures

This study conducted plant growth experiments using sandy soil as the substrate, with a primary focus on assessing the effects of EICP on seed germination, seedling growth, and the vegetation’s resistance to wind erosion. Each sample was uniformly seeded with 30 seeds at a depth of 0 to 1 cm. Immediately after seeding, samples underwent EICP solidification treatment. To isolate the reinforcement effect of the EICP process, the control group (G1) was treated with an equal volume of seawater and planted with Kalimeris indica, but without EICP treatment (i.e., no urease or urea–calcium source). This setup ensures that any observed enhancement in wind erosion resistance can be directly attributed to the biomineralization process and its interaction with the vegetation. The experiment systematically investigated the influence of variables including urease activity (G2), spray volume (G3), treatment frequency (G4), and seawater concentration (G5) on both plant growth and the wind erosion resistance of the sand. Specifically, spray volume denotes the total amount of solution applied per spraying event, while treatment frequency indicates the number of applications into which that volume was partitioned. Detailed treatment protocols are provided in

Table 2. Experimental groups for plant growth and wind erosion testing.

Testing Group	Urease Activity (mmol·L−1·min−1)	Urea Concentration (mol·L−1)	Sea Salt Concentration (mol·L−1)	Reaction Solution Volume (mL)	Treatment Times	Plant (Y/N)	Wind Erosion
G1	0.0	0.0	0.2	100	1	Y	√
G2	1.0	0.3	0.2	100	1	Y + N	√
	1.5	0.3	0.2	100	1	Y + N	√
	2.0	0.3	0.2	100	1	Y + N	√
G3	2.0	0.3	0.2	100	1	Y + N	√
	2.0	0.3	0.2	200	1	Y + N	√
	2.0	0.3	0.2	300	1	Y + N	√
	2.0	0.3	0.2	400	1	Y	-
G4	2.0	0.3	0.2	100	1	Y + N	√
	2.0	0.3	0.2	100	2	Y + N	√
	2.0	0.3	0.2	100	3	Y + N	√
	2.0	0.3	0.2	100	4	Y	-
G5	2.0	0.3	0.2	100	1	Y + N	√
	2.0	0.3	0.6	100	1	Y + N	√
	2.0	0.3	1.0	100	1	Y + N	√
	2.0	0.3	1.4	100	1	Y	-
	2.0	0.3	1.8	100	1	Y	-

Notes: “Y” denotes vegetation presence; “Y + N” denotes both vegetated and non-vegetated conditions; “√” denotes conducted wind erosion tests; “-” denotes no wind erosion tests.

. All experimental treatments and measurements were conducted in triplicate (n = 3), and the results are expressed as mean ± standard deviation in figures.

2.5. Germination Test of Seeds

The plant selected for this study is Kalimeris indica, from Jufeng Seed Industry Group Co., Ltd., Yinchuan, China. In the experimental setup, a controlled environment at 25 °C in darkness was simulated to mimic spring and summer seed germination conditions [53]. The experimental specimens were treated according to the protocol outlined in Table 1. Following adequate solidification reactions, the sandy soil was irrigated daily with deionized water at a rate of 0.2 L·m⁻². Continuous observation and recording of seeds with cotyledons protruding 2 mm above the surface of the sand were conducted [54], until the absence of new seedlings for five consecutive days, signifying the conclusion of the experiment. Additionally, calculations pertaining to seed germination percentage and speed were elaborated based on Formulas (1) and (2).

$$\mathrm{SGP}={\displaystyle \frac{n}{N}}\times 100\%$$

(1)

• SGP—seed germination percentage, %;
• n—number of germinated seeds during the experimental period;
• N—total number of seeds.

$$\mathrm{SGS}={\displaystyle \frac{{\displaystyle \sum Dm}}{{\displaystyle \sum m}}}\times 100\%$$

(2)

• SGS—seed germination speed, day;
• m—number of seeds germinated on the observation day;
• D—days after beginning.

2.6. Plant Growth Performance Experiment

In this study, indoor environmental conditions were controlled to maintain a temperature of 25 ± 1 °C and humidity at 70 ± 10%. Simultaneously, a cold light source fluorescent lamp with a wavelength range of 400–700 nm was employed to provide a 12 h photoperiod with an illumination intensity of 5.0 MJ·m⁻², aiming to fulfill the requirements for plant photosynthesis and growth.

The objective of this research is to quantitatively evaluate the growth status of seedlings through a series of key growth indicators. These indicators encompass plant height, Leaf Area Index, vegetation coverage, Root Area Index, and biomass. Plant height refers to the linear distance from the base of the above-ground portion of the plant to the apex of the unfolded leaf blades, serving as a fundamental parameter for assessing seedling growth. Vegetation coverage (VC) represents the proportion of the above-ground portion of plants’ vertical projection on the ground, reflecting the density of vegetation distribution. Leaf Area Index (LAI) refers to the ratio of the total leaf area per unit ground area to the projection of the plant canopy onto the ground. It is a critical ecological indicator used to assess vegetation growth and functional status, holding significant implications for both environmental science and agronomy. In this study, ImageJ software (version 1.54, National Institutes of Health, Bethesda, MD, USA; available at https://imagej.net) was utilized to compute LAI through high-definition image analysis, employing a binary image processing method to accurately determine leaf area [55,56]. Root Area Index (RAI) refers to the total root surface area per unit of ground area. It reflects the seedling’s capacity for water and nutrient uptake and its environmental adaptability. To assess RAI, three seedlings are randomly selected from each group, with impurities and non-active roots removed. The lengths of coarse roots (those exceeding 1 mm in diameter) and fine roots are measured, and the root biomass is determined after high-temperature drying. Calculating the root length density and mass density per unit soil volume provides insights into the development of the seedling’s root system [57]. Biomass refers to the total dry weight of plant bodies per unit area, reflecting the internal energy storage and distribution of seedlings, thus directly manifesting the survival capability and growth potential of plants.

2.7. Wind Erosion Experiment

To quantitatively assess the wind erosion resistance, a series of experiments were conducted using a high-power wind tunnel apparatus (Figure 2). The experimental soil specimens were prepared in trays with dimensions of 25 × 15 × 5 cm according to the designated experimental groups, and the wind speed was maintained at a constant 20 m·s⁻¹, calibrated with reference to the Gale Force 8 range (17.2–20.7 m·s⁻¹) on the Beaufort scale. This parameter represents a near-surface simulation designed to deliver significant kinetic energy to the specimen surface, mimicking extreme erosive conditions in coastal environments. The experimental duration was set at 15 min, a window determined by preliminary testing to be sufficient for capturing the complete transition from initial particle detachment to structural stabilization before the onset of potential edge effects from the specimen tray. During the process, the sandy soil samples were precisely weighed every 3 min to monitor the dynamic mass loss. This experimental configuration has been validated in previous studies [31] and proves effective for assessing the relative erosion resistance of various stabilization methods.

To quantitatively evaluate the wind erosion resistance, the critical wind speed (V_c) is defined based on the distinct physical responses of the sand surface. For untreated sand or samples with vegetative protection only, V_c is determined as a continuous stream of sand particles begins to leap and move across the surface. For EICP-stabilized samples, V_c is characterized by the onset of surface cracking, peeling, or the detachment of biocemented aggregates.

3. Analysis of Experimental Results

3.1. Factors Affecting Soybean Hull Urease Activity

Figure 3a depicts the relationship between soybean hull powder concentration and urease activity under room temperature conditions (20 °C). The experimental results indicate that the activity of soybean hull urease exhibits an approximately linear increase with increasing soybean hull solution concentration, within the activity range of 0.3–2.2 mmol·L⁻¹·min⁻¹. Figure 3a also depicts the activity of urease at different temperatures under a soybean hull powder concentration of 100 g·L⁻¹, revealing that enzyme activity increases with temperature. Temperature stands as a pivotal factor influencing biochemical reactions, with a pronounced regulatory effect on urease activity. Under low-temperature conditions, urease activity experiences a significant reduction. As the temperature returns to ambient levels, enzyme activity gradually restores to within the normal range, which is of paramount importance; with further temperature escalation, urease activity exhibits a notable upward trend. However, when the temperature reached 60 °C, the increase in urease activity decreased significantly in this study.

Figure 3b illustrates the activity of urease under different centrifugation time and speed conditions at a soybean hull powder concentration of 100 g·L⁻¹ under room temperature conditions. The results indicate that both centrifugation time and speed do not significantly influence urease activity. With the increase in centrifugation speed, the urease activity showed almost no change. As the centrifugation time increased, the urease activity increased slightly, but the improvement was minimal. When the centrifugation time was extended from 10 to 120 min, the urease activity increased by only about 17.8%. Statistical analysis confirmed that concentration and temperature had highly significant effects (p < 0.001), whereas centrifugation speed and time showed no significant impact (p > 0.05).

3.2. Effect of EICP on Plant Growth Performance

3.2.1. Seed Germination Status

Figure 4a illustrates the effect of seawater salt solution concentration on the germination of plant seeds. As the concentration of seawater salt concentration increases, the SGP initially rises and then declines, while the SGS continuously increases. This phenomenon can be attributed to the fact that, at lower concentrations of seawater salt solution, the nutrient content in the sand increases, thereby enhancing seed germination. However, once the seawater salt concentration exceeds a certain threshold, the soil salinity surpasses the seeds’ tolerance level, which inhibits germination. For instance, at lower sea salt solution concentrations (0.2 mol·L⁻¹ and 0.6 mol·L⁻¹), the SGP increased by 10% and 7% compared to the control group, respectively. Conversely, when the seawater salt concentration reached 1 mol·L⁻¹ or higher, the SGP significantly decreased. At concentrations of 1.4 mol·L⁻¹ and above, the SGP fell below 10%, indicating almost no germination.

Additionally, increasing sea salt solution concentrations led to a gradual increase in the strength of EICP-cemented sandy soil, which impeded seedling emergence and extended the time required for seedlings to appear. The SGP exhibited an approximately linear relationship with seawater salt concentration. For example, the average SGS for the control group was about 5 days, whereas for the test groups with seawater salt concentrations of 0.6 mol·L⁻¹ and 1.0 mol·L⁻¹, the average SGS increased to 7 and 8 days, respectively. In the test group with a sea salt solution concentration of 1.8 mol·L⁻¹, the average emergence time was extended to 10 days.

Figure 4b illustrates the impact of urease activity on the germination of plant seeds. The results indicate that the SGP increases significantly with higher urease activity. Specifically, when the urease activity level is elevated to 2.0 mmol·L⁻¹·min⁻¹, the SGP is enhanced by 16% compared to the control group. This effect is attributed to the accelerated breakdown of urea and the faster precipitation of calcium carbonate, which reduces the concentration of soluble ions via mineral precipitation and alleviates vegetative osmotic stress, thereby promoting seed germination. However, an increase in urease activity also leads to a slight extension in the germination speed. For instance, at urease activity levels of 1.5 mmol·L⁻¹·min⁻¹ and 2.0 mmol·L⁻¹·min⁻¹, the SGS is extended by 0.5 days and 1 day, respectively, compared to the control group. The cementation effect of EICP reduces the pore spaces between sand particles, which increases the strength of the sand while delaying seed germination. Statistical analysis confirmed that seawater concentration had an extremely significant impact on both SGP and SGS (p < 0.001).

Figure 5 illustrates the effects of volume of single reaction and treatment times on seed germination characteristics under a seawater salt concentration of 0.2 mol·L⁻¹. The results show that the SGP initially increases with the volume of a single reaction but subsequently decreases. This trend is attributed to the presence of nutrients in the reaction liquid that promote seed germination. When the concentration of these nutrients reaches an optimal level, it enhances germination. However, as the volume of single reaction liquid continues to increase, soil salinity also rises, which in turn inhibits seed germination. For instance, when the volume is increased to 200 mL or the treatment is doubled, the SGP improves by 13.3% compared to the control group, reaching 70%. However, when the volume of the reaction liquid reaches 400 mL, the germination rate drops to 50% or lower. Additionally, the SGS gradually slows with increasing volumes of reaction liquid, resulting in an extended average time to germination. At a reaction liquid volume of 400 mL, the SGS extends to 9 days, which is 1.8 times longer than that of the control group. This extension is due to the increased cementation effect of EICP resulting from the higher volume of the reaction liquid or more treatment times. Statistical analysis confirmed that both reaction solution volume and treatment times had extremely significant impacts on SGP and SGS (p < 0.001).

3.2.2. Plant Growth Status

Figure 6a illustrates the impact of seawater salt concentration on the growth status of plants. The results indicate that plant growth initially increases with rising seawater salt concentrations but subsequently declines. This phenomenon can be attributed to the fact that moderate sea salt levels provide essential nutrients for plant growth. However, excessively high sea salt concentrations lead to elevated soil salinity, which inhibits the effective absorption of water and nutrients by the plants, thereby suppressing their growth. When seawater salt concentration increases from 0 to 0.2 mol·L⁻¹, plant growth reaches its peak. For instance, at a seawater salt concentration of 0.2 mol·L⁻¹, plant height increased by 9 mm compared to the control group. Additionally, LAI, RAI, and plant coverage all reached their maxima at 6.17, 1.1, and 54.35%, respectively. However, further increases in sea salt concentration lead to a gradual deterioration in plant growth. Furthermore, the biomass of plants also decreases with increasing sea salt concentration. As the sea salt solution concentration rises from 0 to 1.4 mol·L⁻¹, biomass declines nearly linearly from 1.23 kg·m⁻² to 0.59 kg·m⁻². Even at the optimal seawater salt concentration of 0.2 mol·L⁻¹, the biomass remains lower than that of the control group. This suggests that while lower seawater salt concentrations can enhance plant growth, they do not favor biomass accumulation. Additionally, a comparison with the results in Figure 4 indicates that the seawater salt concentration optimal for seed germination does not necessarily benefit plant growth. For instance, at a seawater salt concentration of 0.6 mol·L⁻¹, although SGP improves, plant growth status is inferior to that of the control group. This may be due to the higher salt tolerance of seeds compared to the plants.

Figure 6b illustrates the impact of urease activity on the growth status of plants. The results indicate that as urease activity increases, the growth status of plants also significantly improves. This effect can be attributed to the fact that, with fixed seawater volume (100 mL) and concentration (0.2 mol·L⁻¹), increased urease activity accelerates the consumption of urea and sea salt, which reduces the concentration of soluble free ions via mineral precipitation and alleviates vegetative osmotic stress. This reduction in salinity enhances the plant’s ability to absorb water and nutrients effectively, thus promoting plant growth. The growth status of the plants peaks as urease activity rises from 1 to 2 mmol·L⁻¹·min⁻¹. For instance, at a urease activity level of 2 mmol·L⁻¹·min⁻¹, plant height increased from 144 mm to 153 mm compared to the control group. Additionally, LAI increased from 5.7 to 6.17, RAI rose from 0.9 to 1.1, and plant coverage improved from 51.28% to 54.35%. However, despite the positive effect of increased urease activity on plant growth, the biomass of the plants decreases as urease activity increases. Specifically, as urease activity rises from 0 to 2 mmol·L⁻¹·min⁻¹, biomass declines from an initial 1.23 to 1.18 kg·m⁻². This suggests that while higher urease activity supports plant growth, it is detrimental to biomass accumulation. This phenomenon may be due to the rapid formation of mineral crusts under accelerated cementation reactions, which restricts the expansion space for biomass accumulation, thereby leading to a decrease in biomass. Statistical analysis confirmed that both seawater concentration and urease activity exerted extremely significant impacts on all five plant growth parameters (p < 0.001).

Figure 7 illustrates the effects of volume of single reaction and treatment times on the growth of plants. The results indicate that plant growth initially increases and then decreases with increasing reaction liquid volume. This trend is attributed to the fact that an optimal volume of reaction liquid provides necessary nutrients for plant growth. However, excessive reaction liquid can lead to high soil moisture and salinity, which adversely affects root oxygen supply and growth. Additionally, elevated salinity increases soil osmotic pressure, thereby inhibiting the effective absorption of water and nutrients by plants and consequently stunting their growth. When the reaction liquid volume increases from 0 to 200 mL, plant growth reaches its peak. For instance, at a reaction liquid volume of 200 mL, plant height in the G3 and G4 groups increased by an average of 15 mm compared to the control group. Moreover, LAI, RAI, and plant coverage reached their maximum values, with G3 group values of 7.25, 1.05, and 55.62%, and G4 group values of 8.2, 1.1, and 56.98%, respectively. However, further increases in reaction liquid volume lead to deteriorating plant growth. Furthermore, the trend in biomass changes for plants mirrors that of plant growth. As the reaction liquid volume increases from 0 to 200 mL, the average biomass for the G3 and G4 groups rises from an initial 1.23 to 1.24 kg·m⁻². However, with a further increase to 400 mL, the biomass for both groups decreases to 0.82 kg·m⁻² (G3) and 0.92 kg·m⁻² (G4). This indicates that, under conditions of high urease activity and a sea salt solution concentration of 0.2 mol·L⁻¹, an optimal reaction solution volume not only promotes the growth of plants but also supports the accumulation of biomass. Statistical analysis confirmed that both reaction solution volume and treatment times exerted extremely significant impacts on all five plant growth and biomass parameters (p < 0.001).

3.3. Effect of EICP and Vegetation on Wind Erosion Resistance

3.3.1. Effect of EICP on Wind Erosion Resistance

Figure 8 illustrates the impact of EICP technology on the wind erosion resistance of Sandy soil Protected by EICP and Vegetation (SPEV). The results indicate that EICP technology significantly enhances the wind erosion resistance of sand, with its efficacy positively correlated with the degree of EICP sand stabilization. This enhancement is attributed to EICP’s ability to increase cohesion between sand particles, resulting in the formation of a hardened crust on the sand surface, which significantly improves its resistance to wind erosion. Even when EICP sand stabilization is less effective, the wind erosion amount of sandy soil is still substantially lower compared to samples stabilized solely through vegetative sand control. For instance, despite a urease activity of 1.0 mmol·L⁻¹·min⁻¹, a seawater salt concentration of 0.2 mol·L⁻¹, and a reaction liquid volume of only 100 mL, the wind erosion amount was merely 426 g, significantly lower than the 947 g observed in the control group. Moreover, the increasing seawater salt concentration, reaction liquid volume, and urease activity substantially improve sand’s wind erosion resistance. For example, Figure 8b shows that after 15 min of wind erosion, the sample with a seawater salt concentration of 1 mol·L⁻¹ had a wind erosion amount of only 2 g, markedly lower than that of the sample with a sea salt solution concentration of 0.2 mol·L⁻¹. Similarly, Figure 8c,d demonstrate that samples with a reaction liquid volume of 200 mL experienced a significantly slower rate of increase in wind erosion amount compared to those with 100 mL of reaction liquid. Additionally, Figure 8a indicates that the sample with a urease activity of 2.0 mmol·L⁻¹·min⁻¹ had a wind erosion amount of only 69 g, substantially lower than the 426 g observed for the sample with a urease activity of 1.0 mmol·L⁻¹·min⁻¹.

Therefore, increasing urease activity, sea salt solution concentration, and reaction liquid volume effectively promotes carbonate precipitation, thereby enhancing the cohesion between sand particles and improving the structural stability and wind erosion resistance of the sand compared to vegetative sand control methods.

3.3.2. Effect of Vegetation on Wind Erosion Resistance

Figure 9 illustrates the impact of plants on the wind erosion resistance of SPEV. The results indicate that the inclusion of vegetation significantly enhances the wind erosion resistance of EICP sand stabilization. At the end of the wind erosion test, the total wind erosion amount for SPEV was lower compared to those treated with EICP alone. However, during the early stages of the erosion process (0 to 3 min), the wind erosion amount for EICP sand stabilization was less than that for soils treated with both EICP and vegetation. This discrepancy can be attributed to the disruption of the hardened crust formed by EICP treatment due to plant growth, leading to surface irregularities and protrusions around plant roots. These uneven areas are more susceptible to erosion during the early stages of wind erosion. Nevertheless, in the mid-to-late stages of the wind erosion process, SPEV began to exhibit superior wind erosion resistance. During this period, the total wind erosion amount for EICP sand stabilization started to exceed that of the SPEV. For instance, in the group with a sea salt solution concentration of 0.2 mol·L⁻¹, the total wind erosion amount for soils with vegetative sand control was lower than that for EICP sand stabilization. This suggests that vegetation alters the upper soil structure, effectively reducing wind speed over the sand surface and thereby enhancing its resistance to wind erosion.

3.3.3. Microscopic Surface Characterization of EICP-Cemented Sand

Spraying the EICP solution onto the surface of the cured sand layer bonds the loose sand particles together to form an integrated whole, which is the foundation for reducing soil wind erosion. The scanning electron microscope (SEM) can be used to magnify the surface of the cured sand layer at different scales to observe the microscopic surface conditions and investigate the micro-mechanism of EICP curing. Figure 10a shows that numerous sand particles are cemented together on the surface, forming a whole. This gives the originally loose sand particles a certain strength, enabling them to resist wind erosion. Concurrently, there are distinct pores between the sand particles on the surface. These pores provide space for the growth of plant roots later. Figure 10b is a local magnification of the sand particle cementation shown in Figure 10a. Significant cementing material can be observed at the contact gaps between the sand particles, binding them together. However, when the gaps between sand particles are large, insufficient cementing material is generated to bind them together, resulting in the formation of large and deep pores. Further magnification of the sand particle contact gaps and the sand particle surface in Figure 10b is shown in Figure 10c. It can be observed that the cementing material between the sand particles contains numerous cracks. This is likely due to the gold sputtering treatment required for SEM scanning, which causes the cementing material to develop cracks. However, the sand particle surfaces also have numerous fine cracks. This reflects that cementing material also forms on the sand particle surfaces, but the amount is less, and it is distributed more thinly on the surface. Therefore, these surfaces also develop cracks, which are finer. The cementation at the sand particle contact gaps in Figure 10b,c is effective cementation. Due to capillary action, the solution is more easily retained in the gaps between the sand particles. Therefore, the cementing material is more easily generated on the sand particle surfaces at both sides of the gaps and then gradually develops. When the cementing material on both sides contacts, it forms an integrated cementation mass, creating a bridging structure that bonds the loose sand particles together into an integrated whole with a certain strength. Simultaneously, some solution also remains on the sand particle surfaces, leading to the generation of cementing material. However, being far from other sand particles, it cannot form adhesion and ultimately becomes ineffective cementation. The final sand particle cementation pattern is shown in Figure 10d.

SEM is utilized to characterize the micro-morphology of the EICP-treated sand, providing insights into the stabilization mechanism. Subsequently, Energy-Dispersive Spectroscopy (EDS) analysis was performed on selected regions of the stabilized surface to investigate the elemental distribution. This analysis not only corroborates the SEM observations but also enables a quantitative assessment of the chemical composition of the cementing products. According to Figure 11a, the actual scan image is divided into regions corresponding to the sand particles and the inter-particle gaps. Figure 11b is the elemental distribution map for silicon. Observing the regions according to this division clearly shows that the sand particle areas are brightly colored, as the sand particles are primarily composed of silicates. In contrast, the inter-particle gap areas show only sporadic distribution, which can be ignored. This indicates that the inter-particle gaps are indeed bound together by cementing material. Figure 11c is the elemental distribution map for magnesium. Besides the sand particle areas, the inter-particle gap areas also contain a significant amount of magnesium. This is because magnesium ions in the seawater are incorporated into stable carbonate mineral lattices derived from urea hydrolysis to form the cementing matrix. This is verified by the high degree of overlap with the carbon element distribution in Figure 11f. Figure 11d is the elemental distribution map for calcium, which shows a low content. This is because seawater is primarily rich in magnesium ions, with calcium ions being relatively less abundant. Figure 11e is the elemental distribution map for nitrogen, reflecting the distribution of catalytic enzymes in the EICP process. The enzymes are distributed on both the sand particle surfaces and within the gaps. They catalyze the hydrolysis of urea and the reaction with magnesium ions to form cementing material. Therefore, the cementing material is distributed on both the sand particle surfaces and within the gaps, forming effective and ineffective cementation.

According to the EDS analysis (Figure 12) of the mineralized sand surface, the elemental composition provides direct evidence of the biomineralization products and the organic binding matrix. Carbon (30.1 wt%), oxygen (22.7 wt%), and silicon (17.0 wt%) exhibit the highest proportions, primarily originating from the induced carbonate precipitates and the baseline SiO₂ substrate of the coastal sand particles. Elements such as chlorine (9.8 wt%), sodium (4.5 wt%), sulfur (6.0 wt%), and potassium (1.6 wt%) represent the crystallized residual salts inherited from the seawater solvent used in the EICP process. Notably, the simultaneous detection of magnesium (1.7 wt%) and calcium (0.5 wt%) confirms the incorporation of marine divalent ions into the multi-ion carbonate lattices. Furthermore, a substantial proportion of nitrogen (5.9 wt%) was detected on the mineralized surface. This high nitrogen signal serves as direct microchemical evidence of the residual soybean hull organic matrix (e.g., soluble proteins and lipids) and urea hydrolysis byproducts adsorbed onto the sand matrix, thereby justifying the nutrient-enrichment capability of the crude EICP solution. The co-existence of these mineral crystals and the organic matrix substantiates the efficacy of seawater-based EICP in forming a robust, self-sustaining intergranular bonding network.

4. Discussion

4.1. Dual Role of Seawater: Balancing Stabilization Strength and Ecological Viability

The integration of seawater into the EICP–vegetation system introduces a complex trade-off between mechanical reinforcement and ecological sustainability. As a sustainable solvent, seawater provides essential Ca²⁺ and Mg²⁺ ions that act as precursors for calcium carbonate precipitation. However, the concurrent presence of high salinity (primarily NaCl) poses significant physiological challenges to the vegetation. Our multi-dimensional analysis of plant growth metrics (Figure 8, Figure 9 and Figure 10) reveals that the ecological viability of the system is highly sensitive to the seawater salt concentration. At low concentrations (≤0.2 mol·L⁻¹), the salinity does not significantly impede growth, allowing for optimal vegetation coverage (VC) and Leaf Area Index (LAI). In this synergistic zone, the EICP-induced crust formation enhances stabilization strength by binding sand particles into a wind-resistant matrix, while the increased Root Area Index (RAI) provides secondary biological anchoring. This balance aligns with the findings of Mortensen et al. [44] and Cheng et al. [45], who reported that moderate salinity and multi-ion environments do not severely inhibit ureolytic pathways but can instead alter the crystal morphology, leading to more compact mineral packing. Conversely, when the salinity exceeds the critical tolerance of Kalimeris indica (>0.6 mol·L⁻¹), the osmotic stress leads to a sharp decline in biomass and RAI, effectively creating an ecological bottleneck. This adverse threshold effect echoes the ecological observations by Tobe et al. [54] and Bybordi [55], where elevated osmotic potential restricts water uptake and nutrient assimilation, severely impeding radicle elongation and root network architecture. Although higher seawater concentrations theoretically offer more calcium sources for mineral precipitation, the resulting loss of root reinforcement and vegetation cover diminishes the overall long-term integrity of the coastal dune. Furthermore, as noted by Giri et al. [41] and Yang et al. [46], excessive baseline salinity and competing magnesium ions can distort the kinetics of pure calcite nucleation, potentially precipitating more soluble carbonate phases or delaying strength development. Therefore, the efficacy of seawater-based EICP depends on identifying an optimal saline window where the gains in mineralized crust strength outweigh the biological costs of salt inhibition, ensuring a robust and self-sustaining coastal defense system.

4.2. Analysis of the Improvement in Wind Erosion Resistance of Sand by Combined Vegetation and EICP Protection

Compared to untreated sand, wind erosion tests demonstrate that both EICP and vegetative stabilization methods significantly enhance the wind erosion resistance of sandy soils. Notably, for SPEV, the combined application of EICP and vegetation substantially improves wind erosion resistance, as illustrated in Figure 8 and Figure 9. However, a comprehensive analysis of the synergistic effects of EICP and vegetative sand control when used together remains lacking. Therefore, based on the experimental results, a detailed analysis of the performance of different stabilization treatments has been conducted, as presented in

Table 3. Calculated and measured values of wind erosion resistance for SPEV.

Group	$\mathbf{\Delta }\mathit{V}(\mathit{g})$	$\mathbf{\Delta }\mathit{E}(\mathit{g})$	$\mathbf{\Delta }\mathit{C}(\mathit{g})$	$\begin{array}{l}\mathbf{\Delta }{\mathit{C}}^{\prime }\mathbf{(}\mathit{g})\\ =\mathbf{\Delta }\mathit{V}+\mathbf{\Delta }\mathit{E}\end{array}$	$\begin{array}{l}\mathbf{\Delta }\mathit{L}(\mathit{g})\\ =\mathbf{\Delta }\mathit{C}-\mathbf{\Delta }{\mathit{C}}^{\prime }\end{array}$
G2	274.10	636.70	759.20	910.80	−151.60
	274.10	899.94	955.49	1174.04	−218.59
	274.10	1140.54	1152.14	1414.64	−262.54
G3	274.10	1140.54	1152.14	1414.64	−262.54
	274.10	1195.30	1196.80	1469.40	−272.60
	274.10	1214.10	1211.90	1488.20	−276.30
G4	274.10	1140.54	1152.14	1414.64	−262.54
	274.10	1198.50	1200.90	1472.60	−384.70
	274.10	1219.50	1219.90	1493.60	−386.70
G5	274.10	1140.54	1152.14	1414.64	−262.54
	274.10	1206.00	1206.80	1480.10	−386.30
	274.10	1220.30	1220.40	1494.40	−387.00

.

In Table 3, the change in wind erosion amount due to plant sand control, denoted as $\Delta V$, is calculated by subtracting the wind erosion amount after plant stabilization from the 15 min wind erosion amount of the bare sand (1221 g). Similarly, $\Delta E$ and $\Delta C$ are derived by subtracting the wind erosion amounts after EICP and SPEV stabilization, respectively, from the initial 15 min wind erosion amount of the bare sand. It is evident from Table 3 that the computed value $\Delta C^{\prime }$ exceeds the measured value $\Delta C$, indicating that the wind erosion control effect of SPEV stabilization is not simply the sum of the effects of plant stabilization and EICP stabilization. In SPEV samples, EICP cements the loose sand particles together, enhancing their wind erosion resistance and providing a stable environment for early plant growth. When plant roots begin to grow within the pores of the cured sand particles, they partially disrupt the overall integrity of the cured sand, leading to a reduction in the wind erosion resistance provided by EICP. However, the growth of plant foliage (stems and leaves) at the surface reduces the wind speed over the sand surface, improves the wind field conditions, and consequently significantly enhances the overall wind erosion resistance of the sample. Consequently, the combined wind erosion control effect of plants and EICP in SPEV is less than the sum of their individual effects. Therefore, the wind erosion control effect of SPEV should be determined by $\Delta V$, $\Delta E$, and the reduction $\Delta L$ caused by the combined effects of EICP and plants, as represented by the following equation:

$$\Delta C=\Delta V+\Delta E-\Delta L$$

(3)

Specifically, as the efficacy of EICP in sand stabilization increases, the reduction ($\Delta L$) achieved through the combined application of EICP and vegetation gradually grows and stabilizes. This phenomenon is particularly evident in the comparison between $\Delta E$ and $\Delta C$. In the groups where EICP demonstrates more pronounced sand stabilization effects, the data differences between the two parameters are nearly negligible. This indicates that the EICP technology plays a more central role in enhancing the overall stability of the sand.

4.3. Sustainability and Resource-Efficiency Analysis

In response to the growing demand for sustainable coastal engineering, the resource-efficiency of the proposed EICP–vegetation method was evaluated relative to conventional stabilization techniques. The economic and ecological advantages are primarily anchored in a local-sourcing and waste-valorization model. Firstly, the utilization of soybean hulls—a low-value agricultural byproduct—as the urease source represents a strategic shift from the high-cost reliance on industrial-grade enzymes or food-grade soybeans. This transition not only minimizes material expenditures but also aligns with the principles of the circular economy by repurposing waste into high-value engineering precursors. Secondly, the in situ application of seawater significantly reduces the logistical complexity and carbon footprint associated with the transport of industrial calcium salts to remote coastal sites. While the concentration of seawater via natural evaporation in shallow salt pans requires time, its operational cost is negligible compared to the purchase and delivery of chemical reagents. Furthermore, unlike traditional chemical stabilizers that often lead to soil deadening and necessitate expensive subsequent remediation, this method fosters a self-sustaining ecosystem. The synergistic effect of the mineralized crust and the biological anchoring of Kalimeris indica provides a long-term, low-maintenance solution for wind erosion control. This ecological viability aligns with the field-scale observations of Meng et al. [50] and the restoration models of Li et al. [48], which demonstrated that biomineralized crusts provide a transient buffer matrix that resists wind shear while gradually degrading to allow root penetration and organic matter accumulation. By optimizing the resource-efficient use of local precursors, this approach offers a resilient and cost-effective framework for large-scale ecological restoration in saline–alkali coastal environments.

4.4. Limitations and Future Research Directions

While the laboratory results demonstrate the short-term efficacy of the seawater-based EICP–vegetation system, several inherent limitations must be acknowledged for its transition to large-scale coastal applications. First, long-term durability under dynamic conditions remains a challenge. Environmental stressors such as rainfall and tidal action may lead to the gradual physical or chemical degradation of the mineralized crust. This is particularly relevant when using seawater, as the presence of Mg²⁺ ions may result in carbonates with lower thermodynamic stability compared to standard CaCO₃. Second, the environmental safety of the technology requires further transparent evaluation. The urea hydrolysis process inevitably induces temporary pH elevation and ammonium (NH₄⁺) accumulation. Although direct tracking of ammonia volatilization kinetics was not executed, the robust germination and sustained growth metrics of Kalimeris indica observed within the tested crude enzyme range macroscopically demonstrate that byproduct concentration remained within the ecological tolerance threshold, potentially mitigated by the sandy substrate’s diffusion capacity. Finally, due to experimental equipment constraints, the wind tunnel testing simplified the complex, log-by-height natural boundary layer wind field into a uniform 20 m·s⁻¹ flow. Although this uniform boundary condition provides a rigorous, conservative engineering safety margin by directly subjecting the specimen surface to maximum velocity, it limits the direct scalability of the laboratory metrics to real-world coastal aerodynamics. To address these inherent material and boundary limitations, we propose a stage-based protection strategy to ensure the overall viability of the system. In this framework, the EICP crust serves a dual purpose during the critical early stages: it acts as an immediate erosion-resistant barrier for the bare sand while simultaneously providing the necessary surface stability and moisture retention for Kalimeris indica to establish itself. As the crust undergoes natural weathering and physical degradation, the stabilization mechanism is supplemented by the developing canopy and root system, which provide near-surface wind velocity attenuation and deep-seated biological anchoring, respectively. For unexpected extreme events such as typhoons, the system’s resilience can be further maintained through targeted re-spraying to reinforce the surface barrier. Future research should focus on integrating field-scale boundary layer wind fields, quantifying nitrogen transformation kinetics, and optimizing the precise timing of this functional transition to enhance the long-term sustainability of coastal ecological restoration.

5. Conclusions

The aim of this study is to propose an EICP sand solidification technique using soybean hulls as the urease source and seawater as the calcium source, and to investigate the effects of this technique on plant growth performance and the wind erosion resistance of sandy soil. The research focuses on analyzing the effects of urease activity, the volume of the reaction liquid, and seawater salt concentration on SGP, SGS and seedling growth of Kalimeris indica plants. Finally, the experimental setup was subjected to a wind speed of 20 m·s⁻¹ to assess the wind erosion resistance of the treated soil. Based on the results and discussion, the following conclusions can be drawn:

• Urease Extraction and Activity: EICP using soybean hulls and seawater is a resource-efficient method for sand stabilization. Urease activity increases linearly with soybean hull dosage but is significantly inhibited at low temperatures, showing a rapid increase between 10 and 55 °C. Extraction parameters (centrifugation time/speed) have negligible effects on activity.
• Seawater Concentration Thresholds: Seawater salinity is a determinant for ecological viability. Low concentrations (0.2–0.6 mol·L⁻¹) enhance the seed germination potential (SGP). Notably, while the biochemical mineralization process facilitates the transition of soluble Ca²⁺ and Mg²⁺ into solid mineral phases (reducing localized osmotic stress), seawater levels exceeding 0.6 mol·L⁻¹ markedly inhibit growth, with SGP approaching zero at 1.4 mol·L⁻¹.
• Optimal Growth Parameters: Plant growth status (RAI, LAI, and VC) peaks at a seawater concentration of 0.2 mol·L⁻¹ and a reaction liquid volume of 200 mL. While increased liquid volume initially promotes growth, excessive volumes can lead to inhibition. Higher urease activity consistently improves seedling vigor.
• Wind Erosion Resistance: Sandy soil Protected by EICP and Vegetation (SPEV) outperforms individual EICP or vegetation methods, reducing total erosion by approximately 92.3% compared to untreated sand. Although EICP crusts provide superior early-stage protection, SPEV exhibits higher long-term resilience as erosion progresses.
• Mineralization Mechanism: SEM and EDS analyses confirm that the use of seawater produces a mixture of calcium carbonate and magnesium carbonate (with magnesium content reaching approximately 1.7 wt%). These minerals effectively cement intergranular gaps through ionic sequestration, enhancing erosion resistance while maintaining pore space for root development. Despite the detection of nitrogenous byproducts (e.g., NH₄⁺), the plant–soil system maintains its ecological integrity within the tested dosage.
• Stage-based Protection Logic: Although plant growth partially disrupts the EICP crust, the crust provides a critical temporary protective window. This acknowledges the inherent limitation of crust durability while highlighting the functional transition from artificial mineralization to biological anchoring. This strategy effectively balances the immediate erosion resistance provided by mineral cementation with the long-term environmental safety of the biological restoration.
• Field Application Recommendations: Under wind speeds up to 20 m·s⁻¹, a minimum EICP dosage of 300 mL is recommended for immediate sand stabilization. In hypersaline environments exceeding the tolerance threshold of Kalimeris indica (>0.6 mol·L⁻¹ under current testing conditions), supplementary freshwater irrigation or re-spraying is essential to mitigate osmotic stress and sustain long-term ecological viability.