Experimental Study on the Reinforcement of Calcareous Sand Using Combined Microbial-Induced Carbonate Precipitation (MICP) and Festuca arundinacea Techniques

Abstract

Combining the Microbial-Induced Calcium Carbonate Precipitation (MICP) technique with plants to reinforce calcareous sand slopes on reef islands is expected to achieve both reinforcement and economic benefits. In this study, MICP was combined with Festuca arundinacea (MICP-FA) for calcareous sand reinforcement. Based on water retention and scanning electron microscopy (SEM) tests, the water retention performance and mechanism of MICP-reinforced calcareous sand under different cementation solution concentrations and cementation cycles were analyzed. The growth adaptability of Festuca arundinacea was evaluated under different bacteria solution concentrations, cementation solution concentrations and cementation cycles. The engineering applicability of MICP-FA-reinforced calcareous sand was evaluated by wind erosion tests, and the synergistic reinforcement mechanism was analyzed. The results show that with the increase in the cementation solution concentration and cementation cycles, more calcium carbonate filled and adhered to the calcareous sand particles, significantly improving the water retention performance. MICP-FA can enhance the wind erosion resistance of calcareous sand. This synergistic mechanism lies in the surface cementation effect of MICP and the deep anchoring effect of plant roots. This study provides theoretical basis and technical guidance for engineering applications in calcareous sand areas.

1. Introduction

With the vigorous development of reef construction, the stability and sustainability of reef structures have drawn extensive attention. As a natural foundation material, calcareous sand is widely applied for constructing reef infrastructure, including slopes, airport runways, and military bases [1,2,3,4,5]. Due to its characteristics of irregular particle shapes, high crushability, and high porosity [6,7,8,9,10,11], calcareous sand is prone to erosion under hydraulic and aeolian forces, which renders the slopes constructed with it susceptible to reduced stability and subsequent collapses or landslides. Conventional reinforcement methods such as chemical grouting [12,13] and cement stabilization [14,15] are associated with inherent drawbacks, including high construction costs, substantial energy consumption, and significant environmental pollution. Therefore, there is an urgent need to explore more environmentally friendly methods for reinforcing calcareous sand slopes on reefs.

Microbial-Induced Calcite Precipitation (MICP), a novel soil stabilization technology, has been applied to foundation treatment research [16,17,18,19] and is characterized by low cost, low energy consumption, rapid reaction kinetics, and environmental friendliness [20,21]. The primary mechanism of action [22,23,24,25,26,27] involves microbial production of urease during metabolism, which catalyzes the decomposition of urea to generate ${\mathrm{CO}}_3^{2-}$ ions. These ions then react with ambient Ca²⁺ ions to form CaCO₃ precipitates through the following chemical reaction equations:

$${\mathrm{CO}\left({\mathrm{NH}}_2\right)}_2+2{\mathrm{H}}_2\mathrm{O}\to 2{\mathrm{NH}}_4^{+}+{\mathrm{CO}}_3^{2-}$$

(1)

$${\mathrm{Ca}}^{2+}+{\mathrm{CO}}_3^{2-}\to {\mathrm{CaCO}}_3$$

(2)

Scholars have applied MICP technology to soil stabilization to enhance erosion resistance. Li et al. reported that as the number of cementation cycles increases, sand particles are encapsulated by precipitated calcium carbonate generated from microbial reactions, thereby enhancing the mechanical strength of the sand matrix [28]. Chen et al. found that after MICP treatment, a dense hard-shell layer composed of calcium carbonate and soil particles formed on the surface of the specimen, which effectively enhanced the strength and scouring resistance of the sandy soils [29]. He et al. found that the MICP technique bonded loose sand particles with piles to form bio-enhanced piles with larger diameters in the shallow soil layer, which significantly improved the bearing capacity and strength of the original piles [30]. Zhang et al. found that MICP can enhance the shear strength of rock joints. The peak shear strength increased with curing time [31]. Liu et al. treated loess samples with bacterial solutions and cementing solutions of varying concentrations, resulting in a significant improvement in the mechanical properties of the treated loess [32]. Liu et al. treated sandy slopes by varying the Ca²⁺ concentration in the cementing solution, demonstrating that slopes treated with higher concentration exhibited superior erosion resistance compared to those treated with lower concentrations [33]. Huang et al. reinforced slopes using MICP technology, demonstrating that cemented sands remained intact under simulated rainfall erosion and maintained enhanced slope integrity [34]. Dagliya et al. demonstrated that cementing solutions can enhance the strength of desert sands. Through comparative SEM analysis of MICP-treated and untreated samples, crystalline growth was observed in interparticle pores of the sand matrix. Based on these findings, temperature effect analysis was conducted to lay a foundation for field-scale wind erosion tests [35]. Qu et al. reported that precipitated calcium carbonate generated by MICP reactions is distributed on soil particle surfaces and within interparticle pores, enabling aeolian sands to exhibit satisfactory wind erosion resistance at wind speeds of 13 m/s [36]. Hang et al. demonstrated that with increasing temperature and cementation solution concentration, the surface penetration resistance of the specimens was significantly enhanced, leading to improved wind erosion resistance [37].

MICP technology is affected by many factors due to its complex biochemical process, which leads to differences in the engineering properties of MICP-treated soils, so many scholars have performed many studies on the factors affecting MICP. Wang et al. observed the effect of bacterial density on the growth rate and properties of precipitates based on the pore size model of microfluidics, and concluded that the precipitation rate of calcium carbonate increases with the increase in bacterial density, and that bacterial density also has an effect on the size and number of calcium carbonate crystals [38]. Jiang et al. treated artificial slopes with three cementation solution concentrations (0.2, 1.0, and 2.0 mol/L), and after spraying the slopes for 30 min at a rainfall rate of 5 mm/min, it was concluded that the resistance of the slopes with the 0.2 mol/L and 1.0 mol/L treatments to erosion resistance was significantly enhanced, while a large amount of soil was lost from the 2.0 mol/L-treated slopes and the erosion resistance was not improved [39]. Li et al. studied the effects of bacterial concentration, urea concentration, mass concentration, curing temperature, and curing time on the compressive strength based on the microbial-induced calcite precipitation (MICP) technology. The study found that all these factors can enhance compressive strength; however, an excessively high urea concentration and curing temperature will lead to a decrease in compressive strength [40].

According to the above studies, although MICP technology can significantly improve soil strength and stiffness [41,42,43,44,45] and enhance erosion resistance, the reinforcement effect is susceptible to a variety of factors such as bacteria solution concentration, temperatures, and cementing liquids, which makes the treated specimens prone to uneven curing strength. Therefore, its application in large-scale reef slope reinforcement is constrained by increased treatment costs and limited treatment depth.

Vegetative slope stabilization, a sustainable slope protection technique combining slope reinforcement and ecological restoration [46,47], influences slope stability primarily through mechanical and hydrological reinforcement [48,49,50]. Plant roots intertwine with soil particles to form root-soil composites, enhancing soil strength [51,52] and improving slope reinforcement. Rahman et al. demonstrated that plant roots enhance soil strength in slopes through mechanical interlocking between roots and soil particles [53]. Cardoza et al. demonstrated that the strength of natural silty sand soil was significantly increased when reinforced with plant roots [54]. Festuca arundinacea, a commonly used slope protection plant, features an extensive root system, heat tolerance, drought resistance, and cost-effectiveness [55,56]. During plant growth, the soil is blown by wind erosion, covering and burying plant seedlings, which can easily cause seedling death [57], while MICP technology can stabilize the soil surface layer to provide protection during the pre-growth period. Therefore, integrating MICP technology and vegetative reinforcement can achieve dual objectives of soil reinforcement, erosion resistance enhancement, and environmental remediation.

This study investigates the feasibility and reinforcement mechanism of combining MICP with Festuca arundinacea (MICP-FA) for calcareous sand reinforcement. By systematically investigating the effects of bacterial concentration, cementation solution concentration, and cementation cycles on soil water retention capacity and Festuca arundinacea growth performance, the optimal growth conditions for the vegetation will be determined. Wind erosion tests will then be conducted to identify optimal reinforcement parameters, followed by a mechanistic analysis of the combined MICP-FA treatment. The findings of this research are expected to provide a novel eco-friendly solution for sustainable infrastructure reinforcement in reef slope engineering.

2. Materials and Methods

2.1. Test Materials

(1) Sand selection: The sand used in the experiments was calcareous sand collected from a reef island in the South China Sea. The dry density of the specimens ranged from 1.22 to 1.65 g/cm³, with a void ratio between 0.63 and 1.29. The specific gravity of the sand was determined to be 2.69. The particle size distribution is presented in Figure 1a.

(2) Experimental grass seeds: The germination quality of Festuca arundinacea seeds (Jiangsu Yunzhigu Landscape Engineering Co. Ltd., Suzhou, China) was evaluated using the paper towel method. Germination performance is shown in Figure 1b. Seedlings emerged starting on day 2 of the experiment, and no further germination was observed after day 8, yielding a germination rate of 96.67%. These results indicate that the purchased seeds met high quality standards.

(3) Bacterial strain and cementation solution: The Bacillus pasteurii strain BNCC337394, obtained from the BeNa Culture Collection (Guangdong Province Microbial Culture Collection Center, Guangzhou, China), was used in this study. The strain was preserved as lyophilized powder in ampoules via vacuum-drying. The procedures for bacterial activation, cultivation, and storage are illustrated in Figure 1c,d. The cementation solution was prepared by mixing calcium chloride (CaCl₂, Shanghai Xilong Chemical Co. Ltd., Shanghai, China) and urea (Shanghai Aladdin Biochemical Technology Co. Ltd., Shanghai, China) at a molar ratio 1:1. The solution must be allowed to cool before being combined with the urea solution because the dissolution of calcium chloride in water is an exothermic reaction. These reagents provide ${\mathrm{CO}}_3^{2-}$ and Ca²⁺ ions for microbial-induced calcite precipitation. The cementation solution preparation process is shown in Figure 1e.

2.2. Test Methods

2.2.1. Water Retention Test

This study systematically analyzed the effects of cementation solution concentration and cycles on water retention capacity by applying different treatment protocols. A pressurized spray bottle was used to uniformly apply bacterial and cementation solutions at 0.1 mL/cm² and 0.2 mL/cm², respectively. Each cementation cycle consisted of (1) spraying bacterial solution followed by a 12 h rest and (2) spraying cementation solution followed by a 24 h reaction. Specimen labeling and testing procedures are detailed in

Table 1. Specimen labeling scheme for water retention tests.

	CC	1 Cycle	2 Cycles	3 Cycles	4 Cycles
CSC (mol/L)
0 mol/L		J11	J12	J13	J14
0.1 mol/L		J21	J22	J23	J24
0.25 mol/L		J31	J32	J33	J34
0.4 mol/L		J41	J42	J43	J44
0.5 mol/L		J51	J52	J53	J54

(Note: Each treatment group was replicated twice. Cementation solution concentration is abbreviated as CSC; cementation cycles is abbreviated as CCs. The first number in the labeling scheme J21 indicates that the cementation solution concentration is 0.1 mol/L, and the second number indicates that the number of cementation cycles is one.)

and Figure 2.

2.2.2. SEM Analysis

This experiment observes and analyzes the microstructure of the cementation on the specimen surface through a field emission scanning electron microscope (SEM) with the model JSM-7610FPlus (JEOL Ltd., Tokyo, Japan.).

2.2.3. Growth Adaptability Test of Festuca arundinacea

This study aimed to investigate the effects of bacterial concentration, cementation solution concentration, and cementation cycles on the growth characteristics of Festuca arundinacea (including emergence rate, root length, shoot height, and root–shoot ratio) and determine the optimal treatment conditions for plant growth. The emergence rate was calculated as the percentage of germinated seeds relative to the total number of tested seeds after 28 days of experimentation. Root length was defined as the total length of root systems below the rhizome. Shoot height was measured as the distance from the rhizome emerging above the soil surface to the plant apex. The root–shoot ratio was calculated as the biomass ratio of root systems to above-ground parts (including stems, leaves, flowers, and fruits).

Root length: In combination with the software ImageJ (version 1.52a), we measured the length of the root system of each Festuca arundinacea plant with a ruler and calculated the average value.

Shoot height: In combination with the software ImageJ (version 1.52a), the length of each Festuca arundinacea plant was measured from the surface of the soil to the top of the plant with a ruler, and the average value was calculated.

Root–shoot ratio: We separated the root system of Festuca arundinacea from the above-ground part, put it into the oven at 105 °C for 30 min to kill the green treatment, and then transferred it into the oven at 80 °C to dry to the constant weight, and then we weighed the root system and the above-ground part with the balance, and then calculated the ratio of the two as the root–shoot ratio.

In the experiment, bacterial and cementation solutions were applied using a pressurized spray bottle at 0.1 mL/cm² and 0.2 mL/cm², respectively. After curing, deionized water was sprayed onto the soil surface at 0.3 mL/cm² every other day to provide moisture for plant growth.

Table 2. Specimen labeling scheme for growth adaptability tests.

	BSC	Y	xs10	xs20
CSC (mol/L)
0 mol/L		J11-n	J21-n	J31-n
0.1 mol/L		J12-n	J22-n	J32-n
0.25 mol/L		J13-n	J23-n	J33-n
0.4 mol/L		J14-n	J24-n	J34-n
0.5 mol/L		J15-n	J25-n	J35-n

(Note: cementation solution concentration is abbreviated as CSC; bacterial solution concentration is abbreviated as BSC; Y, xs10, and xs20 represent bacterial concentrations with OD₆₀₀ values of 2.2, 0.22, and 0.11, respectively. “n” represents cementation cycles (1–4). The first number in the labeling scheme J12-n indicates that the bacteria solution concentration OD600 value is Y, i.e., 2.2, the second number indicates that the cementation solution concentration is 0.1 mol/L, and “n” indicates the different cementation cycles).

and Figure 3 present the specimen labeling and testing procedures.

2.2.4. Wind Erosion Test

The wind erosion device used in this experiment is shown in Figure 4. The model of the fan (KALAIKE Pump Industry (Zhejiang) Co. Ltd., Wenling, China) is SF4-4, and the model of the anemometer (UNI-TREND Technology (CHINA) Co. Ltd., Dongguan, China) is UT363-BT. Xu et al. selected representative sites on the South China Sea reefs. Based on data from the National Oceanic and Atmospheric Administration (NOAA) of the United States from 2006 to 2016, their analysis revealed that the lowest annual average wind speed at the sites was 3.5 m/s, and the highest was 5.9 m/s [58]. According to this, different test wind speeds of 3 m/s, 5 m/s, and 10 m/s were set in the experiment. The specimens under optimal treatment conditions were determined by analyzing the changes in the specimens’ mass over time and the mass loss rate at the end of the experiment.

$$\mathrm{Percentage} \mathrm{mass} \mathrm{loss} \mathrm{due} \mathrm{to} \mathrm{wind} \mathrm{erosion}\left(\%\right)={\displaystyle \frac{{\mathrm{m}}_1{-\mathrm{m}}_2}{{\mathrm{m}}_1}}\times 100\%$$

(3)

In the formula, m₁ is the mass of the sample before wind erosion and m₂ is the mass of the sample after wind erosion.

3. Results and Discussion

3.1. Effects of Cementation Solution Concentration and Cycles on Water Content

The results in Figure 5 demonstrate that water content in the soil gradually decreased over time. Increasing the cementation solution concentration within the same cementation cycle led to a higher water retention capacity. Moreover, the decline in water content slowed significantly as the number of cementation cycles increased. Increasing the cementation solution concentration [59] and cementation cycles [60] increased calcium carbonate precipitation. More loose calcareous sand particles were cemented into an integrated structure, and the compactness of the sand increased. This effectively inhibited water evaporation and improved soil water retention capacity. Among all groups, the untreated control (CK) showed the most significant water loss, while the specimen treated with 0.5 mol/L cementation solution for four cycles achieved the highest water content, demonstrating a 56.72% improvement compared with the CK group.

These results indicate that MICP technology significantly enhances soil water retention capacity [61]. This enhancement makes it easier for plants to gain access to enough water during their growth and development, which benefits the plants’ overall development.

3.2. Microstructural Analysis

Figure 6 shows SEM images of calcareous sand treated with 0.1 mol/L and 0.5 mol/L cementation solutions for one and four cycles. The images reveal calcium carbonate precipitation on sand surfaces increases with cementation solution concentration and cycles. This is attributed to the enhanced calcite formation resulting from higher reagent dosages and repeated MICP reactions, which improve particle bonding and water retention capacity.

3.3. Impact on Festuca Arundinacea Growth

As shown in Figure 7 and Figure 8, the seedling emergence rate of the untreated control (CK) was 43%, indicating that Festuca arundinacea can generally grow in calcareous sand substrates. Notably, when the cementation solution concentration was 0 mol/L (i.e., only bacterial solution was applied), specimens treated with low bacterial concentrations (xs10, xs20) exhibited higher emergence rates than CK. This suggests that low bacterial concentrations promote plant growth, likely due to urea hydrolysis by microbes generating NH₄⁺, an essential nitrogen source for plant development [62].

Figure 8 illustrates that the emergence rate of the specimens decreased as the number of cementation cycles and the concentration of the cementation solution increased. Taking J23-3 (a cementation solution concentration of 0.25 mol/L, bacteria solution concentration of xs10, and three cementation cycles) in Figure 8d as an example, it can be seen in conjunction with Figure 7 that the emergence rate of the specimens under this treatment condition was 18%, which was 58.14% lower than that of the CK group. The reason is that as they increase, more calcium carbonate is generated in the reaction, causing more loose calcareous sand to be cemented into a whole, which increases the strength of the sample [63]. This makes it difficult for the seeds to break through the soil, resulting in a decrease in the seedling emergence rate. As shown in Figure 7, the seedling emergence rate declined significantly when the cementation solution concentration exceeded 0.25 mol/L, whereas the suppressive effect was relatively mild at 0.1 mol/L. At this concentration, treatments with 1–2 cementation cycles and bacterial concentrations Y, xs10, xs20 (specimen IDs: J12-1, J22-1, J32-1, J22-2, J32-2) increased emergence rates by 16.28%, 18.6%, 30.23%, 3.49%, and 4.65%, respectively, compared to the CK.

As shown in Figure 7, seedling emergence rates were extremely low or zero in specimens treated with ≥0.4 mol/L cementation solution and three cementation cycles. Therefore, these concentrations were excluded from subsequent experiments.

As shown in Figure 9a, the mean root length of Festuca arundinacea decreased with increasing bacterial concentration, cementation solution concentration, and cementation cycles. This is primarily due to the progressive filling of soil pores by calcium carbonate precipitates, which increases soil compactness [64]. Consequently, restricted growth space in the densified matrix impeded root elongation. Specimens treated with 0.1 mol/L cementation solution exhibited a less inhibitory effect on root length than higher concentrations.

The effect of different treatments on the average shoot height of Festuca arundinacea is presented in Figure 9b. Lower bacterial concentrations promoted shoot elongation, with the xs20 bacterial treatment combined with three cementation cycles showing a 16.79% increase compared to CK. At consistent bacterial concentrations and cementation cycles, shoot height progressively decreased as the cementation solution concentration increased, with higher cycles exacerbating this decline relative to CK. This phenomenon is attributed to the nitrogenous compounds generated during microbial growth acting as fertilizer to enhance plant development, while excessive nitrogen supply inhibited nutrient uptake balance. Additionally, increased alkalization of soil matrices with higher reagent dosages and cycles negatively impacted plant growth.

As shown in Figure 10, within the cementation solution concentration range of 0.1–0.5 mol/L, the root–shoot ratio of Festuca arundinacea generally decreased as the concentration increased, except for a notable upward trend observed for one cementation cycle. Taking the Y bacterial concentration combined with one cementation cycle as an example, specimens treated with 0.1–0.5 mol/L cementation solution showed increases of 0.94%, 1.18%, 9.22%, and 28.98% in the root–shoot ratio compared to the CK. This phenomenon arises from the altered osmotic pressure in soil solutions caused by high cementation solution concentrations, impairs shoot water and nutrient uptake. Roots exhibit enhanced growth to compensate, leading to an increased root–shoot ratio. With additional cementation cycles, accumulating calcium carbonate precipitates and densifies the matrix, reducing porosity and degrading soil aeration and water permeability. This restricts root growth space and impedes respiration, slowing root elongation. As shoot development is less affected, the root–shoot ratio stabilizes or decreases.

In summary, to balance soil reinforcement and promote Festuca arundinacea growth, specimens treated with 0.1 mol/L cementation solution were selected. Although low bacterial concentrations enhanced plant development, diluting bacterial solutions reduced overall urease activity, decreasing ${\mathrm{CO}}_3^{2-}$ production and calcium carbonate precipitation [65] and compromising reinforcement efficacy. Increasing cementation cycles augmented calcite formation. Therefore, the optimal treatments were (1) J12-1: Y bacterial concentration +0.1 mol/L solution +1 cycle. (2) J22-3: xs10 bacterial concentration +0.1 mol/L solution +3 cycles. (3) J32-3: xs20 bacterial concentration +0.1 mol/L solution +3 cycles.

3.4. Wind Erosion Test Results

Based on previous experiments, the optimized treatments combining soil reinforcement and Festuca arundinacea growth promotion were identified as J12-1, J22-3, and J32-3. Figure 11 shows the time-dependent mass changes in these specimens and their wind erosion mass loss rates after 20 min erosion at varying wind speeds. As revealed in Figure 11a, specimen mass increased progressively over time, with more pronounced upward trends at higher wind speeds. Figure 12 demonstrates that at low wind speeds, surface erosion was negligible, whereas higher velocities caused significant surface damage, which was particularly severe in the CK group. These results indicate that MICP-FA-treated specimens exhibit superior wind erosion resistance, confirming effective reinforcement of calcareous sand. This improvement is attributed to vegetation increasing surface roughness to reduce wind velocity [66], root systems stabilizing soil through the anchorage and reinforcement mechanisms, and MICP-treated surfaces forming mineralized layers that resist erosive forces.

As shown in Figure 11b, wind erosion mass loss rates increased with rising wind speeds for all three groups (J12-1, J22-3, J32-3), though the J12-1 group demonstrated a significantly gentler growth trend. From 3 m/s to 5 m/s, J12-1’s mass loss rate increased from 0.37% to 1.48% (Δ1.11 percentage points), while J22-3 and J32-3 only experienced 0.3 percentage point increases. During this phase, J22-3 and J32-3 showed smaller increments, but when wind speed rose from 5 m/s to 10 m/s, J12-1, J22-3, and J32-3 increased by 0.37, 1.1, and 1.48 percentage points. Notably, J22-3 and J32-3 exhibited much steeper increases than J12-1 at high wind speeds. This indicates that J12-1 maintained consistently lower mass loss rates across all wind speeds, demonstrating superior stability in wind erosion resistance and, thus, better reinforcement efficacy.

Qu et al. conducted a wind tunnel test on MICP-treated wind sand, and when the wind speed was 13 m/s, the wind erosion mass loss rate of the specimen was 63.6% [36]. Chen et al. conducted a wind tunnel test on desert sand treated by combining Caragana korshinskii Kom with SICP technology, and after blowing for 1 min at a wind speed of 15 m/s and cementation solution concentration of 0.1 mol/L, the soil mass loss was 7968 g/min/m² [67]. In this study, under the conditions of wind speed of 10 m/s and 20 min of erosion, the wind erosion mass loss rate of J21-1 group was only 1.85%. This indicates that the MICP-FA technique can significantly improve the wind erosion resistance of calcareous sand soils.

3.5. Synergistic Reinforcement Mechanism Analysis

A schematic diagram of MICP-FA reinforced calcareous sand is presented in Figure 13a. As shown in Figure 13b, calcium carbonate precipitated through MICP reactions fills soil pores, cementing loose particles into an integrated structure and forming a dense mineralized layer. This process enhances soil strength and stability, thereby providing effective reinforcement. As shown in Figure 13c, primary and coarse roots provide anchorage during plant growth, while fine roots offer reinforcement. Root systems interpenetrate soil particles to form a root–soil composite, significantly enhancing overall soil stability [68,69]. Root systems treated with MICP were shorter than untreated roots. Additionally, the root systems of the MICP-treated plants were slightly shorter than those of the untreated group when bacterial solution, cementation solution, and cementation cycles were at their appropriate levels. Excessive reagent dosages and cycles further reduced root length. This reduction is attributed to increased soil strength and density restricting root penetration [70]. The combined application of MICP-FA creates a dual-layer reinforcement system where the surface mineralized layer formed by microbial calcite precipitation provides wind erosion resistance. The underlying root networks mechanically stabilize deeper soil layers through anchorage and interlocking. This integrated mechanism enhances overall soil structural integrity across the entire profile.

4. Conclusions

This study uses MICP-FA technology to reinforce calcareous sand and evaluates the water retention ability of the treated soil, Festuca arundinacea growth adaptability, and wind erosion resistance. The key conclusions derived from the research results are as follows:

(1)

Based on the water retention test and scanning electron microscope images, it can be seen that with the increase in cementation solution concentration and the number of cementation cycles, more calcium carbonate adheres to and fills in the surface and interstitial space of calcareous sand particles, which effectively improves the water retention performance of the calcareous sand soil body. The best water retention performance is achieved with a 0.5 mol/L cementation solution concentration and four cementation cycles, which is 56.72% higher than that of control group.

(2)

Based on the results of Festuca arundinacea growth adaptation test, lower bacterial solution concentration, cementation solution concentration, and cementation cycles promote the growth of Festuca arundinacea, and increasing these parameters gradually inhibits the development of plants.

(3)

According to the results of the wind erosion test, MICP-FA reinforcement can enhance the wind erosion resistance of calcareous sand soil body. Under an OD₆₀₀ = 2.2 bacterial concentration, 0.1 mol/L cementation solution concentration, and one cementation cycle (J12-1), the best wind erosion resistance of the reinforced soil was obtained, and favorable plant growth was maintained. At this time, under a 10 m/s wind speed, the specimen wind erosion mass loss rate was only 1.85%, which was 97.5% lower than that of the control group. (4) MICP-FA reinforcement significantly improves the wind erosion resistance of calcareous sand. The synergistic mechanism includes the surface bonding of MICP and the deep soil anchoring/reinforcement of the Festuca arundinacea root system.

5. Discussion

This study shows that MICP-FA-reinforced calcareous sand can significantly improve the wind erosion resistance of soil. However, this experiment was conducted indoors and has some limitations. Therefore, the following points can be considered in future experiments:

(1)

Only Festuca arundinacea was selected as a test plant in this study, and there are significant differences in its root morphology, secretion and growth habit among different plants, and the synergistic mechanism with MICP technology may be different. The combination of different plants and MICP technology can be considered in the subsequent experiments.

(2)

The wind speed set in the wind erosion test did not adequately simulate the extreme wind speed conditions during typhoons on the South China Sea islands. Further evaluation of the wind erosion resistance of MICP-FA0-reinforced calcareous sand can be considered in the subsequent study by referring to the wind speed of the typhoon in the South China Sea islands and reefs.

(3)

The issue of how the enhancement of soil strength following MICP treatment influences plant root extension and distribution can be measured in greater detail in subsequent experiments.

(4)

This study was carried out under controlled conditions in the laboratory, and the uniformity of the microbial solution and the initial state of the calcareous sand were strictly controlled. However, in actual island projects with a wide range of calcareous sand sites and complex environmental conditions (e.g., temperature), the implementation effect of the MICP-FA technique may change due to scale-up. While plant growth space and conditions are controllable in a laboratory setting, plant root competition, microbial community diversity, and interactions with other environmental factors (e.g., wave wash, tidal action) are more complex in real island ecosystems. It is necessary to conduct simulated field experiments to further investigate the influence of scale changes on the effectiveness of reinforcement, in order to realize an effective transition from the laboratory to the actual project.