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Article

Effect of W-OH Material on Water/Fertilizer Retention and Plant Growth in the Pisha Sandstone Area of China

by
Zhishui Liang
1,2,*,
Yue Sun
1,
Xiuwen Fang
1,
Bo Pan
3,4,
Yuan Xiao
1,
Haiying Gao
1,* and
Zhiren Wu
5
1
Key Laboratory of Concrete and Prestressed Concrete Structures of Ministry of Education, Southeast University, Nanjing 210096, China
2
National and Local Unified Engineering Research Center for Basalt Fiber Production and Application Technology, International Institute for Urban Systems Engineering, Southeast University, Nanjing 210096, China
3
Academy of Forestry, Guangxi Zhuang Autonomous Region, Nanning 530002, China
4
Guangxi Forest New Fertilizer Research and Development Center, Nanning 530002, China
5
School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(15), 6369; https://doi.org/10.3390/su16156369
Submission received: 27 May 2024 / Revised: 18 July 2024 / Accepted: 19 July 2024 / Published: 25 July 2024
Browse Figures
Versions Notes

Abstract

:
The Pisha sandstone area in the Yellow River Basin is one of the regions with the most severe soil erosion in China and globally, and its erosion is particularly challenging to control. W-OH, a hydrophilic polyurethane material, possesses controllable degradation properties. It can react with water to achieve soil stabilization and erosion resistance during the curing process. The material has been successfully utilized in erosion control in Pisha sandstone areas. This study aims to investigate the impact of W-OH material on water/fertilizer retention and plant growth through experiments on soil hardness, permeability, soil evaporation, soil column leaching, pot tests, and a small-scale demonstration in practical engineering applications. The results indicate that different concentrations of W-OH solution can effectively permeate Pisha sandstone, solidifying the particles to create a flexible and porous consolidation layer on the surface with a specific depth. As the W-OH concentration (3%, 4%, and 5%) increases, the harnesses of the consolidation layer also increase but remain below 1.5 kPa, which does not impede plant root growth. The soil evaporation rate decreased by approximately 45.2%, 45.8%, and 50.3% compared to the control group. The reduction rates of cumulative total nitrogen (TN) content are around 43.57%, 48.14%, and 63.99%, and, for cumulative total phosphorus (TP), are approximately 27.96%, 45.70%, and 61.17% under the 3%, 4%, and 5% concentrations of W-OH solution, respectively. In the pot tests, concentrations of W-OH solution below 5% are suitable for germination and growth of monocotyledons, while the optimal concentration for dicotyledons is around 3%. In the demonstration, the vegetation coverage of the treated gully increases by approximately 11.35%. This research offers a promising and effective approach to enhance ecological restoration in Pisha sandstone areas.

1. Introduction

The Pisha sandstone is a general appellation in China for a type of fluvial clastic deposition sandstone formed during the Jurassic, Triassic, and Cretaceous periods [1,2]. It is widely distributed in an adjacent region close to Shanxi, Shaanxi, and Inner Mongolia autonomous region, covering an area of approximately 16,700 km2 [3,4]. It is comprised of a thick layer of sandstone, arenaceous shale, and mudstone [5,6]. The Pisha sandstone has a poorly bonded structure and low compressive strength due to incomplete epidoteigenic and epithermal stages [7]. Consequently, the Pisha sandstone is extremely hard when it is dry but rapidly disintegrates when exposed to water [8,9,10]. Owing to its unique properties, erosion frequently occurs in gullies during rainfall, with erosion occurs frequently in gullies and the erosion rate is considerably large, with erosion rates reaching 30,000~40,000 t/(a·km2), making it one of the most eroded areas in China and globally. It serves as the primary source area of the Yellow River’s coarse sediment [6], contributing to the formation of the “hanging river” in the lower reaches [1]. Moreover, the Pisha sandstone area lies in a semiarid zone with a fragile ecological environment, where most vegetation species struggle to survive due to the lack of nutrients and harsh conditions. Consequently, the area exhibits sparse surface vegetation and high surface bareness [11,12], earning it the moniker Ecological Cancer. The ecological balance in the Pisha sandstone area is crucial for protecting the fragile environment, reducing carbon emissions, enhancing carbon sinks, and potentially paving the way for future carbon trading opportunities [13,14,15].
Since the 1950s, the Pisha sandstone area has been listed as one of the key areas for national soil erosion control and ecological environment construction in the Loess Plateau [7]. Significant research efforts have been made to combat this issue, and various measures have been proposed, including engineering, biological, plant flexible dam, and material consolidation measures [16,17,18,19]. The most common and conventional approaches involve building check dams and some small-scale sediment retention reservoirs to manage water and soil loss, but they lack ecological benefits. Among biological measures, sea buckthorn has shown the most significant effects. This method involves using sea buckthorn as the dam body, utilizing the plant’s trunk branches to obstruct water and sediment for flood detention and sediment deposition. This approach effectively achieves sediment detention, overflow, or discharge. While this method is highly effective in gully treatment, it has a limited impact on slope erosion control [20]. The low degree of diagenesis of Pisha sandstone, poor cementation between particles, and weak structural strength result in immediate collapse into sand upon contact with water [19]. Despite these challenges, ecological governance in the area remains weak, and the degradation of the ecological environment persists. Traditional measures struggle to simultaneously achieve the goals of soil erosion control and vegetation restoration. Therefore, finding effective strategies to control soil erosion and restore vegetation remains crucial and urgent for this region.
W-OH is a type of hydraulic polyurethane material that can be mixed with water in any proportion to form aqueous solutions. When the aqueous solution is sprayed on the soil, it can permeate and wrap the soil particles in a short time to form a consolidation layer, which effectively resists erosion and promotes vegetation growth to some extent. Additionally, the fish toxicity of W-OH and its consolidated bodies of soil and sand, ultraviolet degradation, and the impact on the growth of soybean corn have been comprehensively studied [21,22,23]. It has been fully proven that W-OH not only does not harm plants and animals but also does not pollute the ecological environment. Therefore, W-OH has been used in desertification control [24,25], red soil area collapse [26,27], and Pisha sandstone areas. The results show that W-OH has good consolidation mechanical properties, UV degradation controllability, erosion resistance, and freeze–thaw resistance [28]. However, the effect of W-OH on water/fertilizer retention and plant growth is not exactly clear, which restricts its large-scale application in Pisha sandstone areas.
To investigate the effects of different concentrations of W-OH solutions on water/fertilizer retention and vegetation growth, tests were conducted on soil surface hardness, permeability, soil evaporation, soil column leaching, and pot experiments. Additionally, a practical engineering application was implemented in Erlaohu Gully in Ordos, China. The wrapping model and microstructure of the consolidation were also analyzed.

2. Materials and Methods

2.1. Materials

Pisha sandstone samples were collected from the bare Pisha sandstone subarea in Erlaohu gully, a two-level tributary of the Nalin River in the Huangfu River, located in the middle of the Yellow River on the Ordos Plateau, Inner Mongolia Autonomous Region, China (110°36′2.74″ E and 39°47′38.79″ N, Figure 1). Following global and local typical principles and considering the characteristics of Pisha sandstone, a horizontal stratified sampling method was employed, and the samples were obtained at a depth of 10 cm [29]. The dry density of the samples ranges from approximately 1.36 g/cm3 to 1.58 g/cm3, with a moisture content from 6.9% to 12.1%. The liquid limit (WL) is 32.2%, and the plasticity index (IP) is 10.6. The specific gravity (Gs) is 2.65. Moreover, the optimum water content (Wop) is 17%. Through the washing sieve separation method, the particles smaller than 0.05 mm constitute around 48.677%, while those larger than 0.05 mm make up 51.32%.
W-OH is a type of preformed polymer of hydrophilic polyurethane (produced by Toho Chemical Industry Co., Ltd., Tokyo, Japan), and the content consists of preformed polymer (approximately 74%), toluene diisocyanate (approximately 11%), and methyl ethyl ketone (approximately 15%). The density is 1.18 g/cm3, the solid content is approximately 85%, and the viscosity is 650–700 Pa·s. It can rapidly react with water to form an elastic gel that has a strong affinity to fix sandy soil [24]. The mixed solution with a certain amount of water and W-OH has to be quickly sprayed on the surface of Pisha sandstone samples or areas before solidification.

2.2. Permeability Test

The Pisha sandstone was sieved with a 2.36 mm sieve and then placed in a cylindrical glass tube measuring φ20 mm (diameter) × 150 mm (height). Different concentrations (0%, 3%, 4%, and 5%, based on the mass concentration) of W-OH solution were prepared, and the spraying amount was set as 3 L/m2, according to the previous research [21]. The permeation thickness of the solutions over time was measured by the height of the wet peak in the columns, indicating the permeability of different solutions on Pisha sandstone. All experiments were conducted three times.

2.3. Soil Surface Hardness Test

Soil surface hardness is one of the important indicators affecting plant germination [30]. The weathered Pisha sandstone is sandy soil, so the surface hardness is used to represent soil compactness, and the conversion is carried out between the two indices. Soil hardness is connected with structural changes, compaction, and cementing action in soil, which has a relationship with the types of vegetation, especially in the presence of steep slopes covered by weathered soil or sand [31]. When the surface hardness is too great, it will seriously affect the seed germination rate with water and heat changes and provide the plant growth with mechanical resistance. In this experiment, the surface hardness of the solidified body was tested using a Yamanaka-type soil penetrometer (Figure 2). The conical surface was perpendicular to the cross-section of the solidified body, and then it was forced into the body quickly. When the cone could not be removed from the body easily, the surface hardness value, instead of the deformation index, could be read from the scale directly [32]. The formula for pressure intensity can be expressed as follows.
P = 100 X 0.7952 ( 40 − X ) 2
where P stands for the pressure intensity, kg/cm2, and X stands for the deformation index, mm.

2.4. Soil Evaporation Test

The Pisha sandstone was simulated in transparent cylindrical polyvinyl chloride (PVC) columns with a size of 50 mm (diameter) × 500 mm (height). A 15 cm layer of sandy land soil was placed in the bottom of each column, and then a 30 cm layer of Pisha sandstone with a moisture content of approximately 30% was spread on the top. The lower part of the container was connected to the ground. Prior to the experiment, the weathered Pisha sandstone was screened by a sieve, and a diameter below 1.0 mm was chosen. The treated samples were sprayed with different concentrations (3%, 4%, and 5%) of W-OH solution on the surface of Pisha sandstone according to previous research, and the thickness was guaranteed to be approximately 10 mm (calculated by the porosity and also measured by a ruler). These columns were placed in the laboratory at room temperature. Subsequently, the mixed samples were collected at 0, 6, 18, 42, 90, 162, 258, 618, 810, 1050, and 1340 h to measure the moisture contents, and the evaporation capacity was obtained [33,34].

2.5. Soil Column Leaching Test

The test was a type of indoor simulated soil column leaching test [35]. The samples were placed in PVC pipes with a diameter of 120 mm and a height of 30 cm. Filter paper and gauze (200 mesh) were placed at the bottom of the pipes to allow the leachate and water to leak, and a large flask was set at the bottom to collect them (Figure 3). Initially, the quartz sand was pre-covered at both the top and bottom of the pipes at a depth of 5 mm. Then, a 1000 g Pisha sandstone sample was placed into the pipe columns. The adding order was to initially put 750 g Pisha sandstone and add the remaining 250× g mixed samples with W-OH solutions (0%, 3%, 4%, and 5%) and fertilizer (total nitrogen (TN) of 1000 mg and total phosphorus (TP) of 60 mg per kilogram of Pisha sandstone). The applied masses of carbamide and ammonium dihydrogen phosphate were 2140 mg and 220 mg, respectively. Deionized water was then sprayed on the surface of the samples to saturate them with water. The soil column leaching test was conducted every 7 days. A total of 1000 mL of water was used in triplicate in each leaching test. The leaching water samples were collected from the bottom of the pipe columns, and the c TN and TP were tested. Each test was repeated three times with three samples. The TN was tested by alkaline potassium persulfate digestion UV spectrophotometry, and the TP was tested using the ammonium molybdate spectrophotometric method.

2.6. Pot Test

The planting experiment focused on the germination percentage and wilting time of the vegetation in the experimental area, with tests conducted in the laboratory. Pisha sandstone samples were placed in soil bins measuring 175 mm (length) × 105 mm (width) × 113 mm (height). Subsequently, 100 seeds of Astragalus adsurgens Pall, Medicago sativa L. (dicotyledons), Buchloe dactyloides, and Lolium perenne L. (monocotyledons) were sown in each experimental section. Different concentrations (0%, 3%, 4%, and 5%) of W-OH solution were then sprayed on the surface at a spraying amount of 3 L/m2. The germination percentage, growth process, and wilting time were observed and recorded.

2.7. The Practical Engineering Application

Study area: The practical engineering application was conducted in Erlaohu Gully (Figure 1), which primarily comprises sandstone and mudstone. There is sporadic distribution of loess and chestnut soil on the ridge top and slope, with a soil cover thickness ranging from 0.3 to 1.0 m. The slope composition in the watershed is detailed in Table 1. Due to the severe climate and infertile soil, the overall vegetation coverage rate is below 30%.
Construction method: Twenty-three typical slopes were selected, and most slopes were divided into five subsections (top of the slope, consolidation-proof, consolidation-growth promotion, consolidation-green, and bottom of the slope) for control. Sea buckthorn seedlings, pine (Salix cheilophila) branches, seedlings of Agropyron cristatum (Linn.) Gaertn., Elymus dahuricus Turcz., Melilotus officinalis L., and Caragana korshinskii were chosen.
Spraying system: Since W-OH can react quickly with water to cure, it cannot be stored in a single tank with water. Therefore, a separate two-liquid spraying system was utilized. The W-OH spraying equipment system comprises a water and W-OH conveying system and a mixing spraying system (Figure 4). The conveying system consists of a generator, a high-pressure engine pump, and two storage tanks. The mixing spraying system included a mixing pipe approximately 20 cm long and a spray nozzle with a 56,000 fogging degree. The high-pressure engine pump used to convey W-OH was a plunger pump type. By regulating the flow rate of the two pumps, the concentration was maintained at around 3% in this experiment.
The construction process consists of digging intercepting ditches and storage cellars, sowing seeds and seedlings, spraying W-OH solutions, and planting trees and shrubs on the top and bottom of the slopes.
For the statistical analysis, all the experimental data were analyzed using standard statistical techniques in ORIGIN 2019PRO software. The results were analyzed using the average values of all the tests with error mean square deviations (LSD); the LSD used the five least significant differences, which were calculated from the analysis of variance, and it could be acceptable and reflect the actual value.

3. Results

3.1. Effect of W-OH Concentration on Permeability

As illustrated in Figure 5, without spraying the W-OH solution, water permeated rapidly in the Pisha sandstone, and the wet peak reached the bottom of the cylindrical glass tube. The permeation rates showed an almost linear relationship with increasing time. However, when the surface of the Pisha sandstone was sprayed with different concentrations of the W-OH solution, the permeation depth initially increased, then stabilized, and eventually decreased gradually with increasing concentrations of W-OH solution (from 3% to 5%). The permeation depth exceeded 5.0 cm when the concentration of W-OH was 3% and 4%, as the solution did not fully cure within 300 s. With a 5% concentration of W-OH, the permeation depth was approximately 3.7 cm. This was primarily attributed to the curing time of the solutions. Solutions with a concentration of W-OH greater than 5% could cure in 3 min, and the porosity in the Pisha sandstone was filled by the curing body and the long-chain structure, indicating that the uncured solution could not penetrate deeper. Moreover, the permeation thickness could indicate the potential solidifying thickness when the W-OH solution was sprayed on the surface of an actual Pisha sandstone slope. This information can be used to regulate the spraying depth, residence time, and other a supplementary measure on construction sites with varying slopes. In practice, the depth of the consolidation layer should be maintained within the range from 5 to 15 mm, with the optimal depth being 10 mm.

3.2. Effect of the W-OH Concentration on Surface Hardness

The deformation index increased after the Pisha sandstone was solidified by different concentrations of W-OH concentrations (Figure 6), indicating an increase in the compactness of the Pisha sandstone particles. The deformation index of the specimen treated with W-OH gradually increased as the concentration of W-OH increased from 0% to 5%, with values of 3.44, 5.85, 6.06, and 7.64 mm.
Soil compaction, an important aspect of soil quality indicators, can reduce soil porosity and alter pore size distribution. Root elongation is typically affected in soils with values ranging from greater than 0.8 to 2 MPa, and may arrest root growth completely at a resistance of 5 MPa [36]. When the compactness reaches a range from approximately 0.7 to 1.5 MPa, the root length can be decreased by 50%, and the root stops developing when the compactness reaches 4.0 MPa [35]. Similar results have been reported, showing that 2.5 MPa [31] and 1.5 MPa [37,38] of compactness have limited the growth of the root. According to the equation in Section 1, when the concentration of W-OH increased from 0% to 5%, the pressure intensity of the specimen treated by W-OH alone was 0.32, 0.63, 0.66, and 0.92 kg/cm2. Based on these results, the consolidation layer treated with a 5% concentration of W-OH may affect root development.

3.3. Effect of W-OH Concentrations on Evaporation Capacity

As the porosity of weathered Pisha sandstone ranges from approximately 35% to 40%, and the cementing material is scarce, the primary form of water loss is evaporation, and the evaporation rate is quick under natural conditions. Figure 7 illustrates that the evaporation capacity increased rapidly in the control group without spraying the W-OH solution. However, when the Pisha sandstone surface was sprayed with different concentrations of W-OH solution with a thickness of approximately 10.0 mm, the evaporation capacity initially increased rapidly and then slowed down or even stabilized. The decrease in moisture content was also mainly attributed to evaporation. When the concentration of W-OH solution increased from 0% to 5%, the evaporation capacity in 1340 h was approximately 19.37%, 10.62%, 10.12%, and 9.63%, respectively. With an increase in the concentration of W-OH solution from 3% to 5%, the reduction in evaporation capacity was approximately 45.2%, 47.7%, and 50.3% compared to the control group. Wu et al. demonstrated that the wilting water content in Pisha sandstone was around 12.5%. These results were primarily due to the consolidation layer on the surface, which bonded Pisha sandstone particles together, leading to a significant decrease in porosity. The consolidation layer exhibited a relatively high water-holding capacity. However, when the moisture content exceeded 20%, the original structure of Pisha sandstone underwent changes. The consolidation layer can facilitate this condition and requires a relatively long time to achieve a low moisture content.
Additionally, when there is vegetation on the surface, it can utilize the water, prolonging its growth period in dry conditions. Therefore, the consolidation layer can help slow water loss and enhance water retention, ultimately promoting vegetation growth and ecological restoration.

3.4. The Effect of the W-OH Concentration on Fertilizer Retention

The fertilizer in the soil will be lost through the processes of evaporation, leaching, and denitrification. It is especially important for water authorities to reduce the losses of nitrogen and phosphorus in the soil [39].
The TN content in leaching samples decreased significantly when different concentrations of W-OH solution were sprayed on the surface. The cumulative TN content decreased as the W-OH concentrations increased (Figure 8). When the W-OH solution concentration increased from 0% to 5%, the TN content in leaching samples on the seventh day was approximately 280, 130, 99, and 73 mg; on the fourteenth day, the values were approximately 154, 121, 138, and 89.0 mg; and the cumulative TN content on the twelfth day was approximately 510.2, 287.9, 264.6, and 183.7 mg, respectively. The reduction rates of the cumulative TN content loss at 3%, 4%, and 5% were approximately 43.6%, 48.1%, and 64.0% compared to the control group. This suggests that W-OH could fix N and reduce leaching loss. However, on the twenty-first day, the TN content in the 4% W-OH condition was higher than that in the 3% W-OH condition. This could be attributed to nitrogen accumulation at the bottom of the Pisha sandstone over several days. Therefore, when W-OH is used to solidify Pisha sandstone, the TN can be fixed in the consolidation body, protected from loss, and made available for vegetation utilization.
The TP content in leaching samples decreased as the W-OH concentration increased (Figure 9). It is likely that PO43− was less easily transported by interstitial flow in a short period, which the Pisha sandstone could absorb. W-OH could hinder water infiltration and enhance the adsorption capacity of PO43−. As the W-OH concentration increased from 0% to 5%, the TP content in leaching samples on the seventh day was approximately 2.55, 1.93, 1.51, and 1.32 mg; on the fourteenth day, it was approximately 9.26, 6.55, 5.42, and 3.87 mg; and on the twenty-first day, it was approximately 3.64, 2.65, 1.46, and 0.81 g. The cumulative TP content at 21 days was approximately 15.45, 11.13, 8.39, and 6.0 mg, respectively. The reduction rates of cumulative TP loss at 3%, 4%, and 5% were approximately 27.96%, 45.70%, and 61.17% compared to the control group. Therefore, W-OH could partially inhibit phosphorus in Pisha sandstone to enhance the utilization rate of phosphate fertilizer, promoting growth. Consequently, when fertilizers containing nitrogen and phosphate were used in Pisha sandstone areas, the W-OH and its consolidation body could retain the fertilizer, aiding in fertilizer retention and enhancing vegetation absorption and utilization.

3.5. The Effect of the W-OH Concentration on Germination

After the seeds were sown, different concentrations of W-OH solution were sprayed on the surface in three groups. The germination amount was recorded, and the growth conditions were observed. The seeds mainly germinated in approximately 7 days, and the germination amount increased initially with time and then stabilized in approximately 12 days. In the control group, the germination amounts of Astragalus adsurgens Pall, Medicago sativa L., Buchloe dactyloides, and Lolium perenne L. were approximately 27, 35, 78, and 33, with germination percentages of 27%, 35%, 78%, and 66%, respectively (Figure 10). When different concentrations of W-OH solution were sprayed on the surface, the germination amounts changed. Astragalus adsurgens Pall and Medicago sativa L., being dicotyledons, were more sensitive to the consolidation layer on the surface. Therefore, when the concentration of W-OH solution was 3%, the germination amount slightly increased, while it decreased rapidly when the concentration of W-OH increased to 4%. A 5% W-OH concentration restrained the growth, and the highest germination amount was approximately 3% (Figure 10a,b). However, Buchloe dactyloides and Lolium perenne L. were monocotyledons, and they were much easier to germinate from the soil. When the concentration of W-OH increased from 0% to 5%, the germination amount showed an increasing trend and then a slight decrease when the concentration of W-OH was 5%. The highest germination amount was at 3% (Figure 10c,d).
For the growth conditions of the seeds, the Astragalus adsurgens Pall grew slowly, reaching a height of 1.0~1.5 cm, and began to wilt on the tenth day. Medicago sativa L. germinated rapidly but grew slowly with thin stems. When the height reached approximately 2.0 cm, it started to bend and then showed signs of wilting without watering. Lolium perenne L. had a high germination rate and grew quickly, although the stems were slightly thin. Buchloe dactyloides had a relatively high germination rate, grew rapidly with thick stems, and was taller than other plants. The results suggested that Buchloe dactyloides and Lolium perenne L. were more likely to survive in the Pisha sandstone area.
Compared to the control group, the germination percentages in the groups decreased with increasing W-OH concentration, and the highest decreasing rate reached approximately 30%. This is due to the fact that the flexible consolidation layer can also have a certain degree of influence on seed germination and emergence. When the concentration of W-OH material is small (≤3%), the cementation layer is very flexible, there is almost no effect on seed germination, and this can protect the seed germination to a certain extent. Thus, there is a tendency to increase the germination rate. However, with the increasing concentration of W-OH solution, the consolidation layer becomes more and more dense, and the strength is also increased. It is much more difficult for the plant seeds to break through the consolidation layer, leading to a reduction in the germination rate. However, once the seeds germinated, the growth rate was similar to that in the control group and the wilting time was increased without watering. The wilting period was approximately 15~20 days, while the wilting period in treated groups could reach approximately 30~40 days, and the period was increased by more than 15 days. The results showed that the grass in the treated groups was much higher and thicker than that in the control group. This is because the consolidation layer has a certain water and fertilizer retention, and, under certain conditions, it can provide more and longer-lasting water and nutrient supply for the growth of plants, thus prolonging the time of wilting and promoting the growth of plants. This indicated that the consolidation layer with a certain thickness could play a role in water conservation and prolong the growth period of the grass. Therefore, in semi-dry or dry conditions, W-OH could be used to guarantee the final survival period, promote growth, and increase the vegetation coverage in Pisha sandstone.

3.6. The Variation in Vegetation Coverage in Practical Engineering Applications

The vegetation coverage in the Erlaohu Gully watershed was calculated and interpreted using aerial images. Shrubs and plantations increased year by year, while the bare area gradually decreased. By the third year, the shrub and plantation areas increased by 8.17% and 3.31%, respectively, and the bare area decreased by 11.35%. The area with a 60% vegetation coverage rate also increased by 3.54% compared to the first year (Figure 11). The results indicated a decreasing trend in vegetation coverage in the Erlaohu gully watershed without treatment, but a slight increase was observed when the control model was applied. Moreover, the consolidation-proof area remained intact for several months, with the vegetation coverage of the treated areas reaching approximately 65% and an average height of 50 cm. The toxicity and ecological properties of W-OH have been tested by authoritative authorities, concluding that it has no negative impact on the ecosystem. Furthermore, the ecological risk of the material has been formalized through long-term use in Japan, with the degradation products deemed harmless and posing no ecological risk.

4. Discussion

The weathered Pisha sandstone consists of small particles, while the adhesion between the particles is weak. The cementitious materials are few, mainly expansible substances such as montmorillonite. When it is exposed to water or external forces, the cementitious materials expand, causing the particles to disperse and collapse. When subjected to flowing water, the dispersed particles can be carried away. However, when a W-OH solution is sprayed on the particles, it quickly permeates into them, wrapping the surface and the particles (Figure 12) and solidifying upon curing (Figure 13). This process bonds the loose particles together, creating an integrated structure. The wrapping and consolidation layer is flexible and can withstand a certain degree of deformation. The wrapping layer, with varying concentrations of W-OH solution, exhibits different functions; for instance, when the concentration is below 5%, the consolidation layer can support vegetation growth.
The W-OH molecules consist of soft and hard segments, presenting thermodynamic incompatibility and microphase separation. Phase and microphase domains can be formed in space. After the reaction between water and the isocyanate and polar groups in the W-OH molecule [32], hydrogen bonds, polyurethane bonds, urea bonds, and other functional groups will form on the surface of the Pisha sandstone. These bonds and groups have high cohesive energy and can form an adhesive layer with high interfacial tension. Due to the good permeability and affinity of the W-OH solution on weathered Pisha sandstone, the permeation depth, strength, and hardness can be guaranteed, and the consolidation body has good mechanical properties. Additionally, the W-OH solution can permeate into the interior of Pisha sandstone and envelop the particles completely. Water cannot easily enter the consolidation layer, as the most important erosion process is hydraulic erosion [40]. The consolidation layer can reduce and slow down the water permeation rate into Pisha sandstone, while also reducing evaporation. These two indicators are constrained by each other. To achieve erosion resistance and vegetation growth, the concentration must be well controlled. Based on the studies, a 3% concentration of W-OH solution is more appropriate. The growth promotion mechanism mainly involves water and fertilizer retention initially. The structure of the curing layer is crossed and networked as it is a polymer with many molecules. When it reacts with water, CO2 gas is generated and released, forming some connected porosity forms (Figure 14). The water and fertilizer supply comes from the original water and fertilizer applied. It can fix the fertilizer and prevent water evaporation. Additionally, the curing body can slowly absorb and release water repeatedly under rainfall and dry conditions, forming a “reservoir”, and fertilizer can be used by the plants. Therefore, the consolidation layer can provide the necessary water for plant growth, extending the period in semiarid or arid areas, promoting vegetation, and improving vegetation coverage in the Pisha sandstone area. However, other curing materials, such as EN-1, can cure Pisha sandstone effectively, but the surface is very hard, making it difficult for vegetation to grow, which is a significant difference from W-OH [41].

5. Conclusions

Different concentrations of W-OH solution can permeate and solidify Pisha sandstone, forming a flexible consolidation layer with a specific porosity. The surface hardness of the consolidation layer increased as the W-OH concentration increased, reaching 0.92 kPa at a concentration of 5%. This consolidation layer can reduce soil evaporation and prevent fertilizer loss and migration during leaching. When the concentration of the W-OH solution is 5%, the soil evaporation rate decreases by 50.3%, and the retention rate of TN and TP exceeds 60%. As a result, it provides vegetation with necessary water and fertilizer, enabling the vegetation to survive for a longer period in semiarid or arid areas. This can improve vegetation growth and coverage in the Pisha sandstone area. For monocotyledons and dicotyledons, the suitable W-OH concentrations were less than 5% and 3%, respectively. The practical engineering application resulted in an increase in vegetation coverage of approximately 11.35%. Furthermore, the mechanism of growth promotion was revealed through the wrapping structure and microstructure, which can conserve water and fertilization through the flexible and porous structures. Therefore, this method offers a possible and effective approach to enhance ecological restoration in the Pisha sandstone area.

Author Contributions

Writing—original draft preparation, Z.L.; validation, Y.S., X.F. and B.P.; formal analysis, Z.L.; investigation, Y.X.; resources, H.G. and Z.W.; data curation, B.P.; writing—review and editing, Y.S. and X.F. All authors have read and agreed to the published version of the manuscript.

Funding

This research was supported by the National Natural Science Foundation of China (42207394), Guangxi Key Research and Development Program of China (AB21196048); Forestry and Grassland Science and Technology Promotion Demonstration Project of the Central Government ([2023] TG17).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Acknowledgments

The authors are grateful to Guangxi Forest New Fertilizer Research and Development Center, and School of Civil Engineering, Southeast University, and International Institute for Urban Systems Engineering, Southeast University. Special thanks go to the anonymous reviewers for their constructive comments in improving this manuscript.

Conflicts of Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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Figure 1. The Erlaohu Gully watershed and its monitoring point.
Figure 1. The Erlaohu Gully watershed and its monitoring point.
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Figure 2. Schematic diagram of the surface hardness test.
Figure 2. Schematic diagram of the surface hardness test.
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Figure 3. Simulated soil columns in the leaching test.
Figure 3. Simulated soil columns in the leaching test.
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Figure 4. The spraying equipment system.
Figure 4. The spraying equipment system.
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Figure 5. Effects of different W-OH concentrations on permeation depth in Pisha sandstone.
Figure 5. Effects of different W-OH concentrations on permeation depth in Pisha sandstone.
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Figure 6. The variation in the deformation index under different W-OH concentrations.
Figure 6. The variation in the deformation index under different W-OH concentrations.
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Figure 7. The moisture content in different samples.
Figure 7. The moisture content in different samples.
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Figure 8. Total N content changed with time in leaching water samples. (a) TN and (b) cumulative TN.
Figure 8. Total N content changed with time in leaching water samples. (a) TN and (b) cumulative TN.
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Figure 9. Total TP content changes with time in leaching water samples: (a) TP and (b) cumulative TP.
Figure 9. Total TP content changes with time in leaching water samples: (a) TP and (b) cumulative TP.
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Figure 10. The germination amount of the different seeds: (a) Astragalus adsurgens Pall, (b) Medicago sativa L., (c) Buchloe dactyloides, and (d) Lolium perenne L.
Figure 10. The germination amount of the different seeds: (a) Astragalus adsurgens Pall, (b) Medicago sativa L., (c) Buchloe dactyloides, and (d) Lolium perenne L.
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Figure 11. Comparison before and after the treatment. (a) Topical Pisha sandstone slope, (b) the Erlaohu Gully, (c) treated Pisha sandstone slope after 6 months, and (d) vegetation coverage of Erlaohu Gully after 2 years of treatment.
Figure 11. Comparison before and after the treatment. (a) Topical Pisha sandstone slope, (b) the Erlaohu Gully, (c) treated Pisha sandstone slope after 6 months, and (d) vegetation coverage of Erlaohu Gully after 2 years of treatment.
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Figure 12. The structure of the original Pisha sandstone particles.
Figure 12. The structure of the original Pisha sandstone particles.
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Figure 13. The structure of the Pisha sandstone and the consolidation body by W-OH. (a) Original weathered Pisha sandstone particles, (b) consolidation body, (c) cementation pattern.
Figure 13. The structure of the Pisha sandstone and the consolidation body by W-OH. (a) Original weathered Pisha sandstone particles, (b) consolidation body, (c) cementation pattern.
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Figure 14. The structure of the curing body at 15× magnification. (a) Surface and (b) cross-section.
Figure 14. The structure of the curing body at 15× magnification. (a) Surface and (b) cross-section.
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Table 1. The distribution of slope angles in the Erlaohu Gully watershed.
Table 1. The distribution of slope angles in the Erlaohu Gully watershed.
Slope Angle≤5°5–15°15–25°25–35°≥35°Total
Area (hm2)20.076.2373.682.071.0323
Proportion (%)6.223.622.825.422100
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Liang, Z.; Sun, Y.; Fang, X.; Pan, B.; Xiao, Y.; Gao, H.; Wu, Z. Effect of W-OH Material on Water/Fertilizer Retention and Plant Growth in the Pisha Sandstone Area of China. Sustainability 2024, 16, 6369. https://doi.org/10.3390/su16156369

AMA Style

Liang Z, Sun Y, Fang X, Pan B, Xiao Y, Gao H, Wu Z. Effect of W-OH Material on Water/Fertilizer Retention and Plant Growth in the Pisha Sandstone Area of China. Sustainability. 2024; 16(15):6369. https://doi.org/10.3390/su16156369

Chicago/Turabian Style

Liang, Zhishui, Yue Sun, Xiuwen Fang, Bo Pan, Yuan Xiao, Haiying Gao, and Zhiren Wu. 2024. "Effect of W-OH Material on Water/Fertilizer Retention and Plant Growth in the Pisha Sandstone Area of China" Sustainability 16, no. 15: 6369. https://doi.org/10.3390/su16156369

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