Soil organic carbon (SOC) is one of the core components of carbon cycle in terrestrial ecosystem, its dynamic balance plays a vital significance for global climate change and agricultural sustainable development. The Three Gorges Reservoir area (TGRA) is a typical ecologically fragile region with characteristic of abundant rainfall concentrated in specific seasons, faces the challenge of severe loss in soil organic carbon. Enzyme-induced carbonate precipitation (EICP) is an emerging bio-mediated technology, has been applied in geotechnical stabilization and soil reinforcement, while its capacity to influence soil aggregate stability and organic carbon retention remains unclear. Therefore, taking purple soil in the TGRA as material, 4 EICP solution concentrations (0.5, 1.0, 1.5 and 2.0 mol/L) and control check (CK) were set, the variations in aggregate particle size distribution, aggregate stability and soil organic carbon retention capacity were evaluated under 7 maintenance durations (0, 1, 7, 15, 30, 60 and 120 d). Moreover, the influence factor influencing soil organic carbon retention capacity were determined. The results indicated that the microaggregate of purple soil rapidly transferred to macroaggregate (>0.25 mm) with the effect of EICP. Compared to that in the CK, the mean weight diameter (MWD), geometric mean diameter (GMD) and soil organic carbon retention capacity with EICP treated significantly increased by 101.43%-219.47%, 75.21%-271.56% and 20.87%-57.48% (P<0.05), respectively. Moreover, MWD, GMD and soil organic carbon retention capacity increased first and then decreased with increasing EICP solution concentration, and they increased rapidly and then tend to stable with increasing maintenance duration. Among them, MWD, GMD and soil organic carbon retention capacity of purple soil in the CK demonstrated negligible variation during the maintenance duration, while significant increases were observed under EICP treatments, with the 1 d increments of MWD, GMD and organic carbon retention capacity accounting for 53.32%-64.23%, 47.30%-52.92% and 63.61%-76.39% of the total increase, respectively. In addition, fractal dimension decreased initially followed by an increase as EICP solution concentration increases, while it decreased rapidly and then tend to stable with the increase of maintenance duration. Furthermore, correlation analysis indicated that the soil organic carbon retention capacity of purple soil under EICP treatments positively related to MWD and GMD, while negatively related to fractal dimension, with statistically significance (P<0.01). The fitting analysis showed that a highest determination coefficient was observed in the fitting function between MWD and soil organic carbon retention capacity, and can be employed as optimal indicator to evaluate the variation in soil organic carbon retention capacity of purple soil under EICP treatments. The results indicated that EICP can rapidly increase the aggregate stability and soil organic carbon retention capacity of purple soil, can provide a theoretical guidance for soil organic carbon retention in the TGRA.

The hedgerow is a row of dense and upright vegetation that is planted along the contours of the slope. Among them, the crops can be produced between the hedges. Planting hedgerows on slopes can be the function of soil stabilization and fertilizer preservation, sediment containment and pollution control, soil quality improvement, and biological terrace formation. Particularly, the hedgerow is a vital soil and water conservation for the purple soil sloping farmland in the Three Gorges Reservoir Area (TGRA). Much effort has been made into the effects of hedgerows on the erosion process, soil and water conservation benefits. However, it is still unclear on the influence of hedgerows on the sediment particle size distribution and sediment sorting, especially for soil with uneven texture, like the purple soil in the TGRA. Taking the purple soil in the TGRA as the research object, this study aims to explore the effects of hedgerows on sediment particle size distribution and sediment sorting. Therefore, the simulated rainfall experiments were conducted with intensities of 60 and 90 mm/h at a fixed slope gradient (15°) for three slope conditions (CK-Control check, P-Hedgerow, and R-Only hedgerow roots). Then, the influence of hedgerows on the particle size distribution of eroded sediment on purple soil slope was evaluated to compare the sediment particle size distribution, enrichment ratio and agglomeration ratio under different slope conditions. The results indicated that the sand was the main particle in the rainfall erosion sediment. The influence of slope condition on particle size distribution was greater than that of rainfall intensity. Compared with the CK, the R slope condition only reduced the loss of clay and silt particles by 0.25%-5.48%, while the P slope condition significantly reduced the loss of sand particles by 30.75%-47.28% percentage points, resulting in a“diminution effect” of sediment particles. By contrast, the rainfall intensity led to the “coarsening effect” of sediment particles under different slope. The sediment sorting mainly existed at the early rainfall period for the CK slope condition. The influence of R slope condition on sediment sorting for effective particle was more pronounced during early rainfall period, indicating the large value of enrichment ratio of clay, fine silt, and coarse silt, compared with the CK. There was the pronounced influence of P slope condition on sediment sorting for effective particle during the whole rainfall period. But the influence effect was stable with the increase of rainfall duration. In terms of particle transport form, the sediment particles were mainly transported via aggregates on the CK, whereas, more aggregates were broken by raindrops and then dispersed into single grains under the R slope condition. Also, more fine particles were aggregated to form aggregates on the P slope, but the aggregates tended to disperse into single grains in the middle and late periods of rainfall. Overall, the sediment sorting was more outstanding under the hedgerow slope. Therefore, the hedgerow can be cooperated with other measures (such as earth banks, tillage measures, and straw mulching) for better performance. The finding can be used to further understand the effect of hedgerow on the sediment sorting of purple soil slopes. Scientific basis and data support can be offered to construct the regional soil erosion model for the efficient use of soil and water resources.

Purple soil can represent a predominant soil type extensively found in the Three Gorges Reservoir Area (TGRA). Severe soil erosion has also caused environmental problems in the soil properties and local topography. Particularly, the sustainable socio-economic development of the Yangtze River Economic Belt is closely related to the long-term ecological security and functional integrity of the reservoir. Fortunately, the Enzyme-Induced Carbonate Precipitation (EICP) can be expected to serve as a promising bio-cementation technique. The soil strength and erosion resistance can be significantly enhanced to catalytically generate the cementitious calcium carbonate crystals for the high interparticle bonding. While the EICP-treated soil has exhibited brittle failures. Although the fiber incorporation significantly improves the brittleness of the EICP-treated soil, it is often required for the fiber influence on the erosion resistance of the EICP-reinforced soil. This study aims to investigate the effect of the palm fiber on the anti-detachment capacity of the EICP-reinforced purple soil in the TGRA. The test material was taken from the purple soil from the TGRA. Experimental variables included the fiber content (0 and 0.1%), cementation solution concentration (0.5, 1.0, 1.5, and 2.0 mol/L), and curing duration (1, 7, 15, 30, 60, and 120 d). Untreated purple soil served as the control group (CK). Simulated scouring tests were conducted to clarify the influence mechanism of the palm fiber on the soil detachment capacity of the EICP-reinforced purple soil, particularly from the perspectives of the apparent cohesion and calcium carbonate content. The microscopic mechanism of the treatment was also analyzed after measurement. The results indicated that the soil detachment capacity of the EICP-treated soil was reduced by 58.63%, compared with the CK treatment, which further decreased by 25.81% after fiber incorporation. The apparent cohesion and calcium carbonate content of the EICP-treated group increased by 79.73% and 20.58 times, respectively, compared with the CK group, which were further enhanced by 16.50% and 16.01%, respectively, after fiber incorporation. The fiber incorporation promoted the calcium carbonate crystal formation and apparent cohesion in the EICP-treated soil. The curing duration and cementation solution concentration were attributed to the increasing apparent cohesion and calcium carbonate content in the EICP-treated soil. Under the same cementation solution concentration, the fiber incorporation increased the apparent cohesion for different incubation time. The apparent cohesion also increased for the cementation solution concentrations of 0.5, 1.0, 1.5, and 2.0 mol/L, where the best performance was observed at 1.5 mol/L under the curing duration of 7 d. Similarly, the fiber incorporation increased the calcium carbonate content for the curing durations of 1, 7, 15, 30, 60, and 120 d, respectively. The fiber incorporation increased the calcium carbonate content for the cementation solution concentrations of 0.5, 1.0, 1.5, and 2.0 mol/L, with the best effect at 1.5 mol/L under the curing duration of 7 d. Both apparent cohesion and calcium carbonate content exhibited a highly significant exponential function with the soil detachment capacity of the purple soil (P < 0.01). The fiber incorporation further reduced the soil detachment capacity to increase the apparent cohesion and calcium carbonate content. Microscopic analysis indicated that the fiber provided the reaction site for the EICP, in order to promote the calcium carbonate crystals formation and the stability of the soil, thereby reducing the soil detachment capacity. This fiber incorporation further enhanced the erosion resistance of the EICP-treated purple soil in the region. The finding can also provide a theoretical basis for soil and water conservation in the TGRA.

The Three Gorges Reservoir Area (TGRA) has often suffered the most serious soil erosion in recent years. Soil erosion control still remains on the purple soil, the main soil type for the sloping farmland in this area. Among them, enzyme-induced carbonate precipitation (EICP) can rely on the urease enzymes to catalyze urea. Specifically, the urea can be decomposed into ammonium and carbonate anions, and then precipitated the calcium carbonate (CaCO₃) with the presence of carbonate anions and external calcium sources. EICP has been proven to efficiently strengthen the soil for less infiltration, in order to restore the rock and heritage. A promising approach can be regarded to effectively control soil erosion. However, the effect of EICP on the soil detachment capacity is still far from clear, especially for the response of soil detachment to the EICP concentration and maintenance time. Therefore, this study aims to clarify the effect of EICP solution concentration and maintenance duration on the soil detachment capacity of purple soil in the Three Gorges Reservoir Area. The purple soil in the TGRA was taken as the research material. The scouring experiments were conducted under five EICP solution concentrations (0 (for control check), 0.5, 1.0, 1.5, and 2.0 mol/L) at six maintenance durations (1, 7, 15, 30, 60, and 120 d), in order to estimate the soil detachment capacity. Also, the apparent cohesion and calcium carbonate content were measured at different experiment conditions. Moreover, the SEM was employed to reveal the micro-mechanism of erosion reduction by EICP. The results indicated that the soil detachment capacity of purple soil significantly decreased with the application of EICP solution. Compared with the CK, the soil detachment capacity decreased by 19.63%-86.92%, 38.79%-89.41%, 48.13%-89.91%, and 31.78%-84.49%, respectively, at the EICP solution concentration of 0.5, 1.0, 1.5 and 2.0 mol/L, with the most pronounced effect at the concentration of 1.5 mol/L. Additionally, the soil detachment capacity decreased rapidly, whereas, the subsequent slowly decreased as maintenance duration increased. Compared with the maintenance for 1 d, the decreases at 7, 15, 30, 60, and 120 d were 19.79%-84.08%, 33.81%-87.80%, 87.27%-94.21%, 91.41%-93.18%, and 91.18%-92.77%, respectively. Moreover, the reduction amplitude in the soil detachment capacity accounted for 85.79%-92.21% of the total reduction amplitude at the maintenance duration of 7 d under the application of the EICP solution. The apparent cohesion and calcium carbonate content of purple soil showed a trend of increase followed by a decrease with the increase of EICP solution concentration. Compared with the CK, the apparent cohesion increased by 43.70%-77.43%, 58.54%-101.21%, 77.06%-135.68%, and 64.08%-87.86%, respectively, at the EICP solution concentration of 0.5, 1.0, 1.5 and 2.0 mol/L, while the calcium carbonate content increased by 10.29-17.35 times, 11.12-23.00 times, 12.00-29.59 times and 10.88-24.35 times, respectively. Additionally, the apparent cohesion increased by 10.46%-36.38%, 21.98%-46.46%, 35.12%-55.41%, 45.15%-65.49%, and 48.61%-72.76%, respectively, at the maintenance for 7, 15, 30, 60 and 120 d, compared with the maintenance for 1 d. The content of calcium carbonate increased by 54.17%-133.48%, 60.94%-134.39%, 61.98%-134.84%, 61.46%-134.84%, and 62.50%-135.29%, respectively, at the maintenance for 7, 15, 30, 60 and 120 d under the application of EICP solution, compared with the stable content in the CK. The increase amplitude in the apparent cohesion and calcium carbonate content accounted for 37.59%-59.56% and 78.26%-98.66% of the total increase amplitude, respectively, at the maintenance duration of 7 d. Additionally, the microscopic analysis exposed that the soil detachment capacity was reduced to aggregate the calcium carbonate in the soil surface under the application of EICP solution. Furthermore, the soil detachment capacity was better described by the apparent cohesion and calcium carbonate content with an exponential function. The findings can provide theoretical guidance to promote erosion control in the application of EICP on soil detachment of purple soil in the TGRA.

Enzyme induced carbonate precipitation (EICP) is one of the innovative geotechnical techniques to reinforce the soil. Specifically, urease can be directly utilized to catalyze the hydrolysis of urea, and then form the calcium carbonate crystals. EICP has promising potential to control rainfall-induced erosion. However, previous research has focused primarily on the overall effect of erosion reduction. It is still lacking in the influence of EICP on erosion processes under different erosion patterns. Among them, different erosion patterns are characterized by distinct processes and mechanisms, where sheet erosion is the initial phase of slope erosion to serve as the most important evolution of soil erosion. It is one of the most complex processes of slope water erosion, indicating significant sediment sorting. Therefore, this study aims to explore the influence of EICP spray treatment on sheet erosion and sediment sorting. The purple soil slope in the Three Gorges Reservoir Area (TGRA) was taken as the research object. Six spraying treatments (bare slope as control check (CK), only urease spray treatment, and EICP spray treatment with 4 cementing solution concentrations (0.5, 1.0, 1.5, and 2.0 mol/L)) were set. Simulated rainfall experiments with two rainfall intensities (60 and 120 mm/h) were carried out to analyze the sheet erosion (including runoff and erosion) and the sediment sorting of sub-rainfall events. Moreover, the microscopic influence mechanism of EICP on sheet erosion was revealed using scanning electron microscopy. The results indicated that the total runoff for only urease spray treatment increased on average by 10.68%, whereas, the total erosion was reduced on average by 43.74%, respectively, compared with the CK; The total runoff under EICP spray treatments increased on average by 29.51% to 50.76%, whereas, there was the decrease on average by 85.57% to 90.90%, respectively. The only urease spray treatment showed a certain effect on increasing runoff and reducing sediment. While the EICP spray treatment showed a significant effect on increasing runoff and reducing sediment (P<0.05). The increasing runoff and reducing sediment shared an increasing trend followed by the reduction with the increase of cementing solution concentrations. The cementing solution concentration of 1.5 mol/L performed the best. However, there was no significant difference in the total runoff and erosion among EICP spray treatments with different cementing solution concentrations (P>0.05), indicating the low concentration (0.5 mol/L) suitable for the control of sheet erosion. The erosion sediment exhibited a distinct sorting with the enrichment of clay, fine silt, and coarse silt particles and a depletion of sand particles. The degree of sediment sorting was reduced after EICP spray treatment, with a decrease in clay content and an increase in coarse sand particle content in the erosion sediment. Microscopic analysis showed that the spherical calcium carbonate precipitates and urease filaments often appeared in the gaps and/or on the surfaces of soil particles after EICP spray treatment. The soil was reinforced to enhance the erosion resistance, leading to reduced soil erosion. The infiltration was reduced to increase the runoff production. The finding can also provide theoretical support to the sheet erosion reduction of EICP for the soil and water conservation in the TGRA.