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 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.