Biocrust effects on soil stability and hydrodynamic erosion in southern China's red soil

Biocrusts are essential for regulating soil erosion and hydrologic processes. However, the response of biocrusts to soil aggregate stability, erosion processes and their hydrodynamic characteristic in eroded red soil remains underexplored. This study investigated the effects of biocrust cover (BC) on the erosion processes, flow hydrodynamics, and soil aggregate stability under varying rainfall intensities (RI). Simulated rainfall experiments were conducted on bare and BC red soil plots under four RI (0.5, 1, 1.5, and 2 mm min^-1). Structural equation modeling (SEM) was employed to identify the direct and indirect pathways through which BC affects erosion. The results showed that BC significantly reduced runoff and erosion rate and improved soil stability (P < 0.05), as indicated by increased mean weight diameter and geometric mean diameter values. A strong linear relationship was observed between runoff rate and erosion rate (R² = 0.834-0.944), indicating that soil erosion on BC plot is primarily governed by runoff processes. Stream power (VP) was strongly correlated with both runoff rate (R² = 0.994) and erosion rate (R² = 0.797), suggesting its suitability as a key predictor of erosion dynamics. Importantly, soil aggregate loss exhibited a bimodal distribution, characterized by two distinct peaks in particle size, with enrichment in <0.5 mm and 1-2 mm aggregates. SEM further revealed that increased BC (path coefficients = 0.60), enhanced aggregate stability, and decreased stream erosive power were key mechanisms for reducing soil erosion. These findings provide novel insights into the ecohydrological functions of biocrusts and highlight their practical potential as a nature-based solution for erosion control and land restoration in subtropical red soil regions, informing sustainable land management strategies.