Grass cover on karst slopes effectively reduces rill erosion due to concentrated flow

Soil degradation caused by concentrated flow-induced erosion poses a substantial threat to global land and water resource sustainability. Although grass cover can contribute to enhancing the structural stability of soil and mitigates erosion via root reinforcement and surface protection, the mechanisms whereby concentrated flows influence soil erosion in karst areas under grass cover have yet to be sufficiently determined. In this study, we conducted controlled scouring experiments using a flume system to simulate karst slopes with gradients of 5°, 15°, and 25° by comparing bare slopes with slopes vegetated with Cynodon dactylon (L.) Persoon. The results revealed that grass cover spatially redistributes runoff, thereby reducing surface runoff by 10 %–33 %, whilst increasing soil–rock interface runoff by 9 %–79 %. In addition, we found that soil loss under grass cover was 8 %–80 % of that on bare slopes, with the efficacy of erosion mitigation diminishing as the gradient of the slope increased. Furthermore, grass cover contributed to a significant alteration in rill morphology. Although bare slopes were characterized by shorter total rill lengths, their average rill width (1.56–9.31 cm) and depth exceeded those of grass-covered slopes (1.77–5.70 cm) at gradients of 15° and 25°. Under identical flow conditions, grass cover was found to promote increases in the rill width-to-depth ratio by 41.49 %–69.46 %, indicating a stronger suppression of down-cutting erosion. Collectively, these findings revealed that grass cover suppresses rill formation via the dual regulation of surface hydrology and sediment transport, primarily by dispersing concentrated flow energy and stabilizing the soil via root networks. Our findings in this study will contribute to advances in our mechanistic understanding of vegetation–erosion feedback in karst systems and provide insights for soil conservation in vulnerable ecosystems.