Erosion-deposition dynamics shape soil microbial communities and CO₂ fluxes in a tropical island watershed

Soil erosion and deposition reshape carbon pools, but how geomorphic redistribution regulates microbial communities and CO 2 flux remain unclear. In tropical island watersheds, intense rainfall, steep terrain, and coarse-textured soils may intensify soil redistribution and amplify variability in surface carbon dynamics. This study evaluated how erosion-deposition patterns were associated with soil physicochemical conditions, microbial assemblages, and soil CO 2 flux along four transects within the Tengqiao River watershed, Baoting, Hainan Island, China. The Unit Stream Power-based Erosion/Deposition (USPED) model revealed spatial heterogeneity in erosion and deposition rates, ranging from − 7.90 to 22.61 t ha − 1 yr − 1 across the 20 sampling sites. Compared with erosion sites, deposition sites tended to show higher nutrient contents, SOC concentration and surface SOC stock, together with lower bulk density (BD). Microbial community composition and co-occurrence networks also differed between erosion and deposition sites. Deposition sites favored taxa linked to nutrient cycling, whereas erosion sites were associated with stress-tolerant taxa. Across the measured months, soil CO 2 fluxes were significantly higher in the rainy season than in the dry season. Partial least squares structural equation modeling (PLS-SEM) showed that erosion-deposition dynamics reduced soil water content (SWC) and pH, increased BD, and significantly reduced SOC. Soil physical condition (SWC, BD) and pH were further associated with significant decreases in bacterial and fungal diversity, and bacterial diversity showed a significant positive association with soil CO 2 flux. Erosion-deposition processes were associated with surface soil redistribution, microbial community restructuring, and seasonal variation in soil CO 2 flux. In this coarse-textured tropical island watershed, erosion-deposition dynamics reduced SOC and altered soil physical and chemical conditions, thereby weakening bacterial and fungal diversity. The significant linkage between bacterial diversity and soil CO 2 flux suggests that bacterial assemblages may play an important role in mediating short-term carbon release. These results highlight the need to integrate geomorphic and microbial processes when assessing carbon responses in erosion-prone ecosystems.