Hydrological Control of SOC Dynamics via Particle Size Redistribution Along Elevation Gradients in the Water Level Fluctuation Zone of the Three Gorges Reservoir

The water level fluctuation zone (WLFZ) of the Three Gorges Reservoir (TGR) represents a distinctive ecotone with inverted hydrological regimes, where elevation gradients play a critical role in determining the spatial distribution and stability of soil organic carbon (SOC). The objective of this study was to test whether soil particle size mediates the effects of hydrological fluctuations on SOC dynamics across elevation gradients. In this study, soils from three elevation zones (155–165 m, 165–175 m, and non-flooded zones) were collected, and bulk soil and particle-size fractions (sand, silt, and clay) were incubated for 60 days to assess SOC mineralization. The results indicated that the SOC stock in the main stream was greater at middle elevations (3.94 ± 0.26 kg·m−2) than at high elevations (3.20 ± 0.18 kg·m−2), whereas the SOC stock in the tributary was greater at high elevations (3.39 ± 0.18 kg·m−2). Random forest and linear regression analyses revealed that total nitrogen (TN) and sand contents were the primary factors controlling SOC. Despite its lower SOC content, the sand fraction presented significantly higher turnover rates (102.14 ± 36.13 μg CO2-C·g−1C·h−1) than the finer fractions, indicating lower carbon stability. These findings suggest that hydrological fluctuations regulate SOC by altering the soil particle-size composition across elevation gradients.