Vertical Distribution and Stability of Soil Carbon Pools Across Ecological Zones in the Dongting Lake Basin

Understanding responses of soil organic carbon (SOC) fractions to variable hydrological conditions is essential to clarify carbon stabilization in subtropical lake wetlands. This study examined vertical and spatial variations of particulate organic carbon (POC), mineral-associated organic carbon (MAOC) and nutrient stoichiometry across four sites in Dongting Lake (0–200 cm depth, five layers). SOC and total nitrogen (TN) showed significant decreasing trends with depth from the surface to 60 cm (p < 0.05), with the highest values in the 0–40 cm layer and significantly lower values below 60 cm. In contrast, total phosphorus (TP) was vertically stable but differed greatly among sites. C/N, C/P and N/P ratios showed distinct regional disparities (p < 0.05). The proportions of POC/SOC and MAOC/SOC exhibited highly significant regional differences (p < 0.05) but no significant vertical variation or interaction effects (p > 0.05). The POC/MAOC ratio exceeded 1 only at Tuanzhou (1.62–2.20), indicating a vulnerable POC-dominated pool; other sites were MAOC-dominated (ratio < 1). Site-specific nutrient regulation further differentiated carbon pool characteristics: nutrients dominated carbon fraction variation at Tuanzhou and Huanghua, co-functioned with soil texture at Junshan, and reduced carbon stability under nutrient enrichment at West Dongting Lake. The texture control effect intensified with soil depth, and deep soil exhibited the strongest mineral protection capacity. We conclude that relative proportions of POC and MAOC in total SOC are region-specific, while their absolute concentrations decrease with depth. We recommend that wetland restoration prioritizes maintaining natural hydrological regimes and soil texture integrity, with future validation across multiple floodplain wetlands.