Vegetation-mediated surface soil organic carbon formation and potential carbon loss risks in Dongting Lake floodplain, China

Sources and stabilization mechanisms of soil organic carbon (SOC) fundamentally govern the carbon sequestration potential of wetland ecosystems. Nevertheless, systematic investigations regarding SOC sources and molecular stability remain scarce in floodplain wetland environments. This study employed dual analytical approaches (stable isotope analysis and 13 C nuclear magnetic resonance spectroscopy) to characterize surface SOC composition (0–20 cm) across three dominant vegetation communities ( Miscanthus , Carex , and mudflat) in Dongting Lake floodplain wetlands. Key findings revealed: (1) Significantly elevated SOC concentrations in vegetated communities ( Miscanthus : 13.76 g kg −1 ; Carex : 12.98 g kg −1 ) compared to unvegetated mudflat (6.88 g kg −1 ); (2) Distinct δ 13 C signatures across communities, with the highest isotopic values in Miscanthus (−22.67 ‰), intermediate in mudflat (−26.01 ‰), and most depleted values in Carex (−28.25 ‰); (3) Bayesian mixing models identified autochthonous plant biomass as the primary SOC source ( Miscanthus : 53.3 % ±10.6 % , Carex : 52.4 % ±11.6 % , mudflat: 47.5 % ±12.5 % ); (4) Spatial heterogeneity in particulate organic matter (POM)contributions across sub-lakes, showing descending contributions from South (highest) > West > East (lowest) Dongting Lake; (5) Molecular characterization revealed O-alkyl C dominance (30.5 %–46.8 %), followed by alkyl C and aromatic C. Notably, Miscanthus soils exhibited enhanced content (44.75 %) (Alip/Arom: 3.64) and reduced aromaticity (0.22) / hydrophobicity (0.68) indices, suggesting comparatively lower biochemical stability of its SOC pool. These results highlight the critical role of vegetation-mediated SOC formation processes and warn against potential carbon loss risks in Miscanthus -dominated floodplain ecosystems, providing a scientific basis for carbon management of wetland soils.