Inundation depth modulates wetland soil organic carbon stability by altering microbial biomass and mineral protection

Wetlands are essential carbon (C) reservoirs, crucial for climate change mitigation, but their hydrological regimes are increasingly threatened by climate change. Inundation depth, a key hydrological factor in wetlands, influences Soil Organic Carbon (SOC) storage and stability, yet its effects on SOC fractions and stability, especially in subsoils, remain critically underexplored. This study investigated the impact of inundation depth on SOC in the topsoil (0–20 cm) and subsoil (20–40 cm) over a 6-year field experiment. As the inundation depth increased from 0 cm to 40 cm, SOC content increased significantly by 62% in the topsoil and by 222% in the subsoil. Particulate Organic C (POC) and Mineral-Associated Organic C (MAOC) responded differently across soil layers, with topsoil POC driven by plant inputs and MAOC by microbial turnover, while subsoil POC and MAOC were influenced by nutrient availability and mineral protection, respectively. Inundation depth increased the subsoil SOC stability but did not affect topsoil stability, where key mineral protection capacity (i.e., clay and free/amorphous Fe/Al oxides) remained unaltered. In the subsoil, both microbial biomass and mineral protection jointly enhanced SOC stability. We identified 20 cm inundation depth as an operational benchmark where SOC accumulation reaches a saturation plateau. This suggests that, within hydrologically managed restoration settings in the Yellow River Delta, maintaining moderate inundation around this depth may help maximize SOC storage, whereas deeper inundation did not provide additional SOC gains. Collectively, these findings advance our mechanistic understanding of depth-dependent carbon dynamics and provide a practical reference for site-level hydrological regulation, although the applicability of this threshold to other wetlands requires further validation.