The vegetation-tidal gradient regulates sediment organic carbon stabilization via microbial transformation and iron-mediated mineral protection in Yangtze Estuary wetlands.

Estuarine wetlands are important 'blue carbon' sinks, yet their carbon storage exhibits pronounced spatial heterogeneity. The mechanisms by which vegetation distribution and tidal flooding affect sediment organic carbon (SOC) content and stabilization remain unclear. Here, we analyzed the SOC fractions, microbial community composition, and enzyme activities to elucidate the SOC stabilization mechanisms along a vegetation-tidal gradient in the Yangtze Estuary wetlands. Our results showed that the contents of SOC, mineral-associated organic carbon (MAOC), particulate organic carbon (POC), microbial necromass carbon (MNC), iron-associated organic carbon (Fe-OC), microbial biomass carbon (MBC), and enzyme activities decreased from the high-tidal Phragmites australis (HTP) zone to the tidal flat (TF), whereas the relative contributions of Fe-OC and MAOC to SOC rose progressively. Microbial communities shifted from K-strategists to r-strategists and from iron-oxidizing bacteria (FeOB) to iron-reducing bacteria (FeRB) along the gradient. Random forest analysis and partial least squares path modeling revealed that POC accumulation was primarily driven by vegetation-derived inputs and microbial processing, whereas MAOC accumulation was more closely associated with iron-mediated mineral protection. Overall, carbon accumulation in the high-tidal zone is primarily governed by vegetation-derived inputs and microbial transformation, whereas long-term carbon stabilization in low-tidal flats relies more strongly on iron-mediated mineral protection. Increased tidal flooding was associated with greater reliance on mineral-dominated stabilization, yet vegetation preservation remains indispensable for counteracting carbon losses and ensuring sustained sequestration. These findings clarify the spatial patterns and underlying mechanisms of SOC stabilization and provide a scientific basis for blue carbon conservation in estuarine wetland ecosystems.