Abstract: Flooding duration and vegetation structure are key regulators of soil organic carbon (SOC) dynamics in mangrove ecosystems, yet their interactive effects and underlying mechanisms remain poorly understood. Here, we investigated how flooding duration and stand type jointly influence SOC content, molecular composition, and microbial community structure in mangrove soils. Our results show that flooding duration and stand type exert significant and interactive effects on SOC dynamics rather than acting independently. Flooding duration primarily constrained organic matter decomposition by regulating soil redox conditions, whereas stand type controlled carbon inputs through differences in litter quantity and quality. Microbial community shifts mediated the responses and were associated with changes in SOC molecular characteristics, indicating coupled controls on carbon stabilization. By integrating SOC quantity, molecular composition, and microbial mediation, this study provides a mechanistic framework for understanding SOC stabilization in flooded forest soils. Our findings highlight that effective mangrove restoration and blue carbon management should consider the combined and interactive influences of hydrological regimes and vegetation structure under changing coastal environments.

Key words: mangrove restoration, soil organic carbon, flooding duration, microbial community, carbon sequestration