Divergent contributions of microbes and plants to soil organic carbon in the drawdown area of a large reservoir: Impacts of periodic flooding and drying

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The distribution patterns and accumulation mechanisms of plant and microbial residues, along with their potential contributions to soil organic carbon (SOC), remain subjects of considerable debate, particularly within drawdown areas affected by reservoir operation. In this study, surface soil samples (0–10 cm) were collected from three different elevations within the drawdown area of the Three Gorges Reservoir. Amino sugars and lignin phenols served as biomarkers for microbial residues and plant-derived materials, respectively. The results revealed that with increasing duration of flooding, the content of amino sugars increased from 0.26 mg g−1 to 0.64 mg g−1, whereas the content of lignin phenols decreased from 204.09 mg kg−1 to 37.93 mg kg−1. Moreover, as the duration of flooding increased, the contribution of microbial necromass carbon (MNC) to SOC rose from 29% to 47%, while the contribution of plant-derived carbon to SOC gradually declined. Plants biomass and iron minerals influenced the accumulation of lignin phenols, whereas amino sugars were affected by plants biomass, microbial biomass carbon and nitrogen, and clay minerals. The periodic flooding and drying events induced alterations in carbon inputs and environmental characteristics within the drawdown area, resulting in fluctuations in the contributions of plants and MNC to SOC in this region. The findings of this study highlight the critical role played by both plant- and microbial-derived carbon in the retention and turnover of SOC within the terrestrial-aquatic transition zone.