Moisture Drives Soil Organic Carbon Dynamics Through Fungal Necromass Inputs and Community Shifts in Floodplain Wetlands: an Ecohydrological Perspective

Global climate change has expanded floodplain wetland areas and accelerated water recession rates, which are expected to alter soil organic carbon (SOC) content at larger scales via moisture-driven ecohydrological processes. Microorganisms act as central regulators of SOC dynamics through their dual roles in organic matter decomposition and necromass production, both of which are governed by microbial community structure and function. However, how moisture influences these microbial traits and necromass formation to mediate SOC dynamics in floodplain wetlands remains unclear. Here, we performed a comprehensive analysis of microbial necromass carbon (C), community diversity and composition, and C metabolism-associated functions to decipher microbial-mediated SOC dynamics along a moisture gradient in Poyang Lake floodplains. We observed that SOC decreased by 62.1 % as soil moisture dropped from 53.3 % to 16.6 %, primarily attributed to the depletion of fungal necromass C rather than bacterial necromass C. Fungal community composition was the key microbial factor influencing SOC. Fungal taxonomic and functional groups were more closely associated with SOC than bacterial counterparts. Moreover, reduced moisture shifted microbial life-history strategies from high yield to resource acquisition, enhancing the degradation of recalcitrant C (e.g., lignin) rather than labile C (e.g., starch and cellulose). These shifts in microbial metabolic functions contributed to decreased microbial (particularly fungal) necromass and SOC under low moisture. Our study highlights the critical roles of fungal necromass and community composition in driving SOC dynamics under varying moisture conditions, providing novel microbial insights for assessing the responses of floodplain wetland soil C pools to hydrological changes.