Chemically recalcitrant DOM accumulation and distinct microbial assembly under hydrological regulation in a managed...

Hydrological regulation can enhance carbon accumulation in coastal wetlands, yet whether this carbon is merely retained or transformed into chemically recalcitrant forms remains unclear. Here, we investigated sediment geochemistry, DOM molecular composition, and microbial communities across adjacent subtropical coastal habitats in Hong Kong, including semi-enclosed, hydrologically regulated ponds embedded within mangrove wetlands (GW), mangroves, tidal flats, and seagrass habitats. Compared with adjacent open habitats, GW sediments contained substantially higher organic carbon stocks and isotopic signatures indicative of enhanced retention of terrestrial and mangrove-derived organic matter. FT-ICR MS revealed a pronounced enrichment of high-molecular-weight, highly unsaturated, aromatic, and oxidized molecules, indicating the accumulation of chemically recalcitrant DOM. This molecular shift coincided with a distinct microbial community. Redundancy analysis revealed strong associations among microbial community structure, DOM molecular characteristics, and sedimentary δ¹⁵N, while variation partitioning showed that DOM molecular variation was jointly associated with organic matter source, sediment environment, and microbial community composition. Differential abundance analysis further identified 108 genera significantly enriched in GW sediments. Within GW, exploratory structural equation modeling suggested a potential pathway linking substrate conditions and the GW-enriched microbial assemblage with δ¹⁵N and recalcitrant DOM, while Random Forest analysis identified candidate genera strongly associated with specific DOM molecular characteristics. Together, these patterns suggest that hydrological regulation enhances organic matter retention while creating conditions conducive to chemically recalcitrant DOM accumulation, with potential implications for carbon persistence in coastal wetlands.