Eutrophication leads to production and accumulation of recalcitrant dissolved organic matter in an urban-agricultural wetland

Over 30% of global wetlands are threatened due to eutrophication driven by anthropogenic activities that alter dissolved organic matter (DOM) concentrations and composition, and profoundly influence wetland carbon budgets. However, the fate of eutrophication-derived DOM and its ultimate contribution to wetland carbon cycling remain unclear. To address this gap, we conducted a two-year monthly investigation of water quality and DOM optical properties (absorbance and fluorescence) in a eutrophic, urban-agricultural wetland. Dissolved organic carbon increased from spring to summer in both years and was significantly correlated with TLI, TN, COD and Chl a , indicating that eutrophication contributed to the organic carbon pool through both sewage inputs and algal production. Moreover, the loss of protein-like C3, together with a significant negative correlation between microbial humic-like C2 and DO, suggested that microbial deoxygenation transformed eutrophication-derived bio-labile DOM into recalcitrant DOM (RDOM) during summer. In winter, lower temperatures limited microbial carbon transformations, and together with continuous sewage inputs, contributed to an increase in protein-like C3. Concomitantly, RDOM removal through particle adsorption-sedimentation and/or photodegradation exceeded its accumulation during winter. Machine learning analyses suggested that microbial transformation explained 39.1-41.2% of the variations in humic-like components, surpassing terrestrial inputs, sewage sources and algal production (2.1-31.6%). Therefore, microbial transformations are considered the dominant driver of wetland RDOM formation. Although eutrophication has negative effects on wetland ecosystems, our findings highlight its potential to enhance carbon sequestration if sewage and algae are effectively managed. • A two-year monthly investigation revealed long-term DOM dynamics in a eutrophic wetland • Eutrophication enhances transformation of bio-labile DOM into RDOM via microbial deoxygenation • Temperature is the key factor regulating DOM turnover in eutrophic wetlands • Microbial transformation is the dominant source of RDOM rather than terrestrial inputs