Key Biotic Control of DOM Chemodiversity Drives Divergent GHG Flux in China’s Constructed Wetlands

Chemodiversity of dissolved organic matter (DOM) is critical in greenhouse gas (GHG) dynamics in aquatic ecosystems, yet the mechanisms by which specific DOM components and biotic communities are associated with GHG flux variation in constructed wetlands (CWs) remain poorly understood. Here, we integrated ultrahigh-resolution DOM profiling, absolute bacterial quantification, vegetation surveys, and direct water-air GHG flux measurements across 39 field CWs spanning approximately 2700 km in China. We found that DOM chemodiversity was a stronger predictor of paired influent and effluent water-air GHG flux variation, expressed as FCO2-eq, than conventional environmental variables. Based on paired changes in FCO2-eq, CWs were classified into increased-flux CWs (ICWs) and decreased-flux CWs (DCWs), which showed distinct DOM transformation patterns. ICWs decomposed labile low-molecular-weight DOM stimulating higher GHG emissions, while DCWs were dominated by recalcitrant highly aromatic DOM associated with lower fluxes. Microbial diversity showed stronger associations with DOM chemistry and GHG flux patterns than plant diversity. Keystone bacteria such as Sphingomonas and Sphingopyxis dominated ICWs and promoted emissions, whereas Pseudarthrobacter and Micromonospora in DCWs favored carbon stabilization. Path analysis further verified that microbes exerted stronger effects than macrophytes (e.g., Canna indica or Phragmites australis) on DOM and GHG relationships. Our findings highlight the importance of DOM and bacterial community coupling in shaping water-air GHG flux variation in CWs, providing field scale evidence to support future CW design and management for water purification and climate mitigation.