Greenhouse gas emissions and carbon budget estimation in constructed wetlands treating aquaculture tailwater: Insight from seasonal dynamics of dissolved organic matter and microbial community

Constructed wetlands (CWs) effectively treat aquaculture tailwater but face challenges from greenhouse gas (GHG) emissions affecting their carbon sink potential. Few systematic studies have been conducted to identify the drivers of temporal variation of GHG emissions in CWs. Through year-round monitoring of an integrated vertical flow constructed wetland (IVCW), we identified seasonal dynamics in carbon oxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) emissions, with summer fluxes exceeding winter levels by 12-fold for CH₄ and 3-fold for N₂O. Through the Mantel test, we found that the relative abundance of fulvic acid, tryptophan-like components, and functional groups related to the carbon and nitrogen cycle remarkably modified the temporal variation in CH₄ and N₂O emissions. The variation in CO₂ emission was primarily regulated by chromophoric dissolved organic matter (DOM) concentration and was indirectly influenced by water properties. Redundancy analysis revealed that water physicochemical parameters and DOM characteristics jointly explained 36.0 % and 49.7 % of the variation in bacterial composition at the phylum and genus levels, respectively, with aromaticity and molecular weight of DOM as key determinants. The IVCW functioned as a net carbon sink with a rate of annual carbon sequestration averaging 1532.36 g C m⁻² yr⁻¹, offering substantial potential for emission reductions. By elucidating how seasonal variations in DOM characteristics and microbial community structure influence GHG emissions in CWs, this study advances our understanding of the mechanisms driving the performance of CWs in terms of carbon sequestration. We propose that regular plant harvesting can enhance the role of CWs as carbon sinks.