Plant-microbe involvement: How manganese achieves harmonious nitrogen-removal and carbon-reduction in constructed wetlands

Carbon deficits in inflow frequently lead to inefficient nitrogen removal in constructed wetlands (CWs) treating tailwater. Solid carbon sources, commonly employed to enhance denitrification in CWs, increase carbon emissions. In this study, MnO2 was incorporated into polycaprolactone substrates within CWs, significantly enhancing NH4+-N and NO3–-N removal efficiencies by 48.26–59.78 % and 96.84–137.23 %, respectively. These improvements were attributed to enriched nitrogen-removal-related enzymes and increased plant absorption. Under high nitrogen loads (9.55 ± 0.34 g/m3/d), emissions of greenhouse gases (CO2, CH4, and N2O) decreased by 147.23–202.51 %, 14.53–86.76 %, and 63.36–87.36 %, respectively. N2O emissions were reduced through bolstered microbial nitrogen removal pathways by polycaprolactone and MnO2. CH4 accumulation was mitigated by the increased methanotrophs and dampened methanogenesis, modulated by manganese. Additionally, manganese-induced increases in photosynthetic pigment contents (21.28–64.65 %) fostered CO2 sequestration through plant photosynthesis. This research provides innovative perspectives on enhancing nitrogen removal and reducing greenhouse gas emissions in constructed wetlands with polymeric substrates.