Optimal influent COD/N ratio for obtaining low GHG emissions and high pollutant removal efficiency in constructed wetlands

Influent chemical oxygen demand (COD)/nitrogen (N) ratio plays a crucial role in the biogeochemical cycle of carbon and N in constructed wetlands (CWs). This study evaluated the greenhouse gas (GHG) emissions, pollutant removal efficiency and specific microbial gene abundances in subsurface flow CWs (SSFCWs) under various influent COD/N ratios. The GHG emissions and pollutant removal efficiency were markedly affected by influent COD/N ratios. The effluent concentrations of nitrate decreased with the increase of influent COD/N ratios, while the trend was completely reversed for COD. The lowest effluent total nitrogen concentration was observed with a COD/N ratio of 10. For GHG emissions, the CO2 fluxes increased with increasing influent COD/N ratios. However, the lowest CH4 (- 42.49 mu g/m(2)/h) and N2O fluxes (31.77 mu g/m(2)/h) were both observed under a COD/N ratio of 10. A significantly positive correlation between N2O fluxes and abundance of nirS and (nirS + nirK) (r(2) = 0.64 and 0.61, p < 0.05, respectively) was observed. The highest ratio of nosZ/(nirS + nirK) observed at a COD/N ratio of 10 suggested the dominance of nosZ-harboring denitrifiers communities. The highest ratio of nosZ/(nirS + nirK) was also consistent with the lowest N2O flux observed at a COD/N ratio of 10. For the global warming potential (GWP), the lowest value (52.89 mg/m(2)/h) was observed at a COD/N ratio of 10. Overall, the influent COD/N ratio of 10 was determined to be optimal for simultaneously achieving relatively higher pollutant removal efficiency and lower GHG emissions.