Characteristics of Greenhouse Gas Emissions from Constructed Wetlands Vegetated with Myriophyllum aquatic: The Effects of Influent C/N Ratio and Microbial Responses

Abstract

This study designed surface flow constructed wetlands (SFCWs) with Myriophyllum aquaticum (M. aquaticum) to evaluate how different influent C/N ratios (0:1 (C0N), 5:1 (C5N), 10:1 (C10N), and 15:1 (C15N)) affect pollutant removal, greenhouse gas (GHG) emissions, and microbial communities. The results showed that effluent ammonia nitrogen (NH4+-N), nitrate nitrogen (NO3−-N), and total nitrogen (TN) concentrations decreased, but effluent chemical oxygen demand (COD) concentration increased with increasing influent C/N ratios. The highest removal rates of TN (73.17%) and COD (74.56%) were observed with C5N. Regarding GHG emissions, a few changes in CO2 fluxes were caused by the influent C/N ratio, whereas CH4 fluxes obviously increased with the increasing influent C/N ratio. The highest N2O emission occurred with C0N (211.03 ± 44.38 mg-N·m−2·h−1), decreasing significantly with higher C/N ratios. High-throughput sequencing revealed that different influent C/N ratios directly influenced the microbial distribution and composition related to CH4 and N2O metabolism in SFCWs. The highest abundance (46.24%) of denitrifying bacteria (DNB) was observed with C5N, which helped to achieve efficient nitrogen removal with a simultaneous reduction in N2O emissions. Methanogen abundance rose with higher C/N ratios, whereas methanotrophs peaked under C5N and C10N conditions. Additionally, the random forest model identified influent C/N ratio and Rhodopseudomonas as primary factors influencing CH4 and N2O emissions, respectively. This highlights the importance of the influent C/N ratio in regulating both pollutant removal and GHG emissions in constructed wetlands.