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

This study designed surface flow constructed wetlands (SFCWs) with <i>Myriophyllum aquaticum</i> (<i>M. aquaticum</i>) 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 (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">H</mi></mrow><mn>4</mn><mo>+</mo></msubsup></semantics></math></inline-formula>-N), nitrate nitrogen (<inline-formula><math xmlns="http://www.w3.org/1998/Math/MathML" display="inline"><semantics><msubsup><mrow><mi mathvariant="normal">N</mi><mi mathvariant="normal">O</mi></mrow><mn>3</mn><mo>−</mo></msubsup></semantics></math></inline-formula>-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 <i>Rhodopseudomonas</i> 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.