| Summary: | 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 (C<sub>0</sub>N), 5:1 (C<sub>5</sub>N), 10:1 (C<sub>10</sub>N), and 15:1 (C<sub>15</sub>N)) 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 C<sub>5</sub>N. Regarding GHG emissions, a few changes in CO<sub>2</sub> fluxes were caused by the influent C/N ratio, whereas CH<sub>4</sub> fluxes obviously increased with the increasing influent C/N ratio. The highest N<sub>2</sub>O emission occurred with C<sub>0</sub>N (211.03 ± 44.38 mg-N·m<sup>−2</sup>·h<sup>−1</sup>), 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 CH<sub>4</sub> and N<sub>2</sub>O metabolism in SFCWs. The highest abundance (46.24%) of denitrifying bacteria (DNB) was observed with C<sub>5</sub>N, which helped to achieve efficient nitrogen removal with a simultaneous reduction in N<sub>2</sub>O emissions. Methanogen abundance rose with higher C/N ratios, whereas methanotrophs peaked under C<sub>5</sub>N and C<sub>10</sub>N conditions. Additionally, the random forest model identified influent C/N ratio and <i>Rhodopseudomonas</i> as primary factors influencing CH<sub>4</sub> and N<sub>2</sub>O emissions, respectively. This highlights the importance of the influent C/N ratio in regulating both pollutant removal and GHG emissions in constructed wetlands.
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