Seasonal variation and influencing factors of gas emission from free water surface flow constructed wetlands

Orthogonal experiments were conducted to investigate the gas emission patterns of free water surface flow constructed wetlands (FWS CWs). The correlation between the emission fluxes of four wetland gases—ammonia (NH₃), carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O)—and several characteristics, including design parameter, hydraulic performance, purification effect, soil nutrients, and microbial communities were analyzed. The results indicated that gas emission fluxes were significantly higher in the warm seasons than in the cold season. In warm seasons, water depth and inflow total nitrogen (TN) concentration emerged as the most crucial design parameters influencing CO₂ and CH₄ emissions, and NH₃ and N₂O emissions, respectively. Higher water depth effectively reduced gas emission, as well as hydraulic and treatment performance. Furthermore, gas emission fluxes exhibited positive correlations with hydraulic performance and negative correlations with nitrogen removal. The emission fluxes of CO₂ and CH₄ demonstrated significant positive correlations with the soil carbon to nitrogen ratio in cold season. However, there was no consistent significant correlation between gas emissions and soil microbes. The relative abundance of Nitrospirota showed moderate to significant inhibitory effects on CH₄ emissions in warm seasons, while that of Planctomycetota exhibited significant positive correlations with NH₃ and CH₄ emissions in early winter. This study highlighted the seasonal variations in gas emissions influenced by multiple factors, underscored the complexities involved in selecting design parameters for optimal wetland performance, and pointed out the challenges in achieving multi-objective optimization of FWS CWs.