Microbial community dynamics behind major release of methane in constructed wetlands

It is global practice to construct wetlands in urban environments to treat pollutants from stormwater and overland runoff. However, constructed wetlands can also trigger climatic consequences by releasing a considerable amount of greenhouse gas emissions via diffusion and ebullition. While diffusive emissions are broadly measured, assessing the extent of ebullitive emissions is less common. This is due to the stochastic nature of ebullitive emissions and the complexity in their measurement. In addition, the soil microbiome community involved in greenhouse gas transport pathway has been generally understudied in Constructed wetlands although widely recognized in natural wetlands. Here we investigated diffusive and ebullitive CO2 and CH4 and ebullitive N2O emissions and the prokaryotic community from four municipal Surface Flow Constructed wetlands in southeast Australia over a 4-week period. We found that the total Global Warming Potential over a 100-year horizon of diffusive and ebullitive emissions was 0.44 kg CO2-equivalents m-2 day-1, which is 3-fold higher than the European estimates that accounted for diffusive emissions alone. Among the microbial functional groups that drove the main differences in beta-diversity, putative methanotrophs represented the 3.6%, nitrogen-cycling microbes the 44.5% and sulfate/iron cycling microbes the 13.5%, indicating that several taxa, other than methanogens, can be key in microbial dynamics, nutrient turnover and in the shift in the production of greenhouse gas thus amplifying the role of constructed wetlands as sources of CH4. We present evidence that Constructed wetlands can be major sources of greenhouse gases, particularly ebullitive CH4 that accounted for more than two-thirds of the total emissions. As such, ebullitive flux rate measurements are critical to assemble national greenhouse gas budgets, particularly from Constructed wetlands.