High methane emissions as trade-off for phosphorus removal in surface flow treatment wetlands

Constructed wetlands (CW) treating runoff from agricultural catchments reduce the nutrient load of water, however, they can also be significant sources of greenhouse gases, especially methane (CH₄). We simultaneously assessed CH₄ emission potentials and phosphorus (P) removal efficiency in a 0.45 ha in-stream surface flow CW to determine the main drivers of CH₄ emissions, and to analyze the temporal dynamics of CH₄ emissions and P removal during an almost 4-year period. The TP (total phosphorus) removal efficiency had a clear seasonal dynamic, with the highest removal occurring during summer and early autumn (monthly average 60.5%), when the flow rate was lowest and water residence time longest. Due to increasing sedimentation and related anaerobic conditions, the mean hourly CH₄ emissions for each year demonstrated an increasing trend over the years: from 88 µg CH₄-C m⁻² h⁻¹ in 2018–2505 µg CH₄-C m⁻² h⁻¹ in 2021. There was a clear seasonality in CH₄ emissions: up to 90% of CH₄ fluxes occurred during the warm period (from May to October). We assume that maintenance of treatment wetlands is essential and predominantly regular removal of aboveground vegetation at the second half of the growing season would decrease CH₄ emissions. Nevertheless, due to the P saturation in sediments, regular sediment removal in the long term is also necessary.