Temporal and seasonal changes in greenhouse gas emissions from a constructed wetland purifying peat mining runoff waters

Constructed wetlands (CW), widely used to remove nutrients from runoff waters, transform some of the carbon and nitrogen they receive into greenhouse gases, carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O), and may therefore have adverse atmospheric impacts. We studied seasonal and temporal changes in C degradation and emissions of CH4 and N2O of a boreal CW used to purify peat mining runoff waters 5 (in 1992) and 15 (in 2001–2002) years after construction. There was a remarkable change in the cycling of carbon in the wetland as the number of years in operation increased: the mean CH4 emission tripled from 140 to 400 mg CH4 m−2 d−1 and the mean CO2 release (respiration) doubled from 7270 to 13 600 mg CO2 m−2 d−1 in the 10-year period. The reasons for the increased C gas production were the increased plant biomass, which doubled in 10 years, and a 3 °C higher average temperature in 2002 than in 1992. The N2O fluxes did not change during the study period: the mean emissions were 340 and 450 μg N2O m−2 d−1 in 1992 and 2002.