Nitrous oxide emissions and the use of wetlands for water quality amelioration

Wetlands ameliorate nitrate pollution but have also been recognized as a source of the greenhouse gas nitrous oxide. Nitrate and N2O fluxes were studied in an experimental wetland in mid-Wales. Diversion of water inflows caused a 200% increase in nitrate release and a >95% decline in nitrous oxide emission over a 20-week period. The responses were attributed to the onset of drier (more aerobic) conditions causing (i) aerobic mineralization and nitrification of N-containing compounds that had previously been immobilized within the wetlands (releasing nitrate) and (ii) the absence of anaerobic denitrification (a potent mechanism for nitrate elimination and source of N2O), allowing NO3-N to leave the wetland. The responses were instantaneously reversible upon re-initiation of the nitrate inflow, indicating a close hydrological coupling between nitrate removal and nitrous oxide emission processes.