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 N 2 O 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 mech anism for nitrate elimination and source of N 2 O), allowing NO 3 -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.