Denitrification as a sink for dissolved nitrous oxide in a freshwater riparian fen

Denitrification as a sink of dissolved nitrous oxide (N2O) was investigated in a freshwater riparian fen. In a 15-m transect extending from the hillslope and into the fen the groundwater concentrations of nitrate (NO3-) declined from 1.8 mM NO3- (25 mg N L-1) to less than 0.01 mM NO3-, dissolved oxygen (O-2) and nitrous oxide (N2O) declined from approximately 110 mu M O-2 (3.5 mg O-2 L-1) and 4.0 mu M N2O-N (56 mu g N L-1), respectively, to zero and the dissolved Nt concentration increased by 589 mu M N-2-N (8.2 mg N L-1). The NO3- reduction was 0.42 mu M cm(-3) d(-1) or 7.71 mu M cm(-2) d(-1) in sediment columns with continuous upward groundwater flow through the sediment. Concomitant with NO3- reduction, N2O was produced at a rate of 5.4.4 nM N2O-N cm(-2) d-(1) in this same 18-cm narrow sediment zone. However, the N2O produced was subsequently reduced at the same rate closer to the sediment surface. In (NO3-)-N-15 experiments on chloramphenicol-treated anaerobic sediment slurries, the denitrifying enzyme activity (DEA) was estimated to be 118 +/- 16.7 nmol N (N-2-N + N2O-N) g fresh weight(-1) d(-1), of which 36% accumulated as N2O. Thus, in this permanently water-covered riparian fen, denitrification served as a sink for both the dissolved N2O in groundwater recharging the ten and the N2O produced within the riparian sediment.