Denitrification as a Sink for Dissolved Nitrous Oxide in a Freshwater Riparian Fen

Denitrification as a sink of dissolved nitrous oxide (N 2 O) 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 (NO 3 - ) declined from 1.8 mM NO 3 - (25 mg N L -1 ) to less than 0.01 mM NO 3 - , dissolved oxygen (O 2 ) and nitrous oxide (N 2 O) declined from approximately 110 μM O 2 (3.5 mg O 2 L -1 ) and 4.0 μM N 2 O-N (56 μg N L -1 ), respectively, to zero and the dissolved N 2 concentration increased by 589 μM N 2 -N (8.2 mg N L -1 ). The NO 3 - reduction was 0.42 μM cm -3 d -1 or 7.71 μM cm -2 d - 1 in sediment columns with continuous upward groundwater flow through the sediment. Concomitant with NO 3 - reduction, N 2 O was produced at a rate of 54.4 nM N 2 O-N cm -2 d -1 in this same 18-cm narrow sediment zone. However, the N 2 O produced was subsequently reduced at the same rate closer to the sediment surface. In 15 NO 3 - experiments on chloramphenicol-treated anaerobic sediment slurries, the denitrifying enzyme activity (DEA) was estimated to be 118 ± 16.7 nmol N (N 2 -N + N 2 O-N) g fresh weight -1 d -1 , of which 36% accumulated as N 2 O. Thus, in this permanently water-covered riparian fen, denitrification served as a sink for both the dissolved N 2 O in groundwater recharging the fen and the N 2 O produced within the riparian sediment.