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

Abstract

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 (NO−3) declined from 1.8 mM NO−3 (25 mg L−1) to less than 0.01 mM NO−3, dissolved oxygen (O2) and nitrous oxide (N2O) declined from approximately 110 µM O2 (3.5 mg O2 L−1) and 4.0 µM N2O‐N (56 µg N L−1), respectively, to zero and the dissolved N2 concentration increased by 589 µM N2‐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, N2O was produced at a rate of 54.4 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 15NO−3 experiments on chloramphenicol‐treated anaerobic sediment slurries, the denitrifying enzyme activity (DEA) was estimated to be 118 ± 16.7 nmol N (N2‐N + N2O‐N) g fresh weight 1−1 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 fen and the N2O produced within the riparian sediment.

The global nitrous oxide budget revisited

“…In addition, riparian zones have been identified as potential sinks for N 2 O (e.g. Blicher-Mathiesen and Hoffmann, 1999;Dhondt et al, 2004). For aquatic systems it was difficult to draw conclusions.…”

Nitrous oxide generation, denitrification, and nitrate removal in a seepage wetland intercepting surface and subsurface flows from a grazed dairy catchment

“…Various workers (Tiedje 1988;Patrick and Jugsujinda 1992;Achtnich et al 1995;Blicher-Mathiesen and Hoffmann 1999) have reported that NO 3 À reduction occurs mainly when subsurface water DO falls below 0.5-1.6 mg O 2 /L.…”

WETTRANS: a flow‐path‐oriented decision‐support system for the assessment of water and nitrogen exchange in riparian peatlands

“…Owing to the special hydrological, hydrochemical and ecological conditions in these ecosystems, wetlands are generally considered to possess a high potential for nutrient retention, nutrient transformation, flood protection and biodiversity (e.g. Devito et al, 1989;Johnston et al, 1990;Leonardson et al, 1994;Blicher-Mathiesen and Hoffmann, 1999;Rosenblatt et al, 2001). However, wetland restoration measures often fail due to the rather incidental selection of sites and water management measures, mainly caused by neglecting the geohydrological characteristics (landscape position) of the specific wetland (Bedford, 1999;Zedler, 2000).…”