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Abstract.We investigated the dynamics of denitrification and nitrous oxide (N 2 O) accumulation in 4 nitrate (NO − 3 ) contaminated denitrifying sand and gravel aquifers of northern Germany (Fuhrberg, Sulingen, Thülsfelde and Göttingen) to quantify their potential N 2 O emission and to evaluate existing concepts of N 2 O emission factors. Excess N 2 -N 2 produced by denitrification -was determined by using the argon (Ar) concentration in groundwater as a natural inert tracer, assuming that this noble gas functions as a stable component and does not change during denitrification. Furthermore, initial NO

Abstract. Knowledge of the kinetics of N 2 O production and reduction in groundwater is essential for the assessment of potential indirect emissions of the greenhouse gas. In the present study, we investigated this kinetics using a laboratory approach. The results were compared to field measurements in order to examine their transferability to the in situ conditions. The study site was the unconfined, predominantly sandy Fuhrberger Feld aquifer in northern Germany. A special characteristic of the aquifer is the occurrence of the vertically separated process zones of heterotrophic denitrification in the near-surface groundwater and of autotrophic denitrification in depths beyond 2-3 m below the groundwater table, respectively. The kinetics of N 2 O production and reduction in both process zones was studied during long-term anaerobic laboratory incubations of aquifer slurries using the 15 N tracer technique. We measured N 2 O, N 2 , NO with initial rates between 0.2 and 13 µg N kg −1 d −1 . The process was carbon limited due to the poor availability of its electron donor. In the autotrophic denitrification zone, initial denitrification rates were considerably higher, ranging between 30 and 148 µg N kg −1 d −1 , and NO − 3 as well as N 2 O were completely removed within 60 to 198 days. N 2 O accumulated during heterotrophic and autotrophic denitrification, but maximum concentrations were substantially higher during the autotrophic process. The results revealed a satisfactory transferability of the laboratory incubations to the field scale for autotrophic denitrification, whereas the heterotrophic process less reflected the field conditions due to considerably lower N 2 O accumulation during laboratory incubation. Finally, we applied a conventional model using first-order-kinetics to determine the reaction rate constants k 1 for N 2 O production and k 2 for N 2 O reduction, respectively.

Production and accumulation of the major greenhouse gas nitrous oxide (N 2 O) in surface groundwater might contribute to N 2 O emissions to the atmosphere. We report on a 15 N tracer study conducted in the Fuhrberger Feld aquifer in northern Germany. A K 15 NO 3 tracer solution (60 atom%) was applied to the surface groundwater on an 8 m 2 measuring plot using 45 injection points in order to stimulate production of 15 N 2 O by denitrification and to detect its contribution to emissions at the soil surface. Samples from the surface groundwater, from the unsaturated zone and at the soil surface were collected in regular intervals over a 72-days period. Total N 2 O fluxes at the soil surface were low and in a range between -7.6 and 29. , indicating that indirect N 2 O emissions from the surface groundwater of the Fuhrberger Feld aquifer occurring via upward diffusion are hardly significant. Due to these observations we concluded that N 2 O dynamics at the soilatmosphere interface is predominantly governed by topsoil parameters. However, highest 15 N enrichments of N 2 O throughout the profile were obtained in the course of a rapid drawdown of the groundwater table.We assume that such fluctuations may enhance diffusive N 2 O fluxes from the surface groundwater to the atmosphere for a short time.