Recovery of groundwater N2O at the soil surface and its contribution to total N2O emissions

Production and accumulation of the major greenhouse gas nitrous oxide (N2O) in surface groundwater might contribute to N2O emissions to the atmosphere. We report on a N-15 tracer study conducted in the Fuhrberger Feld aquifer in northern Germany. A (KNO3)-N-15 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 (N2O)-N-15 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 N2O fluxes at the soil surface were low and in a range between -7.6 and 29.1 mu g N2O-N m(-2) h(-1). N-15 enrichment of N2O decreased considerably upwards in the profile. In the surface groundwater, we found a N-15 enrichment of N2O between 13 and 42 atom%. In contrast, N-15 enrichment of N2O in flux chambers at the soil surface was very low, but a detectable N-15 enrichment was found at all sampling events. Fluxes of groundwater-derived N-15-N2O were very low and ranged between 0.0002 and 0.0018 kg N2O-N ha(-1) year(-1), indicating that indirect N2O emissions from the surface groundwater of the Fuhrberger Feld aquifer occurring via upward diffusion are hardly significant. Due to these observations we concluded that N2O dynamics at the soil-atmosphere interface is predominantly governed by topsoil parameters. However, highest N-15 enrichments of N2O throughout the profile were obtained in the course of a rapid drawdown of the groundwater table. We assume that such fluctuations may enhance diffusive N2O fluxes from the surface groundwater to the atmosphere for a short time.