Emission of groundwater-derived nitrous oxide into the atmosphere: model simulations based on a 15N field experiment

Indirect emissions of the major greenhouse gas nitrous oxide (N2O) occurring from aquatic ecosystems are considered to be a highly uncertain component in the global N2O budget. In this study, we investigated the fate of N2O produced by denitrification in a sandy shallow aquifer in northern Germany. The experimental data from a previous 15N field study and site-specific diffusion coefficients were used to simulate upward fluxes of groundwater-derived (15N-)N2O in the soil as well as its ultimate emission into the atmosphere. The one-dimensional simulation model considered gas diffusion and gas retardation by dissolution in the water phase. The modelled concentration gradients and emissions were in good agreement with the experimental data, indicating that diffusion was the dominant transport process in the soil, and that our model approach was thus suitable for simulating N2O fluxes from the unsaturated zone to the atmosphere. Furthermore, the results revealed that there was no evidence for consumption of 15N-N2O during upward diffusion from the surface groundwater to the atmosphere. Simulated concentrations and emissions of groundwater-derived N2O were found to be very small and a negligible component of total N2O.