Spatial variability of N2O concentrations and of denitrification-related factors in the surficial groundwater of a catchment in Northern Germany

SummaryN2O concentrations and denitrification-related factors (NO3, SO4, dissolved organic carbon (DOC) and CO2) were investigated in the surface groundwater of a catchment in northern Germany, the Fuhrberger Feld Aquifer (FFA). We sampled 79 plots that were selected according to the three criteria of land use, historical land use conversion (1954–1995) and groundwater level. We sampled three sites within each plot. The sampling depth was 0.5 m below the groundwater surface.We found no indication for the occurrence of autotrophic denitrification in the surface groundwater. Heterotrophic denitrification was identified as the main process for N2O accumulation. The variability of N2O concentrations on the plot-scale was extremely high and was poorly explained by the three sampling criteria. Other denitrification-related variables such as NO3, SO4 and DOC were less variable. The selection criteria land use and groundwater level clearly influenced the order of magnitude of N2O concentrations in the surface groundwater. Under arable land, high NO3 concentrations resulted in high N2O concentrations. The surface groundwater under forest and pasture was almost NO3-free and had also very small N2O concentrations. Plots where the distance from the soil surface to the groundwater surface was large (>1 m up to 3.4 m) showed higher N2O concentrations in the surface groundwater than plots where the distance was small (<1 m). A larger distance from the soil surface to the groundwater leads to a longer residence time and more decomposition of DOC in the soil. Consequently the less bioavailable DOC could inhibit the efficiency of the heterotrophic denitrification in the groundwater, yielding more N2O. Elevated organic carbon levels in plots with historic land use conversion (pasture to arable) were very stable and did not influence N2O concentrations. The high within plot variability showed that an upscaling of N2O from the plot-scale to the catchment-scale is possible as long as the groundwater level regime and the land use do not change.