The dynamics of N2O near the groundwater table and the transfer of N2O into the unsaturated zone: A case study from a sandy aquifer in Germany

The research area was the Fuhrberger Feld aquifer (FFA) in northern Germany. It is situated about 30km northeast of the city of Hannover and covers about 300km2. Six multilevel sampling wells along a representative strip under predominantly arable land along a groundwater flow-line were sampled from the groundwater table down to a depth of 10m below the soil surface. We measured N2O, CO2, NO3−, SO42−, DOC, pH, redox potentials and O2 concentrations. N2O accumulated at four out of six wells close to the groundwater table. About 20% of N2O that occurred between the groundwater table and 7–8m below it resided in the top 0.4m. An exchange zone for N2O at the interface between the saturated and the unsaturated zone extended 0.55 ± 0.22m below the groundwater table and acted as a source and sink for N2O. N2O below the exchange zone cannot be transferred from the groundwater to the atmosphere. The upward fluxes from the exchange zone into the unsaturated zone at the six wells ranged between 0.0009 and 0.3kg N2O ha− 1 year− 1. The yearly downward fluxes into the exchange zones had about the same order of magnitude as the upward fluxes. The upward and downward fluxes of N2O at the (fluctuating) water table did cancel out each other, but this does not yet imply, that the N2O fluxes at the soil surface also cancel out each other. N2O–N:NO3–N ratios were highly variable ranging from 0.0002 to 0.0417. A multiple regression for the monthly N2O amounts in the exchange zone could explain 66% of the yearly variation. The significant variables were NO3−, CO2, pH, and O2. Therefore, a combination of the land use (NO3−), the geochemical boundary conditions (pH) and the type of denitrification reaction (O2 and CO2 indicate the importance of a heterotrophic denitrification process) governed the N2O dynamics in the surface groundwater of the FFA and its transfer into the unsaturated zone.