Indirect emissions and isotopologue signatures of N2O from agricultural drainage water of a Pleistocene lowland catchment in North-Eastern Germany

Artificial drainage of low- and wetlands is a common practice in many agricultural regions to facilitate crop production. Agricultural drainage water was shown to be supersaturated with nitrous oxide (N2O), a major greenhouse gas thought to contribute to global warming and to the destruction of stratospheric ozone. Therefore, drainage of agricultural land has potential for indirect N2O emissions which are a highly uncertain component of the global N2O budget. This case study focuses on these emissions and further tries to unravel the source processes of N2O as well as the impact of its hydrological controls by applying an isotopologue approach. The research area was an intensively tile drained agricultural catchment embedded in the Pleistocene lowland of the federal state Mecklenburg-Vorpommern (North-Eastern Germany). Water sampling was conducted during the consecutive hydrological winter periods 2007/2008 and 2008/2009 by sampling a collector drain outlet and an adjacent drainage ditch. Besides concentrations of dissolved N2O and NO3- we determined the isotopologue signatures of N2O by measuring δ15Nbulk and δ18O as well as the 15N 'site preference', which characterizes the intramolecular distribution of the N isotopes within the asymmetric N2O molecule and is a promising tool to distinguish between the main source processes of N2O, nitrification and denitrification. The investigated hydrological winter periods varied considerably concerning the weather and hydrological conditions. During the comparatively wet winter period 2007/2008, indirect N2O emissions accounted for 0.17 kg N2O-N ha-1 a-1 and were thus higher than during the colder and comparatively dry 2008/2009 period, where we found 0.12 kg N2O-N ha-1 a-1. The emission factors for both sampling periods were 0.23 % and 0.17 % of the N input, respectively, and therefore in good agreement with the current IPCC default value of 0.25 %. The isotopologue signatures of N2O reflected the different hydrological conditions during the investigated winter periods. Isotopologue data gave no unequivocal information on the source processes nitrification and denitrification. However, there is evidence that denitrification is dominating the N2O dynamics in the near-surface groundwater of the research area, as the 15N site preference and δ18O of N2O in the drainage water were significantly positively correlated which indicates N2O reduction to N2. By taking into account hydrological information, concentration- and isotopologue data, we conclude that reducing NO3- losses from artificially drained catchments will not only mitigate the eutrophication of adjacent aquatic ecosystems, but also indirect N2O emission from agricultural drainage water.