Simulation of terrestrial nitrogen fluxes in Mecklenburg-Vorpommern and scenario analyses how to reach N-quality targets for groundwater and the coastal waters

In a research project on behalf of the Federal State Agency for Environment, Nature Protection and Geology Mecklenburg-Vorpommern (LUNG), the GROWA–DENUZ–WEKU model system has been used to simulate nitrogen (N) inputs into groundwater, surface waters and the coastal waters in Mecklenburg-Vorpommern. Model analyses have been performed area-covering and at the same time in high spatial resolution (100 m × 100 m) for six diffuse and four point-source input pathways. In the framework of a status quo analysis, “hot-spot-areas” of N-pollution and priority areas for the implementation of N-reduction measures according to the requirements of the EU Water Framework Directive have been identified. It becomes apparent that at present 95% of all N-inputs into surface waters (ca. 32,270 t N/a) originate from diffuse sources with N-inputs from drainage systems as the dominating input pathway. The sum of all N-inputs from point sources contributes to 5% of all N-inputs (ca. 1470 t N/a). The agricultural N-balance surplus would have to be reduced by ca. 41,500 t N/a in order to comply with the EU target value for groundwater (50 mg NO₃/L). Analyses of scenarios about the impact of agricultural management adaptations indicate that individual N-reduction measures are not sufficient to reach the target value for groundwater. Even the introduction of an area-covering limit of 50 kg N/(ha a) for the agricultural N-balance surplus would entail a further reduction in the N-balance surplus by ca. 21,000 t N/a in order to reach the EU target value for groundwater. The target values for the North Sea (2.8 mg N/L) and the Baltic Sea (2.6 mg N/L), however, would already almost be reached by a reduction in the N-balance surplus in this order of magnitude. Against this background, we recommend to include area-covering N-reduction measures for groundwater protection into management strategies for reaching the N-target values for the North Sea and the Baltic Sea.