How much did the Nitrates Directive contribute to changes in surface water nitrate concentrations across German catchments?

Nitrate originating from agricultural lands is identified as one of the main causes of water pollution in Europe. As a result, the European Nitrates Directive (ND) was introduced in 1991 to protect water bodies from nitrate pollution by reducing diffuse nitrogen inputs. Despite decades having passed since its implementation, there are diverging observations of nitrate concentration changes in surface waters in Europe. Recent work suggested that the success of input reduction can be strongly blurred due to long water and nitrogen transit times and the built-up of nitrogen stores in catchment soils, making it difficult to evaluate the effectiveness of the ND. Therefore, it is still unclear to what extent the ND contributed to changes in surface water nitrate concentrations.  Such understanding could help to develop better management policies. In this study, we used previously calibrated nitrate export models for various German catchments (Nguyen et al., 2022) based on the principle of water transit times and observed inputs (baseline scenario). We then force these models with different N input trajectories that assume no implementation of the ND (hypothetical scenarios). Here, we will compare simulation results from the baseline scenario with hypothetical scenarios to evaluate the effectiveness of the ND implementation as well as its controlling factors. In addition, we will also check if different catchments respond differently to changes in N inputs to see whether different management strategies are needed for different catchments.

Nguyen, T. V., Sarrazin, F. J., Ebeling, P., Musolff, A., Fleckenstein, J. H., & Kumar, R. (2022). Toward understanding of long-term nitrogen transport and retention dynamics across German catchments. Geophysical Research Letters, 49, e2022GL100278. https://doi.org/10.1029/2022GL100278