Implications of nitrogen legacy on the effectiveness of management measures in central European river catchments

Despite far-reaching legal regulations and extensive management measures, nitrogen and phosphorus are still elevated in many river systems causing the degradation of ecosystems and the failure to achieve good ecological status following the EU-WFD. Our study analyzes the long-term change in nitrogen surpluses, and quantifies the denitrification rates as well as the residence time in soil and groundwater. The goal is to assess the effect and the time lag of management measures and to evaluate the achievability of environmental goals, e.g. EU-WFD or EU-MSFD. 

The calculations were carried out with a completely revised version of the widely applied nutrient emission model “MONERIS” with a resolution of 1 km x 1 km on a monthly basis from 2003 to 2020 for all German rivers including their hydrologically connected catchment areas in neighboring countries. The runoff and residence times were modeled using an integrated precipitation-runoff model and the retention processes in soil and groundwater were calculated via a coupled three-layer denitrification module, based on soil characteristics such as pH, soil texture, soil temperature, leakage water concentration. The effect of oxygen-reduced conditions in soils is represented by the water saturation.

The residence time in the soil ranges usually between a few days and a month, with local peaks of up to several months. The residence time in groundwater shows strong spatial variations. It ranges between less than 5 years and more than 100 years, however, for the N-balance history, a maximum of 50 years was taken into account. Although longer residence times generally lead to higher total denitrification, the rates are strongly controlled by local site characteristics such as pH value, N leachate concentration and soil texture. 

Due to the highly variable denitrification rates (< 1 – 92 kg/ha/yr, mean 44.1 kg/ha/yr), nitrogen emissions vary despite resulting from similar N surpluses. However, the proportions of the emission pathways surface runoff, interflow, and groundwater determined both the total emissions due to different denitrification rates as well as the resulting average lag time between fertilizer application and nutrients entering a surface water. Locally, the total residence time as mean over all pathways is determined by the proportions of runoff components and the respective residence times involved. Whereas areas with a high proportion of direct runoff and sealed urban areas react within months or even days, the lag time in surface waters results as a runoff-weighted average of local residence time in its upstream reaches.

The management to reach environmental quality goals and the need to rapidly reduce N surpluses and N concentrations in surface waters require comprehensible links between reduction measures and their effects on concentrations in surface waters. Our results indicate that the efficiency of measures to reduce nutrient concentrations in surface waters should not be assessed solely on the basis of the quantitative reduction potential, but also taking into account the time component. This also opens up the possibility of achieving a higher level of acceptance among the public and politicians if the time delays are known and considered during implementation.