Long-term simulation of effects of energy crop cultivation on nitrogen leaching and surface water quality in Saxony/Germany

The production of energy crops in Germany is a growing agronomic sector and is expected to occupy a substantial share of farmland in the near future. At the same time, there are concerns that energy crops might cause increased nitrogen pollution of soil water, surface water and groundwater. Therefore, the Federal State of Saxony, Germany, funded a study on potential effects of an intensified cultivation of energy crops. In frame of this study, we used the Web GIS-based model STOFFBILANZ to simulate N leaching from the rooting zone and N loads of surface water for a reference scenario and an energy crop scenario. For the reference scenario, we used data representing the crop cultivation for the year 2005 at municipality level. We found that the total loads for N leaching from the rooting zone of cropland are highest for the loess region (8,067 t year(-1)), followed by mountainous region (6,797 t year(-1)) and lowland (5,443 t year(-1)). However, highest N fluxes in the leachate from rooting zones have been simulated for lowland (40.6 kg ha(-1) year(-1)) and mountainous region (37.1 kg ha(-1) year(-1)), while nitrate concentrations of leachate were highest for the lowland (101.8 mg l(-1)). In terms of diffuse N input into surface water, the mountainous region is the most important source area (total N load 6,380 t year(-1), flux 34.6 kg ha(-1) year(-1)). Retention by in-stream processes accounts for 15 % (3,784 t year(-1)) of the total N load leaving the study area (25,136 t year(-1)). In the 2020 energy crop scenario, shares of rape and silage maize (id., ensiled corn) were limited for each municipality to a maximum of 25 and 33 %, respectively. The conversion of grasslands to crop farming was not allowed. Under these conditions, we found slight to substantial reductions of nitrogen loads for leachate from the rooting zone and for surface waters. The simulated reduction depends strongly on local conditions. Only small reductions (ca. 4-8 %) were found for the lowlands and mountainous regions of Saxony, while reductions for the loess region were substantial (ca. 22 %). A major outcome of our study is that the cultivation of energy crops might reduce N loss if certain preconditions are assumed, for example, without conversion of grasslands to crop farming. However, effects might vary widely depending on local conditions.