Minimizing nitrate leaching while maintaining crop yields: insights by simulating net N mineralization

Nitrate leaching from agricultural fields still is a serious threat in temperate regions, as it often causes an exceeding of the legal nitrate threshold in the groundwater. It was often proposed to lower mineral nitrogen (N) fertilization, but suboptimal N rates are usually associated with severe yield losses. Here it was hypothesized that the crop type has a strong impact on the relation between N leaching and yield, besides N fertilizer rates, due to crop-specific use of N mineralized from soil organic matter. We analysed N leaching and yield data of a field trial in NW-Germany with five mineral N fertilizer levels and a crop rotation of silage corn, winter barley and winter rye from 12 years. Net N mineralization was calculated with an N mineralization model, that allowed site-specific estimations, and with a balance approach based on measured field data. Yield and N leaching of the three crops strongly depended upon N supply, but N leaching could not be totally avoided even without any N fertilization, since for each crop type N leaching showed relatively constant values below an N supply of ca. 150 kg N ha−1. Secondly, the possibility of minimizing N leaching with concurrent modest reduction in yield (not more than 10 %) depended upon crop-specific use of mineralized N. Thirdly, these reductions appear to be most probable for silage corn due to 2–4 times higher N uptake in unfertilized plots (mean: 87 kg N ha−1) compared to the cereals (mean: 31 kg N ha−1). Thus there is a strong need to include estimates for net N mineralization in fertilizer recommendations, especially for corn, in order to more efficiently use mineralized N and reduce nitrate leaching. This is evident for the kind of sandy soil studied, and even stronger for soils with finer texture and consequently higher net N mineralization potential.