Reduction of Nitrate Leaching by Suboptimal Mineral N Fertilization Depends Upon Crop-Specific Use of Mineralized N: Insights by Simulating Field Net N Mineralization.

Nitrate leaching of agricultural fields still is a serious threat as it often causes an exceeding of the drinking water limit for Nitrate in the groundwater recharge. It was often proposed to considerably lower mineral nitrogen (N) fertilizer rates, because these fertilizers have been implicated to be a main origin of leached N. Interestingly, lowering N application from surplus to about optimum levels has been shown to generally decrease N leaching, whereas further lowering below the optimum levels seems to be not as effective. Simultaneously, suboptimal N rates are usually associated with immediate, relatively large yield reductions by up to 50 % of the optimum yield. As it is crucial for determining the optimum fertilizer rate to include highly variable current net N mineralisation, and as uptake of mineralized N might vary due to variations in the length of the crops’ growth period (e.g. winter cereals vs. corn), the crop type might have an important impact on N leaching and yield as a function of N rates.  

We analysed yield data and N leaching of a field trial with five mineral N fertilizer levels and a crop rotation of silage corn, winter barley and winter rye from 12 years. An N mineralization model was used to evaluate, whether differences between crops depended upon crop-specific use of mineralized N.

There were three main results: N leaching and yield strongly depended upon crop type and N fertilizer rates, but N leaching cannot be totally avoided even without mineral N fertilization. Differences between crops indeed largely depended upon crop-specific use of mineralized N, as revealed by simulated net N mineralization. And, of the three crops studied, only for corn ecologically (minimized N leaching) as well as economically (minimized yield losses) optimized fertilization is possible - in case mineralizable N is considered.