Evaluation of different agronomic strategies to reduce nitrate leaching after winter oilseed rape (Brassica napus L.) using a simulation model

Winter oilseed rape (OSR) demands high levels of N fertilizer, often exceeding 200 kg N ha(-1). Large amounts of residual soil mineral nitrogen (SMN) after harvest are regularly observed, and therefore N leaching during the percolation period over winter is increased. In this study agronomic strategies (fertilization level, crop rotation, tillage intensity) to control nitrate leaching after OSR were investigated by combining field measurements (soil mineral nitrogen, soil water content, crop N uptake) of a 2-year trial and another 5-year field trial with simulation modeling. The crop-soil model uses a daily time step and was built from existing and partly refined submodels for soil water dynamics, mineralization processes, and N uptake. It was used to reproduce the complex processes of the N dynamics and to calculate N concentration in the leachate and total volume of percolation water. Some parameters values were thereby newly identified based on the agreement between measured data and model results. Although SMN in the 60-90 cm layer was overestimated, the model could reproduce the measured data with an acceptable degree of accuracy. Overfertilization of OSR increased N leaching and therefore the precise calculation of N fertilizer doses is a first step towards prevent N leaching. Compared to ploughing, minimum tillage decreased N leaching when winter wheat was grown as the subsequent crop. Volunteer OSR and Phacelia tanacetifolia were grown as catch crops after OSR harvest. N leaching could be decreased especially when Phacelia was grown, but nitrate concentrations in the drainage water were higher and exceeded the European Union (EU) threshold for drinking water when volunteer OSR was grown. The results of this study provide strong evidence that reduced tillage or growing of noncruciferous catch crops decrease N leaching and may be used as an agricultural measure to prevent N pollution.