Modelling nitrogen dynamics after growing winter oilseed rape in different cropping systems

摘要

In Germany the acreage grown with winter oilseed rape (OSR) increased in the last decades because of its profitability, the beneficial value as preceding crop for cereals, and the opportunity to grow OSR for biofuel production on set-aside. OSR demands high levels of nitrogen (N) fertilizer often exceeding 200 kg N ha-1 to achieve maximum yields. However, N offtake by the seeds is comparatively low leading to high positive N balances. There are also significant N losses from the plant before harvest due to leaf abscission. Additionally, the early harvest potentially enhances high mineralization of both crop residues and soil N in the favourable structured soil after harvest The subsequent winter wheat crop normally takes up 20 - 25 kg N ha-1. As a result soil mineral N content (SMN) can increase during autumn and large amounts of nitrate are likety to be leached with percolation water.However, there are several well known agronomic opportunities like minimum tillage and catch crop growing to reduce nitrate leaching after growing OSR. The aim of our study is to quantify the effects of several agronomic strategies on nitrate leaching losses after OSR by combining an experimental and a modelling approach. The experiment is a field trial with different tillage operations (mouldboard ploughing vs. conservation tillage) and different succeeding crops (winter wheat, spring oats, catch crop). The model was constructed from several existing and newly designed modules, all based on an object oriented class library called HUME. Modules for calculating the N dynamics including nitrate leaching, crop residue decomposition, tillage operations, and N uptake by the crops following OSR were thereby coupled to a system simulation model. First results of the leaching period 2005/2006 show that nitrate leaching during winter was only marginally affected by the amount of crop residues of OSR. N mineralization after OSR therefore originates mainly from the soil mineral N pool which was highly influenced by the intensity of tillage in autumn. The results indicate that catch crops have a high potential to reduce the SMN pool in autumn because of N uptake up to 60 kg N ha-1 before winter. The model was used to extrapolate the experimental results to different soil and weather conditions.