Nitrogen dynamics in crop sequences with winter oilseed rape and winter wheat

With the aim of improving fertiliser nitrogen (N) strategies and reducing N leaching, seasonal changes in soil N supply in crop sequences with winter wheat after winter oilseed rape were examined through field experiments. Break crop effects on wheat after oilseed rape, compared with after peas and oats, were determined as regards residual N, yield and optimum N rate (Opt-N). The impact of residues of the previous crops on net N mineralisation-immobilisation during autumn and winter was studied in field incubation experiments. The influence of spring N fertilisation on winter wheat yields was investigated. Nitrate leaching under winter oilseed rape, peas and oats, and under subsequent wheat was quantified, and measures against leaching were studied. Opt-N to winter wheat was 25 and 15 kg ha-1 lower after winter oilseed rape and peas, respectively, than after oats, despite a yield increase of 700 kg ha-1 after both. The uptake of soil N by wheat until maturity was 26 and 20 kg N ha-1 larger after oilseed rape and peas. Net N mineralisation (Nnet) between spring and maturity was higher after oilseed rape and peas than after oats. Nnet corresponded to 84% of the uptake (or supply) of soil N. The variations in wheat yield at optimum, together with either uptake of soil N or Nnet, explained 70% of the variation in Opt-N. Thus predicting wheat yield and supply of soil N, or Nnet, after previous crops is crucial for the calculation of Opt-N. Due to the large N uptake of winter oilseed rape during the autumn, N leaching during the following winter was lower than during the winter with subsequent winter wheat. More soil mineral N was found at harvest of winter oilseed rape and peas than after oats, affecting later leaching. Above-ground residues of oilseed rape, peas and oats incorporated into soil in September caused N immobilisation during autumn and winter, with the shortest duration for peas. Thus the residue N did not increase leaching risks. Perennial ryegrass as a catch crop undersown in spring in oilseed rape and peas reduced N leaching during winter, whereas direct drilling of winter wheat did not. Fertilisation of oilseed rape above Opt-N enhanced leaching by 0.5 kg N ha-1 per kg fertiliser N. It was concluded that optimising the spring N rate to winter oilseed rape was the most important measure against leaching under subsequent wheat.