Effects of various cover crops after peas on nitrate leaching and nitrogen supply to succeeding winter wheat or potato crops

In organic farming systems, it has been demonstrated that grain pulses such as peas often do not enhance soil N supply to the following crops. This may be due to large N removals via harvested grains as well as N‐leaching losses during winter. In two field‐trial series, the effects of legume (common vetch, hairy vetch, peas) and nonlegume (oil radish) cover crops (CC), and mixtures of both, sown after peas, on soil nitrate content, N uptake, and yield of following potatoes or winter wheat were studied. The overall objective of these experiments was to obtain detailed information on how to influence N availability after main‐crop peas by adapting cover‐cropping strategies. Cover crops accumulated 56 to 108 kg N ha–1 in aboveground biomass, and legume CC fixed 30–70 kg N ha–1 by N2 fixation, depending on the soil N supply and the length of the growing period of the CC. Nitrogen concentration in the aboveground biomass of legume CC was much higher and the C : N ratio much lower than in the nonlegume oil radish CC. At the time of CC incorporation (wheat series) as well as at the end of the growing season (potato series), soil nitrate content did not differ between the nonlegume CC species and mixtures, whereas pure stands of legume CC showed slightly increased soil nitrate content. When the CC were incorporated in autumn (beginning of October) nitrate leaching increased, especially from leguminous CC. However, most of the N leached only into soil layers down to 1.50 m and was recovered more or less by the following winter wheat. When CC were incorporated in late winter (February) no increase in nitrate leaching was observed. In spring, N availability for winter wheat or potatoes was much greater after legumes and, after mixtures containing legumes, resulting in significantly higher N uptake and yields in both crops. In conclusion, autumn‐incorporated CC mixtures of legumes and nonlegumes accomplished both: reduced nitrate leaching and larger N availability to the succeeding crop. When the CC were incorporated in winter and a spring‐sown main crop followed even pure stands of legume CC were able to achieve both goals.