Vorfruchtwirkurigen unterschiedlicher Rotationsbrachen auf Winterweizen

Effects of rotational fallows (‘set‐aside land’) on subsequent winter wheat. The aim of the present study was to investigate the effects of different fallow treatment on subsequent winter wheat. The field trials included rotational fallows planted with Trifolium repens, Festuca rubra and Lolium perenne sown under winter barley compared to complete fallow and natural fallow without seed application and fallows planted with Trifolium pratense, Festuca rubra and Dactylis glomerata sown under winter wheat. After ploughing up the fallow vegetation, winter wheat was planted for 2 succeeding years at two levels of N‐fertilization. Herbicides and fungicides were not applied. The following criteria were investigated: biomass‐production, N‐uptake, yield, weed infestation, nitrate and water content of the soil. In autumn, after ploughing up the fallow vegetation, the nitrate content of the soil (0—150 cm) increased by up to 210 kg NO 3 ‐N/ha after complete fallow, by up to 60 kg NO 3 ‐N/ha after natural fallow and by up to 75 and 130 kg NO 3 ‐N/ha after fallows cropped with Trifolium repens and Trifolium pratense , respectively. Low nitrate levels of 20—27 kg NO 3 ‐N/ha were observed after fallows planted with grass. N‐immobilization caused by ploughing up grass fallows continued until the first harvest of the subsequent winter wheat. In the second year of winter‐wheat, no differences of N‐mineralization dependent on the previous fallow crop occurred, except in the case complete fallow which showed lower N‐mineralization. It can be concluded that fallows cropped with grass lead to a higher nitrogen fertilizer requirement m the succeeding crops. Festuca rubra was able to form dense swards in strong competition with weeds and to decrease the abundance of Alopecurus myosuroides and Apera spíca‐venti in subsequent winter wheat, while natural fallow and fallow planted with Trifolium repens and Lolium perenne caused epidemical increases in grass‐weed density. Preceding crop effects on grain yield of the winter wheat showed a close relation to N‐supply and were compensated by mineral N‐fertilization. After natural fallow and fallow covered with Trifolinm repens , yield reductions due to grass‐weed competition occurred. Undersown Festuca rubra seems to possess a special suitabihty for cultivation in routional fallows. It establishes itself strongly under different cover crops and is able to form dense swards in strong competition with weeds. Grass‐weed density in the succeeding crops will be reduced and nitrate leaching will still be prevented after ploughing up the fallow vegetation. N‐fertilization of the subsequent crops must be carried out under considerations of higher N‐requirements which is probably not entirely due to a stronger N‐immobilization.