EFFECTS OF ROTATIONAL FALLOWS (SET-ASIDE LAND) ON SUBSEQUENT WINTER-WHEAT

The aim of the present study was to investigate the effects of differe nt fallow treatment on subsequent winter wheat. The field trials inclu ded rotational fallows planted with Trifolium repens, Festuca rubra an d Lolium perenne sown under winter barley compared to complete fallow and natural fallow without seed application and fallows planted with T rifolium pratense, Festuca rubra and Dactylis glomerata sown under win ter wheat. After ploughing up the fallow vegetation, winter wheat was planted for 2 succeeding years at two levels of N-fertilization. Herbi cides and fungicides were not applied. The following criteria were inv estigated: biomass-production, N-uptake, yield, weed infestation, nitr ate and water content of the soil. In autumn, after ploughing up the f allow vegetation, the nitrate content of the soil (0-150 cm) increased by up to 210 kg NO3-N/ha after complete fallow, by up to 60 kg NO3-N/ ha after natural fallow and by up to 75 and 130 kg NO3-N/ha after fall ows cropped with Trifolium repens and Trifolium pratense, respectively . Low nitrate levels of 20-27 kg NO3-N/ha were observed after fallows planted with grass. N-immobilization caused by ploughing up grass fall ows 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 co mplete fallow which showed lower N-mineralization. It can be concluded that fallows cropped with grass lead to a higher nitrogen fertilizer requirement in the succeeding crops. Festuca rubra was able to form de nse swards in strong competition with weeds and to decrease the abunda nce of Alopecurus myosuroides and Apera spica-venti in subsequent wint er wheat, while natural fallow and fallow planted with Trifolium repen s and Lolium perenne caused epidemical increases in grass-weed density . Preceding crop effects on grain yield of the winter wheat showed a c lose relation to N-supply and were compensated by mineral N-fertilizat ion. After natural fallow and fallow covered with Trifolium repens, yi eld reductions due to grass-weed competition occurred. Undersown Festu ca rubra seems to possess a special suitability for cultivation in rot ational fallows. It establishes itself strongly under different cover crops and is able to form dense swards in strong competition with weed s. Grass-weed density in the succeeding crops will be reduced and nitr ate leaching will still be prevented after ploughing up the fallow veg etation. N-fertilization of the subsequent crops must be carried out u nder considerations of higher N-requirements which is probably not ent irely due to a stronger N-immobilization.