In order to study the residual nitrogen effects of alternatives of handling set-aside acreage, 14 two-year field experiments, with two types of fallows, four green manure crops and spring barley in the first year, were carried out in south and central Sweden in 1986/87, 1987/88 and 1988/89. After ploughing in August or September, winter wheat was sown. The fallows were summer fallow, either with soil tillage for weed control ("mechanical fallow") or with use of herbicides ("chemical fallow"). The green manure crops were (1) oats + fodder peas + white mustard, (2) Persian clover, (3) red clover + Italian ryegrass, and (4) Italian ryegrass. Due to mineralization, an average of 105 and 77 kg ha-1 of mineral nitrogen (ammonium and nitrate N) accumulated within 0-90 cm soil depth up to late autumn in the first year after the mechanical and chemical fallows, respectively. After spring barley, 45 kg N ha-1 was found. After Italian ryegrass, the mean amount was 12 kg N ha-1 less than after barley. Thus, the net nitrogen mineralization did not seem to increase in the autumn after ploughing of the ryegrass. In contrast, on average 12-32 kg ha-1 more mineralized nitrogen was found in late autumn after the legume green manure crops than after barley. In the fallow treatments, the amounts of mineral N remained almost unaltered up to the following early spring in the 1986/87 trials due to a cold winter, whereas the mineral N storages decreased considerably during mild winters, in 1987/88 and 1988/89, on light soils and in the southernmost experiments. In treatments with legume green manures, part of the autumn-mineralized nitrogen was evidently lost during the mild winters. After the mechanical and chemical fallows, winter wheat (in two trials replaced by spring barley after out-wintering) could utilize, on average, 45 and 36 kg ha-1, respectively, more soil nitrogen than after barley. After the legume manure crops, almost 30 kg ha-1 more plant-available soil nitrogen was found than after barley. This was partly a result of increased N release during the growing season of the second year. The differences in these respects between Italian ryegrass and spring barley as preceding crops were insignificant. After fallows and green manure crops containing clover, the mean grain yields of the winter wheat, cultivated without nitrogen fertilization in the second year, were about 1600 kg ha-1 higher than after spring barley. After Italian ryegrass, the corresponding wheat yields were about 250 kg ha-1 higher. Following application of moderate amounts of fertilizer nitrogen, wheat grain yields were also higher after the fallows and legume green manures than after barley, but to a reduced extent. In these cases, the total nitrogen percentages in the harvested grain were also higher than after spring barley. The lowest concentrations of N were found after the Italian ryegrass.
