Åkerbönor som förfrukt till vårsäd i ekologisk odling

In eight 3-year field trials in south-west Sweden, 2002-2006, residual nitrogen effect of faba beans during two following years with spring cereals were investigated. In the first experimental year oats, faba beans (harvested) and faba beans (not harvested) were grown with three replicates in each trial. In three of the experiments also corresponding treatments with harvested and non-harvested field peas were included. The oats in the first experimental year was fertilised with 72 kg NH4-N per hectare, on average, as cattle slurry. The crop residues of the harvested crops and the whole not harvested crops were ploughed under in mid-October. In the second and third year spring barley and oats, respectively, were grown. Grain yield of cereals, beans and peas were measured by combine harvesting in each plot. At GS 91-92 all above ground material were cut within three areas of each 0.25 m2 in each plot. Rations of straw, kernels, beans and peas were measured and analysed for total nitrogen content. In the straw total carbon content were determined to determine C/N-ratio. To determine total nitrogen content in spring barley and oats it was assumed that the nitrogen amount in the roots were 25% of the crops total nitrogen content (Hansson et al., 1987). To determine soil mineral nitrogen (NH4-N and NO3-N) at different times, soil samples were taken to 90 cm depth. Soil samplings were performed at the following times: At full ripeness in each crop and at ploughing in autumn (mid October) in year 1 and in early spring (mid April) and at full ripeness GS 91-92 in year 2. In year 3, no soil samplings were carried out. Two incubation experiments were performed to study the effect of the previous crops on nitrogen mineralisation and immobilisation from the harvest of these crops in year 1 until spring in year 2. For this, soil and straw from oats, faba beans and field peas were sampled shortly after harvest in year 1 in each of the main plots in three trials. A soil and a soil-straw mixture were incubated in plastic bottles. The straw/soil ratios were set in relation to the biological straw harvest in each crop and trail and assuming that straw was incorporated into a 7 cm thick soil layer. The bottles were placed in the topsoil of a field in west Sweden. They were covered with soil and equipped with a ventilation tube reaching about 30 cm above the soil surface. By placing the bottles in the field they were exposed to natural temperature variations. The incubation study was divided into two series, with soil and straw from two and one field experiments, respectively. The first series was performed during the period 10.10.2003- 29.04.2004 and the second 8.10.2004-28.04.2005. The changes in soil mineral nitrogen in the bottles were determined at five times during the incubation period. Year 1 the average yield of faba beans were 2710 kg dry matter (DM)/ha and of oats 3050 kg DM/ha. Spring barley yield after the preceding crops were largest after not harvested faba beans 4470 kg/ha (15% moisture content). Harvested faba beans yielded 3630 kg/ha and oats as preceding crop yielded 2980 kg/ha. There was significantly higher yield after harvested faba beans, 650 kg/ha (P=0.0021) and not harvested faba beans, 1490 kg/ha (P=0.000) compared to oats that is 20 to 50% higher yield. There were no significant differences in oats yield the third year. In mid October, year 1, there were 29 kg N/ha in soil after oats and after faba beans harvested and not harvested 46 and 49 kg/ha, respectively. There was significantly more nitrogen in soil after faba beans, P=0.0330, and not harvested faba beans, P=0.0018, compared to oats. During the winter nitrogen content raised although nitrogen losses also must have occurred. In mid April nitrogen levels after oats were 40 kg N/ha. Harvested faba beans had 18 kg N/ha more, P=0.0025, and not harvested faba beans 29 kg N/ha more, P=0.0000. During the growing season soil mineral nitrogen decreased to 22-24 kg N/ha after oats and harvested faba beans. In not harvested faba beans it remained 32 kg N/ha. Nitrogen in spring barley, incl. roots 25%, at GS 91-92 were 74, 94 and 119 after oats, faba beans and not harvested faba beans, respectively. Residual nitrogen effect after faba beans harvested and not harvested were 20 and 45 kg N/ha larger than after oats. Soil mineral nitrogen at spring and at GS 91-92 and nitrogen uptake by barley in the second year were used to calculate apparent net nitrogen mineralisation during the growing season using the formula (Lindén et al., 1992): Nnet =Nplant +Nmd -Nmv, where Nnet=net nitrogen mineralisation during the growing season, Nplant=Nitrogen in crop at GS 91-92, Nmd=Nitrogen in soil (0-90 cm) at GS 91-92 and Nmv=Nitrogen in soil (0-90 cm) in spring. After oats net nitrogen mineralisation during the growing season were 57 kg N/ha. For faba beans harvested and not harvested the mineralisation were 60 and 83 kg N/ha, respectively. The small difference in nitrogen mineralisation following oats and harvested faba beans indicated that the nitrogen effect of the bean crop were mainly caused by more soil mineral nitrogen present in early spring. In the third year with oats the nitrogen levels in crop, at GS 91-92, were evened out as there were no differences between oats or faba beans as preceding crop. In not harvested faba beans there was a small increase of 4 kg N. The incubation studies revealed that both a mixture of straw from oats or faba beans in the soil caused net nitrogen immobilisation. This may be explained by the high C/N ratio in the oat and faba bean straw, 43 and 80, respectively. Nitrogen immobilisation seemed to stop first in the faba beans (as winter begun). Immobilisation was still taking place in oats at the end of the incubation study in April. This agrees with the larger amounts of soil mineral nitrogen in the spring in the field experiments after oats than with faba beans as preceding crop. The nitrogen immobilisation in the autumn, caused by the bean straw, implies that this straw should not have contributed to the larger amounts of soil mineral nitrogen in the field trials in late autumn following faba beans than after oats. This additional mineral nitrogen was present in the soil already during ripeness and harvest of the faba beans. In conclusion, there were large residual effects on the nitrogen supply and yield of the spring barley in the treatment where the bean yield of the faba crop was incorporated into the soil. This effect corresponded to about 18% of the nitrogen content of the bean yield. The residual nitrogen effect was less following harvested faba beans. In this case, the effect seems to be due mainly to the extent to which the increased amounts of soil mineral nitrogen in the autumn remains in the rooting zone after the winter. No significant residual effects were found in the second year after the faba bean crops.