The introduction of soybean in an organic crop rotation in the Nemoral zone: the impact on subsequent spring wheat productivity

The sustainability of an organic crop rotation frequently depends on the residual effects of legume pre-crops.However, the contribution of legumes varies considerably depending on their species as well as local soil and climatic conditions.This research aims to compare the pre-crop effects of traditional grain and forage legumes with those of soybean (Glycine max (L.) Merr.), which is not widespread in Europe latitude 55° due to climatic constrains, on the productivity of subsequent cereals.A three-year (2016-2018) crop rotation was investigated using four pre-crops: spring barley, spring barley intercropped with red clover, pea and soybean.The effect of pre-crops on grain yield and quality and chemical composition of subsequently grown spring wheat was explored.Nitrogen (N) content in grain and straw, mineral nitrogen (N min ) content in the soil were measured, the carbon to nitrogen (C:N) ratio and net N balance were calculated.The rotation was repeated twice and the results showed that pea and soybean did not produce sufficient biomass and amount of fixed N to maintain a stable positive residual effect in both rotation replications.Under favourable conditions, pea derived 66.1 kg ha -1 N and soybean -64.7 kg ha -1 N from the atmosphere, while red clover intercropped with spring barley produced 65.4-82.7 kg ha -1 N yearly.Most of the total N (N tot ) accumulated by pea and soybean plants, 69% and 80%, respectively, was removed with grain yield, while comparable percentages of N (59-68%) were left by the residues of red clover and spring barley mixture.As a result, the greatest effect on subsequently grown spring wheat was exerted by the mixture of spring barley and red clover (yield increase 354-1414 kg ha -1 ).Comparing grain legumes, only efficiently nodulated soybean produced a positive +20.6 kg ha -1 net N balance and increased the yield of spring wheat by 920 kg ha -1 .Pea resulted in the negative net N balance and increased the yield of spring wheat by 534 kg ha -1 .Mineral nitrogen in the soil increased under all legume pre-crops only in the second year of the crop rotation.