Nitrogen fixation and residual effects of field pea intercropped with oats

Due to an increased demand for excluding external nitrogen fertilization there is a need for cultivation measures which optimize use of legume nitrogen fixation. We have, during field conditions in central Sweden, evaluated the contribution of intercropped peas (Pisum sativum L.) and oats (Avena sativa L.) to the overall N economy. We have done this by determining the N2 fixation by the difference method and by the 15N-dilution method using two reference crops. N2 fixing efficiency was calculated as fixed nitrogen of total pea nitrogen and of dry matter, respectively. We complemented this by determining the mineral nitrogen at three soil levels before and after the growing seasons. Our results indicate that nitrogen yield sometimes, but not dry matter yield, was affected by pea ratio. The efficiency of the fixation increased with decreasing pea ratio. This could be explained by competition for N from the intercropped oats, in which the N-concentration decreased with decreasing pea ratio. Results indicate that peas supplied the intercropped oats with nitrogen. Residual soil mineral nitrogen after harvest was similar in the intercrop to the pure stand of oats, while it was higher in a pure stand of peas. This was repeated in one out of three trials following the winter season. The nitrate to total mineral N ratio was higher in the pure stand of peas and lower in the intercrop and the pure oats. The studied crops did not give different residual effects in nitrogen and dry matter contents in a following oat crop. Conclusions are that intercropping may increase the efficiency of fixation and reduce risk of N leaching, but this is not necessarily reflected in current yields or subsequent crops.