Fate of legume-derived nitrogen in monocultures and mixtures with cereals

Cropping grain legumes can cause benefits due to symbiotic nitrogen (N) fixation, but on a downside there are environmental risks like nitrogen losses especially after grain harvest. Until now, hardly any knowledge has been available on these effects of N derived from a preceding crop like spring peas (Pisum sativum L.) mixed with cereals or from winter peas. Therefore, field experiments were conducted in order to examine N mineralization and N preceding crop effect from six different winter pea genotypes (five regular-leaf-type peas and one semi-leafless pea) and one semi-leafless spring pea in three subsequent years (2003/04–2005/06) in pure and mixed stands with cereals on a silty loam, at the research farm of the University of Kassel, Germany. Immediately after grain harvest, total above-ground biomass was removed apart from the stubble, and a catch crop of either mustard and oil radish or mustard was sown, which was harvested in the first half of October.N uptake of catch crops was usually significantly higher after regular-leaf winter peas in pure stands (80–90 kg ha−1) when compared with semi-leafless winter and spring peas (69 and 49 kg ha−1), respectively. In mixtures, the N preceding crop effect of regular-leaf-type peas was more pronounced than that of the semi-leafless peas. It could be shown that winter peas make a higher contribution to the N supply of a subsequent crop in rotation than spring pea.For regular-leaf-type winter peas there was a significant correlation between N uptake of the catch crop and the amount of mineralized N at grain harvest. At this time under pure stands, on average 44, 15 and 12 kg Nmin (NO3-N + NH4-N) ha−1 in top soil and 25, 7 and 22 kg Nmin ha−1 in 30–90 cm soil depth were recorded for regular-leaf-type winter pea, semi-leafless winter pea and spring pea, respectively. Under mixtures, similar values for the different genotypes were recorded (15 and 7 kg Nmin ha−1 for topsoil and subsoil, respectively). This might be explained by the different uptake of mineralized N, the differentiated above-ground biomass and a probably higher root biomass, which may have caused increased rhizodeposition, assuming that root biomass is positively correlated with above-ground biomass.Based on these experiments it can be concluded that, after pea pure stands, the cultivation of catch crops is an absolutely essential measure to conserve mineralized N, which can be expected to be as high as 100 kg ha−1. In the present experiments even these high amounts could be reduced to an environmentally irrelevant level of 30 kg ha−1 by cultivation of a catch crop.