The fate of nitrogen from winter-frozen rapeseed leaves: mineralization, fluxes to the environment and uptake by rapeseed crop in spring

For environmental purposes, very early sowing of winter rapeseed may reduce winter nitrate leaching thanks to the high N uptake capacities of rapeseed in autumn. However, freezing could lead to high losses of leaf nitrogen, a mounting to more than 100 kg N ha(-1) (Dejoux et al., 1999). Here we invest igated the agronomic and environmental consequences of the decomposition of fallen leaves, based on field and laboratory studies with N-15 labeled lea ves (C:N=9). The potential kinetics of decomposition of leaves were measure d by incubation in the laboratory. In the field, all leaves were removed at beginning of winter and replaced by labeled leaves, artificially frozen at -15 degrees C , which were laid on the soil surface. Compared on a thermal time basis, decomposition proceeded as quickly in the field as in the incu bations and was complete after 116 normalized days at 15 degrees C. The pro portion of N-15 derived from labeled leaves, absorbed again by the rape pla nts, was 28% at flowering and 24% at harvest. This high N recovery is assum ed to result from the synchronization of leaves decomposition and active N absorption by rape in spring. Leaf N mineralization did not increase soil N mineral content at flowering or at harvest, but we observed a 40% loss of N-15. As no leaching was simulated, this loss was supposed to be gaseous. S uch a high percentage could be explained by the fact that the decomposing l eaves lay on the soil surface, and by climatic conditions conducive to such emissions. For environmental purposes, the quantity and nature of these ga seous N emissions have to be studied for other climatic conditions and type s of leaves. As a proportion of N is reabsorbed, N fertilizer application r ates could be reduced accordingly.