Decomposition in the field of residues of oilseed rape grown at two levelsof nitrogen fertilisation. Effects on the dynamics of soil mineral nitrogen between successive crops

The decomposition of oilseed rape residues of different quality and its eff ects on the mineral N dynamics of the soil in the period between crops were studied in situ. The residues studied were obtained by growing an oilseed rape crop at two levels of N fertilisation, 0 and 270 kg N ha(-1). The stud y was carried out using two types of experiment: field plots and cylinders filled with disturbed soil and inserted into the soil. The decomposition of the residues was followed using an approach involving the dynamics of both carbon and nitrogen, the parameters measured being the CO2 emitted from th e soil, the soil mineral N content, the C present in soluble form or in the form of microbial biomass, and the C and N present in the form of plant re sidues. The two residues studied, of similar biochemical composition, and differing only in their N content, were rapidly mineralised: approximately 50% of th e carbon in the residues was decomposed during the first two months followi ng incorporation into the soil. The carbon mineralised in the form of CO2 w as largely related to the C present in the residues, no relationship having been found with the C present in soluble form or in the form of microbial biomass. Calculation of net N mineralisation from the residues using a model of mine ralisation and leaching has provided evidence of an immobilisation phase fo r soil mineral N, during the first steps of residues decomposition. Labelli ng the high-N residues with N-15 has moreover enabled us to demonstrate the low availability of the organic N from this residue, 20.8% of the organic N being mineralised in the course of 18 months of experimentation. Eventual ly, only the highest-N content residue resulted in a mineral N surplus in t he soil, equivalent to 9 kg N ha(-1), by comparison with the control soil. Finally, this study has provided good evidence of the complementarity betwe en the two experimental methods. The cylinders of disturbed soil gave a pre cise measurement of the decomposition of the residues, especially by means of monitoring soil respiration. The field plots were used to monitor the dy namics of soil mineral N which were calculated with the aid of a mathematic al model of mineralisation and leaching of nitrogen in the presence and abs ence of residues.