INTERACTIONS BETWEEN DECOMPOSITION OF PLANT RESIDUES AND NITROGEN CYCLING IN SOIL

The processes of N mineralization and immobilization which can occur i n agricultural soils during decomposition of plant residues are briefl y reviewed in this paper. Results from different incubation studies ha ve indicated that the amounts of N immobilized can be very important a nd that the intensity and kinetics of N immobilization and subsequent remineralization depend on the nature of plant residues and the type o f decomposers associated. However, most of the available literature on these processes refer to incubations where large amounts of mineral N were present in soil. Incubations carried out at low mineral N concen trations have shown that the decomposition rate of plant residues is d ecreased but not stopped. The immobilization intensity, expressed per unit of mineralized C, is reduced and N remineralization is delayed. N itrogen availability in soil can therefore strongly modify the MIT kin etics (mineralization-immobilization turnover) by a feed-back effect. The mineralization and immobilization kinetics have been determined in a two-years field experiment in bare soil with or without wheat straw . Mineralization in plots without straw seemed to be realistically pre dicted by accounting for variations in soil temperature and moisture. Immobilization associated with straw decomposition was clearly shown. It was increased markedly by the addition of mineral N throughout deco mposition. It is concluded that mineral N availability is an important factor controlling plant residues decomposition under field condition s. A better prediction of the evolution of mineral N in soil may there fore require description and modelling of the respective localization of both organic matter and mineral N in soil aggregates.