DECOMPOSITION OF N-15-LABELED CATCH-CROP RESIDUES IN SOIL - EVALUATION OF N MINERALIZATION AND PLANT-N UPTAKE POTENTIALS UNDER CONTROLLED CONDITIONS

The decomposition of N-15-labelled catch-crop materials (rape, radish and rye), obtained from field experiments, was studied in a chalky Champagne soil during a 60-week incubation at 28 degrees C. Mineralized N was assumed to come from either labile or recalcitrant fractions of plant residues. The labile fraction represented about one-third of the catch-crop N; its mineralization rate constant varied from 0.06 to 0.12 d(-1). The decomposition rate of the recalcitrant N fraction ranged from 0.03 x 10(-2) to 0.06 x 10(-2) d(-1). Catch-crop species and rate of incorporation had no effect on N residue mineralized at the end of incubation. The decomposition of labelled rye was monitored in the same soil during a 5-month pot experiment to determine the N availability to an Italian ryegrass crop and the effect of plants on the decomposition processes. The N-15-rye decomposed rapidly both in the presence or absence of Italian ryegrass, but the amounts of N mineralized were influenced by the presence of living roots: 42% of the N-15 in labelled rye was present as inorganic N in the pots without plants after 5 months, compared with only 32% in the ryegrass crop. Comparison of microbial-biomass dynamics in both treatments suggested that there had been preferential utilization by soil micro-organisms of materials released from the living roots than the labelled plant residues.