Decomposition of 15 N‐labelled catch‐crop residues in soil: evaluation of N mineralization and plant‐N uptake potentials under controlled conditions

The decomposition of 15 N‐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°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 × 10 −2 to 0.06 × 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 15 N‐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 15 N 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.