The dynamics of N-15-labelled mature, pea (Pisum sativum L.) residue turnover in soil were studied in two 3 yr experiments, using residue sizes of < 10 mm or < 3 mm in unplanted soil in the field. During the initial 10 days of decomposition there was a sharp decline in the amount of N-15 in organic form in the topsoil in the two experiments, i.e. 14% (Expt 1) and 28% (Expt 2). Simultaneously, 14% (Expt 1) and 22% (Expt 2) of the residue N-15 was incorporated into the microbial biomass. The labelled N incorporated in the biomass was estimated to account for 66 and 100% of the increase in the biomass N, indicating that unlabelled soil N was immobilized in the biomass in the second experiment, despite the high residue N concentration. The total residual organic N-15 and the N-15 in the biomass declined with average decay constants of 0.44 and 0.65 yr-1, respectively, for the 10 d to 1 yr period, and 0. 15 and 0.29 yr-1, respectively for 1-3 yr period, showing a faster turnover rate of the biomass N-15 than of the total residual organic N-15 pool. The pool of potentially-mineralizable organic N-15, as determined by an anaerobic incubation procedure, varied with time in a similar way to the biomass N-15, but the size of the potentially-mineralizable N-15 pool was much smaller than the biomass N-15 pool. After 3 yr of decomposition, 28% (Expt 1) and 45% (Expt 2) of the residue N-15 input was present in the topsoil in organic forms. Only 1-2% of the residual organic N-15 was potentially mineralizable after 2 yr of decomposition, indicating that the remaining residue N-15 was present in rather recalcitrant soil organic matter. After 90 days of decomposition N-15 unaccounted for corresponded to 10% of the input, increasing to 20-30% after 2-3 yr of decomposition. The main part of the N unaccounted for may have been lost by leaching of organic N and denitrification, since the soil had high nitrate concentrations during the late summer and autumn.
