Dynamics of mature pea residue nitrogen turnover in unplanted soil under field conditions

Abstract

The dynamics of 15N-labelled mature, pea ( Piston salivum 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 15N 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 15N 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 15N and the 15N 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 15N than of the total residual organic 15N pool. The pool of potentially-mineralizable organic 15N, as determined by an anaerobic incubation procedure, varied with time in a similar way to the biomass 15N, but the size of the potentially-mineralizable 15N pool was much smaller than the biomass 15N pool. After 3 yr of decomposition, 28% (Expt 1) and 45% (Expt 2) of the residue 15N input was present in the topsoil in organic forms. Only 1–2% of the residual organic 15N was potentially mineralizable after 2yr of decomposition, indicating that the remaining residue 15N was present in rather recalcitrant soil organic matter. After 90 days of decomposition 15N 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.