FACTORS AFFECTING NITROGEN IMMOBILIZATION IN SOIL AS ESTIMATED BY SIMULATION-MODELS

Factors that control immobilization of inorganic N in soils not amended with organic C-rich materials have not been fully established. Different N flow schemes for N immobilization were tested by comparing the kinetics of CO2 evolution, NH4+ -N, NO3--N, inorganic N-15, and organic N-15 excess, measured in soils incubated with 200 mg kg-1 of N-15-labeled NH4+-N, with data simulated by the computer model NCSOIL structured to representseveral pathways of C and N turnover. Estimates of those parameters most likely to induce or enhance N immobilization, namely the mineralizable soil organic matter content, its C/N ratio, and the efficiency with which C is assimilated by the soil microbial biomass were obtained by optimization. Cumulative N immobilization was up to 10% of the N added and was greater when NH4+ was added to dry soils than when added to preincubated soils. The immobilization was primarily due to the microbial biomass drawing N from the inorganic pool, rather than utilizing N directly from the decomposing soil organic matter. Nitrate formed by nitrification seemed to have also been immobilized. A microbial efficiency factor of 0.4 and a C/N ratio of 20 or more for mineralizable soil organic matter were needed to account for the observed rate of N immobilization.