Simulation of nitrogen assimilation by heterotrophic soil microbial biomass

Abstract

Assimilation of N by heterotrophic soil microbial biomass is associated with decomposition of organic matter in the soil. The form of N assimilated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH+4 and NO−3 from the soil inorganic N pool, into which mineralized organic N is released (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation models. NCSOIL was constructed to represent the two assimilation schemes. The rate of N assimilation depends on the rate of C assimilation and microbial C/N ratio, thereby rendering it independent of the assimilation scheme. However, if any of the N forms is labeled, a different amount of labeled N assimilation will be simulated by the different schemes. Experimental data on inorganic N and 15N and on organic 15N dynamics in soils incubated with 15N added as NH+4 or organic N were compared with data simulated by different model schemes. Direct assimilation could not account for the amount of 15N assimilated in any of the experimental treatments. The best fit of the model to experimental data was obtained for the mineralization immobilization turnover scheme when both NH+4 and NO−3 were assimilated, in proportion to their concentration in the soil.