SIMULATION OF NITROGEN ASSIMILATION BY HETEROTROPHIC SOIL MICROBIAL BIOMASS

Assimilation of N by heterotrophic soil microbial biomass is associate d with decomposition of organic matter in the soil. The form of N assi milated can be either low molecular weight organic N released from the breakdown of organic matter (direct assimilation), or NH4+ and NO3- f rom the soil inorganic N pool, into which mineralized organic N is rel eased (mineralization immobilization turnover). The kinetics of C and N turnover in soil is quantifiable by means of computer simulation mod els. 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 assimilat ion scheme. However, if any of the N forms is labeled, a different amo unt of labeled N assimilation will be simulated by the different schem es. Experimental data on inorganic N and N-15 and on organic N-15 dyna mics in soils incubated with N-15 added as NH4+ or organic N were comp ared with data simulated by different model schemes. Direct assimilati on could not account for the amount of N-15 assimilated in any of the experimental treatments. The best fit of the model to experimental dat a was obtained for the mineralizaton immobilization turnover scheme wh en both NH4+ and NO3- were assimilated, in proportion to their concent ration in the soil.