| Popis: |
Nitrogen assimilation by microbial, biomass during the decay of organic material in soil may follow two patterns: (1) direct assimilation of low molecular weight organic N compounds (Direct hypothesis) or (2) immobilization of mineral N, while organic N is completely mineralized (MIT hypothesis). To test these hypotheses equal concentrations of NH 4 -N and alanine-N were added to soils, either one or the other labeled with 15 N, and incubated for 1.2 days. The K 2 SO 4 -extractable organic and mineral N and 15 N and CO 2 release were measured periodically. Experimental results were compared with data computed by two versions of the model NCSOIL, that simulates the C-N turnover and 15 N distribution among soil pools, and is structured to represent either the Direct or the MIT hypothesis. The fitted first order rate constant of mineralization of alanine was 3.2 d −1 , following a delay of 0.25 d. Evolution of CO 2 proceeded at a considerable rate after alanine was decomposed and net N mineralization had ceased, indicating a rapid decomposition of the microbial population that consumed alanine. The isotopic dilution of mineral N proceeded very rapidly and fitted the simulation by MIT better than by the Direct model. The rate of 15 N withdrawal from total extractable N was greater when alanine was labeled and fitted the prediction by the Direct model, but when NH 4 was the source of 15 N, the Direct model failed to predict 15 N consumption. It seemed that both pathways operated concurrently, with the Direct dominating N assimilation by the substrate specific population and the MIT operating at the level of the native soil population. |
