Assimilation of nitrogen by soil microbial population: NH4 versus organic N

• Published in: Soil biology & biochemistry Vol. 24; no. 2; pp. 137 - 143
• Main Authors: Hadas, A, Sofer, M, Molina, J.A.E, Barak, P, Clapp, C.E
• Format: Journal Article
• Language: English
• Published: Oxford Elsevier Science 01.02.1992; New York, NY
• Subjects: Agronomy. Soil science and plant productions; alanine; amino nitrogen; ammonia; ammonium nitrogen; assimilation; biodegradation; biogeochemical cycles; Biological and medical sciences; carbon nitrogen ratio; Chemical, physicochemical, biochemical and biological properties; comparisons; computer simulation; Fundamental and applied biological sciences. Psychology; immobilization; measurement; mineralization; nc soil model; nitrogen; nitrogen metabolism; Organic matter; Physics, chemistry, biochemistry and biology of agricultural and forest soils; simulation models; soil microorganisms; soil organic matter; Soil science; use efficiency; Assimilation; Isotope enrichment; Organic matter; Alanine; Organic nitrogen; Isotope labelling; Microflora; Modeling; Microbial biomass; Soils; Vertisols; Carbon nitrogen ratio; Nitrogen cycle; Mineralization; Aminoacid; Aridisols; Turnover; Ammonium Sulfates
• ISSN: 0038-0717; 1879-3428
• DOI: 10.1016/0038-0717(92)90269-4

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 NH4-N and alanine-N were added to soils, either one or the other labeled with 15N, and incubated for 1.2 days. The K2SO4-extractable organic and mineral N and 15N and CO2 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 15N 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 3.2d-1, following a delay of 0.25 d. Evolution of CO2 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 15N withdrawal from total extractable N was greater when alanine was labeled and fitted the prediction by the Direct model, but when NH4 was the source of 15N, the Direct model failed to predict 15N 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.