Keywords: Gross N biotransformation, soil microbial biomass, nitrogen availability, mineralization, C:N ratios.

Summary:
Understanding the factors that govern microbial activities in soils is important, because the heterotrophic soil microbial biomass is driving the carbon (C) and nitrogen (N) cycling, and the net availability of N for plant uptake or losses. The processes by which C and N interact during decomposition of organic matter in soil, and the mineralization-immobilisation turnover are well known but still difficult to predict, due to the complex interactions between soil micro-organisms and their substrates. The ¹⁵N isotope dilution techniques that allow rates of N transformations to be quantified accurately and simultaneously help the understanding of the effects of soil, crop and management conditions on microbial activities. The quantification of the relative importance of gross mineralization, immobilisation and nitrification is also a key for understanding the competition between processes and the susceptibility of systems to lose or to retain nitrogen.

Sylvie Recous, INRA, Unité d'Agronomie , rue F. Christ, 02007 Laon cedex, France.

20 pages, 3 figures, 3 tables, 40 references.