Availability of NH 4+-N to microorganisms and the soil internal N cycle

Abstract

The fate of added 15N-labelled (NH 4) 2SO 4 was determined in high and low NH + 4fixing soils under mineralizing conditions after 30 days of incubation at 15°C. Although soils with a high capacity for NH + 4 fixation had a higher organic 15N content than low fixing soils, none of the added NH 4 + was recovered in microbial biomass N. The soil internal N cycle is largely comprised of two independent processes, mineralization and immobilization, both under microbial control but physically and/or temporally separate. The connection between these two processes is through the inorganic N phase, with both forms of inorganic N (NH + 4 and NO 3 −) participating in immobilization depending on availability to the heterotrophic microbial biomass.