AVAILABILITY OF NH-4+-N TO MICROORGANISMS AND THE SOIL INTERNAL N-CYCLE

The fate of added N-15-labelled (NH4)2SO4 was determined in high and low NH4+ fixing soils under mineralizing conditions after 30 days of incubation at 15-degrees-C. Although soils with a high capacity for NH4+ fixation had a higher organic N-15 content than low fixing soils, none of the added NH4+ 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 (NH4+ and NO3-) participating in immobilization depending on availability to the heterotrophic microbial biomass.