Bacterial immobilization and remineralization of N at different growth rates and N concentrations

Abstract

An experiment was designed to resolve two largely unaddressed questions about the turnover of N in soils. One is the influence of microbial growth rate on mobilization and remineralization of cellular N. The other is to what extent heterotrophic immobilization of NO 3 - is controlled by the soil concentration of NH 4 + . Bacteria were extracted from a deciduous forest soil and inoculated into an aqueous medium. Various N pool dilution/enrichment experiments were carried out to: (1) calculate the gross N immobilization and remineralization rates; (2) investigate their dependence on NH 4 + and NO 3 - concentrations; (3) establish the microbial preference for NH 4 + and NO 3 - depending on the NH 4 + / NO 3 - concentration ratio. Remineralization of microbial N occurred mainly at high growth rates and NH 4 + concentrations. There was a positive correlation between NH 4 + immobilization and remineralization rates, and intracellular recycling of N seemed to be an efficient way for bacteria to withstand low inorganic N concentrations. Thus, extensive remineralization of microbial N is likely to occur only when environmental conditions promote high growth rates. The results support previous observations of high NO 3 - immobilization rates, especially at low NH 4 + concentrations, but NO 3 - was also immobilized at high NH 4 concentrations. The latter can be understood if part of the microbial community has a preference for NO 3 - over NH 4 + .