Microbial mobilization and immobilization of soil nitrogen

Abstract: Microorganisms in forest ecosystems normally recycle nitrogen (N), such that gaseous losses and leaching are limited. Mobilization of organic N, including microbial N, and immobilization of inorganic N, especially NH4+, are the quantitatively most important N transformation processes. Various factors influence their rates, e.g. the amount of carbon and nitrogen, temperature, moisture, and the microbial biomass and activity. The factors are adressed in the thesis with the ambition to suggest mechanistic explanations for their influence on nitrogen mobilization and immobilization rates. The mineralization and immobilization rates are not dependent on the concentration of carbon or nitrogen per se, but rather on the microbial biomass and its activity. This may invalidate the use of first order kinetics to describe mobilization and immobilization rates. Earlier claims that mineralization and immobilization rates are dependent on the concentration of carbon and nitrogen may have overseen the dependence of the microbial biomass on the soil organic matter content. Moisture, temperature, and the quality of the utilized organic matter appear to determine the activity of the microorganisms. Mobilization of microbially bound N seem to occur mainly as a result of predation and drying-rewetting cycles, since microorganisms have an efficient intracellular recycling of nitrogen, especially at low ammonium concentrations. Thus, mobilization and remineralization of microbial nitrogen by other processes than predation and drying-rewetting cycles is likely to occur only when conditions promote high growth rates, i.e. when microorganisms are not substrate limited and temperature and moisture conditions are favourable. The main factors determining microbial mobilization and immobilization of soil N are therefore the size of the microbial biomass, its activity, and the quality of the growth substrate. Those three factors in turn seem to be dependent on soil SOM content, predation, drying-rewetting cycles, temperature, and soil water content.