How do soil organisms affect total organic nitrogen storage and substrate nitrogen to carbon ratio in soils?: A theoretical analysis

To examine how soil organisms affect soil organic nitrogen (N storage and mineralization, we theoretically derive the soil substrate nitrogen to carbon ratio and total soil N storage for both donor-controlled (DC) and Lotka-Volterra (LV) soil systems with three trophic levels: substrate, microbes and grazers. We show that substrate N:C ratio and soil N storage are functions of the structure of the soil food web, of the properties of the organisms in the soil community, and of the properties of litter input from above ground. The carbon production-to-assimilation efficiency of soil organisms is the most important property determining how the substrate N:C ratio and soil N storage are affected by changes in the soil community. The qualitative effects of grazers depend on whether they have an efficiency larger or smaller than some critical efficiencies. In this paper, we derive these critical efficiencies mathematically. Within the normal range of efficiencies, substrate N:C ratio and soil N storage will usually decrease, and the nitrogen mineralization will increase, with addition of grazers, or as a result of a succession of species from a lower to a higher level of metabolic activity. Numerical analyses show that an increase in soil temperature has a pronounced negative effect on soil N storage through the effects on the metabolic activity of organisms at all trophic levels. N dilution in litter, because of increased CO2 levels in the atmosphere, can lead to a moderate increase in soil N storage.