Element cycling as driven by stoichiometric homeostasis of soil microorganisms

Cycling of carbon (C), nitrogen (N) and phosphorus (P) at the ecosystem scale is largely driven by microbial activity. A major factor that controls element cycling is the stoichiometric relationship between the microbial biomass and its substrate. I review recent advances in the field of ecological stoichiometry in terrestrial ecology with emphasis on processes performed by non-mycorrhizal soil microorganisms. The review shows that key processes of element cycling are driven by the property of microorganisms to maintain their biomass element ratio by (I) adjusting rates of element acquisition processes (organic matter decomposition, N-2 fixation and P solubilization) in order to acquire missing elements, and by (II) adjusting element partitioning and turnover times of elements in the microbial biomass to ratios of available elements. The review also shows that turnover times of elements in the microbial biomass have been neglected in ecological stoichiometry so far, although they are likely of high importance for microorganisms that thrive on substrates with extremely high C: nutrient ratios. Long turnover times of nutrients in the microbial biomass may explain why net nutrient mineralization also occurs at very high substrate C: nutrient ratios. In conclusion, ecological stoichiometry provides a conceptual framework for predicting relationships between the cycling of several key elements at the ecosystem scale, whose potential has not fully been exploited in terrestrial ecology yet.