Ecological stoichiometry

Nutrient cycling

Terrestrial microbial decomposer communities thrive on a wide range of organic matter types that rarely ever meet their elemental demands in terms of C, N and P (and other nutrients). The stoichiometric imbalance between microbial communities and their organic substrates generally decreases from wood to leaf litter and further topsoil and subsoil organic matter. Microbial communities can respond to these imbalances in four ways: first, they adapt their biomass composition toward their resource in a non-homeostatic behavior. Such changes are, however, only moderate, and occur mainly because of changes in microbial community structure and less so due to cellular storage of elements in excess. Second, microbial communities can mobilize resources that meet their elemental demand by producing specific extracellular enzymes, which, in turn, is restricted by the C and N requirement for enzyme production itself. Third, microbes can regulate their element use efficiencies (ratio of element invested in growth over total element uptake), such that they release elements in excess depending on their demand (e.g., respiration and N mineralization). Fourth, diazotrophic bacteria and saprotrophic fungi may trigger the input of external N and P to decomposer communities. Theoretical considerations show that adjustments in element use efficiencies may be the most important mechanism by which microbes regulate their biomass stoichiometry. This framework allows for integrating and linking microbial adaptation to resource imbalances to ecosystem-scale fluxes across scales and eosystems.

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