Do soil microbes maximize their growth?

Soil organic matter (SOM) forms the foundation of microbial life in the soil and its processes. However, what drives the organization of organic matter turnover and microbial communities into growth remains unclear. In particular, we ask whether physical conditions in the soil—such as the quantity or quality of litter inputs—exist to which soil microbial processes adapt in order to maximize microbial growth as a proxy for power. We address this question in the frame of the German priority program 2322  by building on the maximum power principle. The principle suggests that biological systems tend to maximize the flux of energy into useful power under given constraints. We study a minimal model of SOM dynamics at steady state. In the model, litter inputs add to the organic matter pool, which is decomposed by microbial enzymes into compounds available for microbial uptake. The flux of Gibbs free energy provided with litter is used to build biomass while dissipating it during cycling, and the microbial decay returns as dead microbial biomass to the soil pool. We explore how different model structures, feedbacks, and parameterizations might lead to a maximum in the flux of free energy to microbial biomass, thereby providing insights into the conditions under which microbial growth is energetically optimized in soils.