Stoichiometrically constrained soil microbial community adaptation modeled with SESAM

Describing the coupling of nitrogen (N), phosphorus (P), and carbon (C) cycles of land ecosystems requires understanding microbial element use efficiencies of soil organic matter (SOM) decomposition. These efficiencies are studied by the soil enzyme steady allocation model (SESAM) at decadal scale. The model assumes that the soil microbial community and their element use efficiencies develop in a way that maximizes the growth of the entire community. Specifically, SESAM approximated this growth optimization by allocating resources to several SOM degrading enzymes proportional to the revenue of these enzymes, called the Relative approach. However, a rigorous mathematical treatment of this approximation has been lacking so far. 

Therefore, this study derives explicit formulas of enzyme allocation that maximize total return from enzyme reactions, called the Optimal approach. When comparing predictions across these approaches, we find that the Relative approach is a special case of the Optimal approach valid at sufficiently high microbial biomass. However, at low microbial biomass, it overestimates  allocation to the enzymes having lower revenues.

The model finding that a smaller set of enzyme types is expressed at low microbial biomass provides another hypothesis for why some substrates in soil are preserved over decades although being decomposed within a few years in incubation experiments. This study is another step in integrating a simple representation of an adaptive microbial community into coupled stoichiometric CNP SOM dynamic models.