Abstract: Modeling decomposition of soil organic carbon is crucial to predict ecosystem feedbacks to global change. Typically, soil C models divide organic C into pools with different decomposition rates. Even though these pools contain complex mixtures of organic molecules, most models assume that decomposition from pools is a function of the average chemical properties of the organic molecules it contains. Soil microbes, however, do not decompose pools of SOC. Rather, they take up and metabolize organic molecules with distinct chemical structures. Different organic substrates are metabolized by different enzymes, and building enzymes for any distinct metabolic pathway can be costly for microbes. In the presence of many substrates, microbes then must either allocate resources toward metabolizing specific substrates while ignoring others, or split resources among enzymes for different available substrates.
To investigate the influence of chemodiversity on decomposition, we developed a theoretical model that explicitly represents decomposition of distinct substrates, corresponding enzymes, and the costs and benefits associated with building enzymes. With this model we show two mechanisms by which chemodiversity could suppress microbial uptake of available organic substrates. In the first, microbes ignore substrates that aren’t abundant enough to justify investing in corresponding enzymes, effectively shrinking the effective size of the decomposing pool. In the second mechanism, microbes divide resources to make multiple enzyme types. If microbial uptake is co-limited by substrates and enzymes, then increasing chemodiversity decreases microbial uptake by simultaneously decreasing the relative abundance of each enzyme and each corresponding substrate. Using this model, we predict that increasing chemodiversity of microbially-available substrates suppresses the overall decomposition rate, particularly when overall substrate availability is low.
