Anthony's Science Blog

Schimel Weintraub model

October 28, 2011 – 3:52 pm

At the Weintraub lab meeting, we’ve been talking with Daryl Moorhead about the Schimel Weintraub model of soil organic matter (SOM) decomposition. The main contribution of this model is to replace the empirical observation of exponential SOM decay with the mechanism that actually dissasembles macromolecules: extracellular enzymes (exoenzymes) produced by saprotrophs.

One interesting question that this modeling exercise brought up was: Why do SOM pools containing energetically favorable material persist in soils? In other words, why don’t microbial communities produce a ton of enzymes and eat everything available? The authors’ first try at simulating enzymes created this kind of unstable decomposition behavior:

…a stable system cannot be constructed when the kinetics are first order on enzymes, there must be a mechanism to produce non-linear kinetics. The specific mechanism in nature remains unclear, but one must exist. A likely mechanism is that as an organism produces more enzymes that must bind to solid substrates, they must diffuse further out from the cells, the substrates must diffuse further back, and enzymes may compete with each other for binding sites.

In this scenario, you have a relatively immobile organism that has to digest its food outside of its body and then hope that the food diffuses back to it. The authors’ hypothesis is that the ensuing dynamics within the soil matrix can greatly slow decomposition over time. This type of theoretical approach based on exoenzymes has spawned a lot of interesting followup work. Daryl for one has been working on making the model more sophisticated by including SOM pools of varying quality as would be encountered by real microbes.