A spatial microbial competition mechanism of soil carbon persistence

Here we present a novel model supplementing existing theories of soil organic matter (SOM) decomposition, based on evolutionary-ecological principles rather than chemical or physical limitations to decomposition. We argue that decomposition of some substrates, in particular nitrogen-rich non-hydrolyzable matter (NHLS), may be constrained by spatial competition from opportunists (Bonner et al., 2022). Our model is based on two linked hypotheses: (1) From an evolutionary point of view, microbes should optimise their enzyme production to maximise the net fitness gain (F), and they should only decompose NHLS if the uptake of decomposition products (S) brings a net fitness gain (F > 0) in terms of growth minus costs of enzyme production. (2) F strongly depends on the fraction of decomposition products absorbed by the decomposer, i.e. the return on enzyme investment, which depends on the distance to the substrate and the competition from opportunistic bacteria. A minimum ‘safe’ distance for oxidative decomposition is included, based on the idea that cost of oxidative stress to the decomposer will surpass potential gain from decomposition when the activity is too close. Although the model predictions have not been tested directly against observations, they provide proof-of-concept that substrate can be spared decomposition and accumulate even when it is physically and chemically accessible. Due to the spatial competition effect, it is not profitable for either bacteria or decomposer fungi to decompose NHLS under certain conditions.  Our framework can help explain a variety of SOM dynamics, including priming and the suppression of decomposition by nitrogen addition.

 

Reference

Bonner MTL, Franklin O, Hasegawa S, Näsholm T. 2022. Those who can don't want to, and those who want to can't: An eco-evolutionary mechanism of soil carbon persistence. Soil Biology and Biochemistry 174: 108813.