Incorporating Microbial Ecophysiology Into New Microbial-based Soil Organic Matter Frameworks.

We now know that microbes play a critical role in soil organic matter (SOM) formation, but their specific contributions remain undefined and even the newest models of SOM formation largely ‘black box’ the microbial community. In order to understand microbial contributions to SOM chemical structure, accumulation and stability, we must incorporate microbial ecophysiology and ecology into SOM formation concepts. In order to stimulate discussion and new research into this area, we identify several knowledge gaps that should be addressed in order to advance our understanding of microbially-mediated SOM formation. We anticipate that microbial ecophysiology will have direct influences on the rates of SOM accumulation and on SOM chemistry and stability, yet how the expression of these traits vary across environments and between microbial communities is unclear. Surprisingly, very little is known about these basic microbial physiological traits in complex communities in soil environments despite their potential influence on SOM processes.  We discuss the importance of several microbial physiological processes to SOM formation, including: 1) differences in microbial growth rates and growth efficiencies, and their tradeoffs between species and under different environmental conditions; 2) variation  in microbial cellular chemistry and its implications for interactions with minerals and inherent recalcitrance; 3) the role of microbial substrates (e.g. litter inputs) in shaping microbial physiological processes related to SOM formation. We consider the relative importance of these processes and subsequent effects on SOM formation across temporal and spatial scales. Finally, to synthesize and highlight key future research needs, we propose a new conceptual model that incorporates microbial ecophysiology into soil C dynamics.