Bridging 20 Years of Soil Organic Matter Frameworks: Empirical Support, Model Representation, and Next Steps.

In the past few decades, there has been an evolution in our understanding of soil organic matter (SOM) dynamics from one of inherent biochemical recalcitrance to one deriving from plant‐microbe‐mineral interactions. This shift in understanding has been driven, in part, by influential conceptual frameworks which put forth hypotheses about SOM dynamics. Here, we summarize several focal conceptual frameworks and derive from them six controls related to SOM formation, (de)stabilization, and loss. These include: (a) physical inaccessibility; (b) organo‐mineral and ‐metal stabilization; (c) biodegradability of plant inputs; (d) abiotic environmental factors; (e) biochemical reactivity and diversity; and (f) microbial physiology and morphology. We then review the empirical evidence for these controls, their model representation, and outstanding knowledge gaps. We find relatively strong empirical support and model representation of abiotic environmental factors but disparities between data and models for biochemical reactivity and diversity, organo‐mineral and ‐metal stabilization, and biodegradability of plant inputs, particularly with respect to SOM destabilization for the latter two controls. More empirical research on physical inaccessibility and microbial physiology and morphology is needed to deepen our understanding of these critical SOM controls and improve their model representation. The SOM controls are highly interactive and also present some inconsistencies which may be reconciled by considering methodological limitations or temporal and spatial variation. Future conceptual frameworks must simultaneously refine our understanding of these six SOM controls at various spatial and temporal scales and within a hierarchical structure, while incorporating emerging insights. This will advance our ability to accurately predict SOM dynamics. Plain Language Summary: Soil organic matter, the remains of plants, animals, and microbes in the soil, performs many important functions for humans and ecosystems, providing habitat for animals, nutrients for plants, climate change buffering, and structure for soil animals and human structures. Thus, it is important to understand how soil organic matter is formed, stabilized, and lost. Here, we review conceptual frameworks that have contributed to our understanding of soil organic matter over the past 20 years. We evaluate their support in experiments and also how well represented they are in computer models. We find the least support and representation for controls of soil organic matter associated with properties of microbes and physical barriers between microbes and soil organic matter. These and novel soil organic matter controls require more research for better understanding of soil organic matter functions. Key Points: Soil organic matter (SOM) research has been advanced by conceptual frameworksConceptual frameworks are associated with different SOM controls with variable empirical support and model representationMicrobial physiology and morphology and physical inaccessibility, as SOM controls, require more empirical work and model representation [ABSTRACT FROM AUTHOR]