Soil organic matter formation, persistence, and functioning: A synthesis of current understanding to inform its conservation and regeneration

Soil organic matter (SOM) provides vital services to humanity. Its preservation and further accrual are key to sustain food production and avoid an irreversible climate crisis. Here we present the processes and drivers of SOM formation and persistence within a coherent state-of-the-art framework. We posit that SOM forms via two distinct pathways depending on whether inputs are water soluble and/or easily solubilized entering the soil as dissolved organic matter (DOM), or they are structural. These distinct inputs form mineral-associated organic matter (MAOM), and particulate organic matter (POM), respectively. Both these SOM fractions have plant and microbial components but in different proportions, with MAOM being more highly microbial. SOM persistence will depend on microbial activity inhibition, the degree of its limitation and carbon use efficiency, and microbial access constraints, primarily due to association to minerals and occlusions in fine aggregates. Climate is the overarching control of SOM persistence, also by affecting ecosystem traits, when persistence is driven by microbial activity inhibition or limitation, largely responsible for POM storage. Soil geochemical traits are the overarching control of SOM persistence driven by microbial access constraints, particularly in the subsoil, specifically controlling MAOM storage. SOM affects soil properties (aggregation, porosity, and cation exchange capacity) which in turn determine the soil's capacity for functioning and ability to provide desired outcomes including erosion and flood prevention, plant productivity, and climate mitigation. The specific properties of SOM which influence its contributions to these functions are discussed, with implications for SOM conservation and regeneration to promote desired outcomes.