Embracing the Heterogeneity Within Particulate and Mineral‐Associated Organic Matter for the Next Generation of Soil Carbon Questions

Soil organic matter (SOM) is a complex and dynamic substrate central to the functioning of terrestrial ecosystems. Because of its heterogeneity, SOM is routinely parsed into physical fractions with more uniform composition and behavior. Over the last several years, particulate (POM) and mineral-associated (MAOM) organic matter have become the most widely adopted of these fractions, which has given the field a shared, comparable framework for resolving the first-order heterogeneity within SOM. As fractionation has become routine, however, it has become clear that POM and MAOM identity alone cannot resolve the finer, second-order variation that additional mechanistic inference requires. This is in part because the fractions are operationally defined composites, whose properties vary across composition, formation pathway, and behavior. Conceptualizing them as the terminal state of analysis collapses each fraction to its central tendency, flattening the distribution and obscuring the structure on which more complex questions depend. Here we propose embracing this within-fraction heterogeneity rather than averaging over it, treating POM and MAOM as composite distributions whose internal structure underlies the variation in how they form, persist, and function. We suggest that this view can be operationalized via two complementary approaches: tailoring fractionation methods to target subpopulations most relevant to a question, and pairing fractions with orthogonal measurements. Together, these resolve the within-fraction distribution that bulk measurements integrate over, narrowing the analysis onto the most explanatory slice of the bulk SOM. Similar to the original framework, we do not aim to prescribe a certain methodology, but rather a way of thinking about heterogeneity within SOM, in which a separation is justified by the understanding it produces rather than the data it generates. This way of reasoning allows POM and MAOM to continue linking organic matter to its functions, and in doing so supports more informed stewardship of soil carbon.