Mineral-associated carbon persistence arises from steady-state dynamics, not saturation

Soil organic carbon (SOC) persistence is central to climate mitigation yet often framed by the debated concept of mineral-associated organic carbon (MAOC) saturation. At the microscale (MAOC, <50 µm), organic carbon associates with minerals to form primary organo-mineral complexes. The enrichment factor (EFc), the ratio of C concentration in the silt and clay (silt+clay) fraction to SOC content, emerged early as a useful measure of MAOC enrichment or saturation. For example, a higher EFc is interpreted as indicating that coarse-textured soils are more saturated than fine-textured ones. This concept parallels Hassink’s saturation theory, which posits that C sequestration is constrained by mineral sorption capacity currently observable in the silt+clay fraction. Here, I show that both assumptions are not supported by global empirical evidence, and an alternative steady-state framework is proposed. This study assessed whether SOC accumulation is driven by site-specific inputs and decomposition rather than by a fixed saturation capacity, drawing on updated global data to reconcile the MAOC:silt+clay and MAOC:SOC approaches across a wide range of pedoclimatic conditions. The analysis further highlights future directions for refining sequestration estimates through the development of a pedotransfer function framework. The slope of the MAOC versus SOC regression from global datasets, previously reported, remains linear up to ~13% SOC, then the observed accumulation of MAOC likely reflects a dynamic steady-state rather than a saturation threshold. By contrast, MAOC versus silt+clay content captures variation in C loading, not strictly a universal fixed saturation. Although rarely observed, MAOC may continue to accumulate under varying C flux scenarios, stabilizing beyond the measurable range. This framework improves SOC sequestration predictions and challenges the paradigm that C saturation is determined solely by silt+clay.