Soluble and insoluble fractions of mineral-associated organic carbon differ in composition, accumulation patterns, and environmental controls

ABSTRACT Mineral-associated organic carbon (MAOC) is one of the largest and most persistent soil C reservoirs, with major implications for soil functioning and climate mitigation. However, bulk MAOC is chemically heterogeneous and may contain organic matter with contrasting solubility, chemical composition, source-related signatures, and environmental controls, which remain poorly understood. Here, we used archived surface and subsurface soils from 43 National Ecological Observatory Network sites across North America to isolate MAOC and partition it into soluble fractions, which were released by repeated hydrofluoric acid (HF) extraction followed by water rinsing, and insoluble fractions which comprised the extraction residue. Across sites, the soluble fraction accounted for 11–96% of total MAOC, whereas the insoluble fraction accounted for 5–89%. The soluble fraction exhibited a Langmuir-type accumulation pattern and was enriched in phenolic and carboxyl functional groups, as well as lignin, tannin, and condensed aromatic-like molecules. In contrast, the insoluble fraction increased nonlinearly without an apparent asymptote across the observed MAOC range and was enriched in alkyl C and non-aromatic molecular classes, including lipids, carbohydrates, and proteins. Composition-based estimates indicated stronger plant-associated signatures in soluble MAOC and stronger microbial-associated signatures in insoluble MAOC. Despite these chemical contrasts, radiocarbon values did not differ detectably between the soluble and insoluble MAOC fractions; instead, Δ 14 C declined with soil depth and increased with mean annual precipitation. Environmental analyses showed that a higher contribution of soluble MAOC to bulk MAOC was associated with deeper soils and wetter, more acidic, Fe/Al-rich conditions. These findings suggest that bulk MAOC contains operationally separable fractions with distinct solubility, accumulation behavior, chemical composition, source-related signatures, and environmental associations. Accounting for this heterogeneity may improve conceptual and process-based models of MAOC dynamics.