Core Ideas
* Bulk soils are mixtures of SOM pools that cycle C and Hg very differently.
* Density separation and C‐14 analysis reveal differences between SOM pools.
* Mineral interactions are an underappreciated mechanism for Hg and C stabilization.
* C‐Hg interactions in Spodosols are consistent across the entire soil order.
* Soil taxonomic orders differ in C‐Hg relationships in pedogenically predictable ways.

Soils comprise the largest terrestrial pool of C and Hg on Earth, and these elements have critical feedbacks to problems ranging from atmospheric pollution and climate change to public health. Empirical evidence suggests these elements cycle closely in a wide range of soils, but mechanistic studies of their interactions within distinct soil organic matter (SOM) pools and between different soil types are needed. Here, we report findings of a novel approach to investigate C–Hg interactions, primarily in Spodosols, in which we: (i) examined density separated topsoil and illuvial horizons of four contrasting Spodosols, and used radiocarbon to investigate interactions between Hg and C cycling in distinct SOM pools; (ii) assessed broader patterns across Spodosols and other soil orders using USDA soil survey laboratory data. Consistent with other studies, C and Hg concentrations of individual soil horizons were positively related across the four contrasting Spodosols. Carbon and Hg were also positively related in the density fractions comprising individual soil horizons, but radiocarbon analysis revealed fundamental differences in Hg retention in modern, C‐rich fractions vs. low‐C fractions containing less modern radiocarbon. The lack of significant site‐to‐site variation in C and Hg across these sites (and Spodosols more broadly), contrasted against significant differences between horizons and fractions, suggests processes controlling C–Hg interactions are consistent across the taxonomic order. Furthermore, significant differences between other soil orders indicate that processes controlling soil formation—as represented by soil taxonomy—can explain differences in C–Hg interactions and their distribution across soils.