Organo-mineral interactions underpin long-term stabilization of soil organic carbon (OC). It remains unclear to which extent soil OC stabilization is coupled to the abundance of fine mineral particles or which compositional changes underlie the formation of OC in the mineral-associated pool. Here, we identify that soil OC content plays a larger role in determining the OC composition of fine fractions than the abundance of fine mineral particles (<20 µm). The formation of soil OC occurs in microscale patches which depend on the selective anchoring of newly forming organic matter to minerals or native organic matter. Using X-ray absorption and mid-infrared spectroscopy, we observed that the fine fraction of low-OC soils (7-16 g OC kg-1 soil) contained more oxidized OC than high-OC soils (46-73 g OC kg-1 soil) regardless of the abundance of fine mineral particles. A majority (mean of 61% (17-86%)) of new 13C-enriched litter-derived OC was found associated with mineral-dominated surfaces, which indicated a greater degree of oxidative transformation (increased carboxylic compounds). In contrast, new OC associated with native organic matter, occurred in larger patches and was less transformed. These results demonstrate that whether new OC is selectively anchored on mineral surfaces or native organic matter, which plays a central role for OC stabilization and consequently it’s resistance to future environmental and climate changes.