Mineral protection of organic carbon (OC) by reactive metal (mainly iron and aluminum) (hydr) oxides is one of the key mechanisms promoting the long‐term stabilization of soil organic carbon (SOC). However, the sources and turnover of (metal‐) bound OC in natural soils remain poorly constrained, hampering our understanding of the behavior of this important pool of stabilized SOC. Here, we examine the 13C and 14C characteristics of SOC and bound OC in 15 wetland and upland soil profiles with contrasting reactive metal contents and environmental conditions. We find that bound OC has similar sources (reflected in δ13C) as SOC, suggesting no preference for plant‐ or microbe‐derived carbon. However, bound OC is older than SOC in wetland but not upland mineral soils, which is attributed to the high content of reactive minerals and low microbial activity in wetlands. Our finding highlights an active exchange of bound OC with SOC in upland soils, calling for a more holistic assessment on its dynamics and a better recognition of reactive metals in stabilizing OC in wetlands.