Iron-bound organic carbon responses to land-use change vary with soil carbon availability and Fe-pool composition

Iron-bound organic carbon (Fe-OC) contributes to soil organic carbon (SOC) persistence, but its broad-scale response to land-use change (LUC) remains unclear. We synthesized paired-site observations from 75 publications worldwide. Degradation and restoration were analyzed separately relative to their respective paired reference sites. Degraded soils contained less SOC, Fe-OC, clay, total nitrogen, and total Fe, but had higher topsoil bulk density and pH. They also showed smaller relative contributions of short-range-order and organically complexed Fe forms. Restored soils contained more SOC, Fe-OC, clay, nutrients, and total Fe and showed greater relative contributions of these Fe forms, but had lower pH. However, Fe-OC accounted for a smaller share of SOC in restored soils, while organic carbon loading relative to extractable Fe was higher. Predictor support differed between LUC categories: SOC, pH, and microbial biomass carbon (MBC) received the strongest statistical support under degradation, whereas SOC, C/N, and the Feo/Fed ratio received the strongest support under restoration. Overall, these independent contrasts indicate that Fe-OC responses vary with soil carbon availability and Fe-pool composition. Higher bulk SOC in restoration comparisons therefore does not necessarily indicate a proportionally greater contribution of Fe-associated carbon.