Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O₂) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding ¹³C dissolved organic matter (DOM) and ⁵⁷Fe^II to soil slurries incubated under static or fluctuating O₂. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive Fe^III phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding ⁵⁷Fe^II alone increases SOM mineralization by 8% following oxidation to ⁵⁷Fe^III. Under O₂ limitation, de novo reactive ⁵⁷Fe^III phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O₂ limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.