Iron-mediated organic matter decomposition in humid soils can counteract protection

Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O(2)) becomes limited. Here we quantify Fe-mediated OM protection vs. decomposition by adding (13)C dissolved organic matter (DOM) and (57)Fe(II) to soil slurries incubated under static or fluctuating O(2). 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 (57)Fe(II) alone increases SOM mineralization by 8% following oxidation to (57)Fe(III). Under O(2) limitation, de novo reactive (57)Fe(III) phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32‒41%, respectively. Periodic O(2) 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.