Abstract
It is known that the association of soil organic matter (SOM) with iron minerals limits carbon mobilization and degradation in aerobic soils and sediments. However, the efficacy of iron mineral protection mechanisms under reducing soil conditions, where Fe(III)-bearing minerals may be used as terminal electron acceptors, is poorly understood. Here, we quantified the extent to which iron mineral protection inhibits mineralization of organic carbon in reduced soils by adding dissolved (13)C-glucuronic acid, a (57)Fe-ferrihydrite-(13)C-glucuronic acid coprecipitate, or pure (57)Fe-ferrihydrite to anoxic soil slurries. In tracking the re-partitioning and transformation of (13)C-glucuronic acid and native SOM, we find that coprecipitation suppresses mineralization of (13)C-glucuronic acid by 56% after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks, owing to ongoing reductive dissolution of the coprecipitated (57)Fe-ferrihydrite. Addition of both dissolved and coprecipitated (13)C-glucuronic acid resulted in increased native SOM mineralization, but the reduced bioavailability of the coprecipitated versus dissolved (13)C-glucuronic acid decreased the priming effect by 35%. In contrast, the addition of pure (57)Fe-ferrihydrite resulted in negligible changes in native SOM mineralization. Our results show that iron mineral protection mechanisms are relevant for understanding the mobilization and degradation of SOM under reducing soil conditions.