Coprecipitation with Ferrihydrite Inhibits Mineralization of Glucuronic Acid in an Anoxic Soil

It is known thatthe association of soil organic matter(SOM) withiron minerals limits carbon mobilization and degradation in aerobicsoils and sediments. However, the efficacy of iron mineral protectionmechanisms under reducing soil conditions, where Fe(III)-bearing mineralsmay be used as terminal electron acceptors, is poorly understood.Here, we quantified the extent to which iron mineral protection inhibitsmineralization of organic carbon in reduced soils by adding dissolved ¹³C-glucuronic acid, a ⁵⁷Fe-ferrihydrite-¹³C-glucuronic acid coprecipitate, or pure ⁵⁷Fe-ferrihydriteto anoxic soil slurries. In tracking the re-partitioning and transformationof ¹³C-glucuronic acid and native SOM, we find that coprecipitationsuppresses mineralization of ¹³C-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 ⁵⁷Fe-ferrihydrite. Addition of both dissolved and coprecipitated ¹³C-glucuronic acid resulted in increased native SOM mineralization,but the reduced bioavailability of the coprecipitated versus dissolved ¹³C-glucuronic acid decreased the priming effect by 35%. Incontrast, the addition of pure ⁵⁷Fe-ferrihydrite resultedin negligible changes in native SOM mineralization. Our results showthat iron mineral protection mechanisms are relevant for understandingthe mobilization and degradation of SOM under reducing soil conditions.