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 dissolved13C-glucuronic acid, a57Fe-ferrihydrite-13C-glucuronic acid coprecipitate, or pure57Fe-ferrihydrite
to anoxic soil slurries. In tracking the re-partitioning and transformation
of13C-glucuronic acid and native SOM, we find that coprecipitation
suppresses mineralization of13C-glucuronic acid by 56%
after 2 weeks (at 25 °C) and decreases to 27% after 6 weeks,
owing to ongoing reductive dissolution of the coprecipitated57Fe-ferrihydrite. Addition of both dissolved and coprecipitated13C-glucuronic acid resulted in increased native SOM mineralization,
but the reduced bioavailability of the coprecipitated versus dissolved13C-glucuronic acid decreased the priming effect by 35%. In
contrast, the addition of pure57Fe-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.