Organic Matter from Redoximorphic Soils Accelerates and Sustains Microbial Fe(III) Reduction

Microbial reduction of Fe(III) minerals is a prominent process in redoximorphic soils and is strongly affected by organic matter (OM). We herein determined the rate and extent of microbial reduction of ferrihydrite (Fh) with either adsorbed or coprecipitated OM by Geobacter sulfurreducens. We focused on OM- mediated effects on electron uptake and alterations in Fh crystallinity. The OM was obtained from anoxic soil columns (effluent OM, efOM) and included.unlike waterextractable OM.compounds released by microbial activity under anoxic conditions. We found that organic molecules in efOM had generally no or only very low electron-accepting capacity and were incorporated into the Fh aggregates when coprecipitated with Fh. Compared to OM-free Fh, adsorption of efOM to Fh decelerated the microbial Fe(III) reduction by passivating the Fh surface toward electron uptake. In contrast, coprecipitation of Fh with efOM accelerated the microbial reduction, likely because efOM disrupted the Fh structure, as noted by Mossbauer spectroscopy. Additionally, the adsorbed and coprecipitated efOM resulted in a more sustained Fe(III) reduction, potentially because efOM could have effectively scavenged biogenic Fe(II) and prevented the passivation of the Fh surface by the adsorbed Fe(II). Fe(III)-OM coprecipitates forming at anoxic-oxic interfaces are thus likely readily reducible by Fe(III)-reducing bacteria in redoximorphic soils.

Automated summary

In redoximorphic soils, microbial reduction of iron (III) minerals is significantly influenced by organic matter, which can either decelerate or accelerate the reduction rate. The organic matter released by microbial activity under anaerobic conditions (efOM) plays a key role in affecting the electron uptake and crystallinity of iron (III) minerals. The presence of efOM, whether adsorbed or coprecipitated, leads to distinct effects on microbial reduction of iron (III) minerals by affecting the surface passivation and altering the mineral structure.