Pyrolysis temperature dependent electron mediating mechanisms of biochar for microbial reduction of Fe(III)-rich smectite

The redox state of structurally-coordinated iron in clay minerals profoundly influences the physicochemical properties of soils. While iron-reducing microorganisms, such as Shewanella oneidensis strain MR-1, can mediate the reduction of structural Fe(III) in clay minerals, the increasing agricultural application of biochar raises questions about its potential involvement in this biogeochemical process. In this study, we conducted bench-scale experiments to investigate the impact of wheat straw-derived biochar, produced at different pyrolysis temperatures (300–600 °C), on the microbial reduction of Fe(III) in nontronite (NAu-2) by S. oneidensis strain MR-1. NAu-2 served as a model Fe(III)-rich smectite representing pre-existing clay phases in soil systems. Our results revealed that increasing pyrolysis temperature enhanced the aromaticity and electrical conductivity of the biochar while decreasing the concentration of its redox-active functional groups, such as quinone moieties. Biosystems amended with low-temperature biochar exhibited a higher initial bioreduction rate and greater final reduction extent. This enhancement was primarily attributed to the electron-shuttling capacity of biochar-associated quinone moieties rather than its electrical conductivity. In addition, the bioreduction kinetics were influenced by biochar dosage, with higher biochar concentrations correlating with faster initial bioreduction rates. The bioreduction of structural Fe(III) in NAu-2 induced mineral dissolution and triggered a structural transformation from dioctahedral to trioctahedral smectite, particularly in biochar-amended systems. Furthermore, high-charge smectite was observed in the bioreduced NAu-2 samples.