Microbial reduction and alteration of Fe(III)-containing smectites in the presence of biochar-derived dissolved organic matter

Clay minerals are important components of solid-phase iron pools in soils, and the redox state of structurally-coordinated iron in clay minerals can significantly influence soil physicochemical properties. Due to increasing use of biochar in agriculture, the dissolved organic matter derived from biochar (BDOM) has become an unignorable fraction of dissolved organic carbon in soil systems. However, our knowledge regarding the effect of BDOM on soil biogeochemical processes (particularly on iron redox cycling) is still very limited. To examine the role of BDOM in the microbial reduction of Fe(III) in clays, BDOM was extracted from wheat straw biochar and selectively added to laboratory reactors in which lactate, Fe(III)-containing smectite (montmorillonite SWy-2 or nontronite NAu-2), and a typical iron-reducing bacterium Shewanella oneidensis strain MR-1 were used as the respective electron donor, electron acceptor, and reaction mediator. The results demonstrated that BDOM contained bio-reactive quinone moieties. BDOM amendment increased the initial rate and final extent of Fe(III) bioreduction by strain MR-1. Such promotion effect of BDOM might be ascribed to its quinone moieties functioning as electron shuttles between MR-1 cells and structural Fe(III) in smectite. Structural changes were observed in the bioreduced smectite systems, especially for NAu-2 reactors in which obvious dissolution and illitization occurred. These observations suggest that BDOM is bio-reactive and is able to stimulate the microbial reduction of Fe(III) in clay minerals, and may offer new insights into the effects of biochar application in elemental cycles in soils. [Display omitted] •The influence of BDOM on microbial reduction of structural Fe(III) in smectites is evaluated.•BDOM amendment can enhance the initial rate and final extent of Fe(III) bioreduction.•The bio-active quinone moieties within BDOM might account for the promotion role of BDOM in microbial iron reduction.•Microbial iron reduction favors nontronite dissolution and illitization.