Networks of Dissolved Organic Matter and Organo-Mineral Associations Stimulate Electron Transfer over Centimeter Distances

Natural organic matter (NOM) dominated electron transfer has been widely studied in wetlands, freshwater sediments, and peatlands, in which a diffusion-electron hopping mechanism consisting of dissolved organic matter (DOM) and particulate organic matter (POM) was found to mediate electron transfer over centimeter (cm) distances. However, it remains unclear whether such long-distance electron transfer also occurs when NOM is associated with minerals, which form organo-mineral associations (OMAs) and thus are less mobile and accessible. In this study, we investigated the roles of DOM and OMAs in transferring electrons by performing a series of microbial Fe(III)-mineral reduction experiments over a 2 cm distance. We found that significant electron transfer only occurred when both DOM and OMAs were present. Generally, we observed a positive correlation between the relative proportion of DOM and OMAs and the extent of Fe(III) mineral reduction. However, varying the proportion of DOM showed a stronger effect on the Fe(III)-mineral reduction compared to OMAs, indicating that DOM played a more critical role in the electron transfer network. Our findings shed new light on how organic carbon facilitates iron transformation and the associated biogeochemical cycling of nutrients and contaminants in forest soil systems.