Rice root Fe plaque increases paddy soil CH4 emissions via the promotion of electron transfer for syntrophic methanogenesis

Iron (Fe) plaque is a concentrated form of microbially available Fe oxide that coats rice plant root surfaces, representing a high density of Fe oxide and which potentially mediates paddy soil CH4 emissions. Using a combination of methods including Fe plaque induction, isotopic labeling, pure microbial strains, and Fe oxide addition experiments, we investigated the impact of Fe plaque on methane (CH4) emissions from paddy soils and explored the associated mechanisms underlying the influence of Fe plaque on CH4 emissions. A 13C–CH4 isotopic labeling experiment showed that Fe plaque did not significantly affect CH4 oxidation and associated gene expression, whereas Fe plaque significantly enriched methanogenic archaea and their expression of genes associated with methanogenesis. Pure microbial strain and Fe oxide addition experiments showed that the enhancement of CH4 production in the presence of Fe plaque was caused by the (semi) conductive minerals within the Fe plaque, specifically, hematite, which promoted the extracellular electron transfer between the methanogenic archaea and their syntrophic bacteria, resulting in enhancement of methanogenesis. Our results imply that the presence of Fe plaque will accelerate CH4 emissions from paddy soils and suppressing Fe plaque has the potential to mitigate CH4 emissions.