Iron oxide minerals: promising materials for sustainable rice production via stimulating iron-reducing diazotrophs.

This mini-review highlights the emerging agricultural applications of iron oxides, the primary form of iron and one of the most abundant elements on Earth. Rice yield is strongly influenced by soil nitrogen fertility, which is supported by biological nitrogen fixation. Using soil metatranscriptomic analysis and isolation-cultivation experiments, we recently discovered iron-reducing Deltaproteobacteria as the predominant but previously overlooked drivers of nitrogen fixation in paddy soil. As these bacteria utilize Fe3+ as an electron acceptor during anaerobic respiration, we hypothesized that amending soils with Fe3+-rich iron oxides would enhance their nitrogen-fixing activity. Laboratory and field experiments confirmed that applying iron oxides with low crystallinity significantly stimulated the diazotrophic activity of iron-reducing bacteria, enabling reduced nitrogen fertilizer input in rice cultivation, with a reduced nitrogen burden to the environment. Recognition of iron-reducing diazotrophs has opened a new research frontier: using metals, particularly crystalline forms of iron oxides, in sustainable agricultural systems.