Low-crystallinity iron oxide minerals generated from iron powder applied to rice paddy enhanced microbial nitrogen fixation in soil

Biological nitrogen (N) fixation is crucial for sustaining the N fertility in rice paddy soils. The N-fixing activity of paddy soils is higher than that of upland soils and increases during the flooded period as soil reduction progresses. Previously, we identified that N fixation in paddy soils is actively driven by iron (Fe) -reducing bacteria belonging to Deltaproteobacteria , which use Fe oxides as electron acceptors for respiration. Applying oxidized Fe powder to paddy soil enhanced the energy metabolism of these microorganisms, thereby increasing their N fixation activity. However, the Fe-oxide minerals responsible for this enhancement remain unknown. In this study, we identified the Fe-oxide minerals generated from Fe powder applied to paddy soils and evaluated their effects on soil N fixation. Structural analyses using XRD, SEM (-EDS)/TEM, and µXRF-XAFS showed that ferrihydrite and goethite predominated near the Fe-oxide particles, whereas lepidocrocite was more abundant farther from the particles. Some ferrihydrite incorporated Si, which was likely obtained from the surrounding soil. Of the identified Fe-oxide minerals, ferrihydrite and lepidocrocite significantly enhanced the N-fixing activity in paddy soil microcosms, suggesting that these low-crystalline Fe oxides were responsible for the enhanced N fixation observed in previous field studies. This study shows that the mineralogical composition of Fe oxide is essential for regulating microbial N fixation, and that Fe oxidation conditions should be optimized to selectively generate ferrihydrite and lepidocrocite, offering a promising strategy for increasing N fertility in paddy soils.