Enhancement of the nitrogen-fixing activity of paddy soils owing to iron application

<i>Anaeromyxobacter</i> and <i>Geobacter</i>, iron-reducing bacteria belonging to Deltaproteobacteria, are newly discovered nitrogen-fixing bacteria predominant in paddy soils. We hypothesized that adding ferric iron oxide as an electron acceptor for respiration of the iron-reducing bacteria could enhance the nitrogen-fixing activity of <i>Anaeromyxobacter</i> and <i>Geobacter</i> in paddy soils. In the soil microcosm study, soil nitrogen-fixing activity significantly increased after adding ferrihydrite or Fe₂O₃, ferric iron oxides, to the soil. In these soils, gene transcripts of <i>nifD</i> from <i>Anaeromyxobacter</i> and <i>Geobacter</i> were detected, whereas those from other general diazotrophs were not detected. <i>Anaeromyxobacter</i> and <i>Geobacter</i> might be involved in the enhanced soil nitrogen-fixing activity. The application of iron powder to paddy field soils also enhanced the soil nitrogen-fixing activity. This study is the first to demonstrate that addition of iron compounds to paddy soil enhanced the soil nitrogen-fixing activity. The results of this study may lead to the development of novel paddy soil management strategies to increase soil nitrogen fertility and ensure rice yields with a reduced nitrogen fertilizer input and a lower environmental nitrogen burden.