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

Anaeromyxobacter and Geobacter, 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 Anaeromyxobacter and Geobacter 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 nifD from Anaeromyxobacter and Geobacter were detected, whereas those from other general diazotrophs were not detected. Anaeromyxobacter and Geobacter 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.