## 1 Introduction  
Rice (_Oryza sativa_ L.) is one of the most important food crops and feeds more than half of the population in the world [1]. Since the green revolution in the 19 th century, chemical nitrogen fertilizers have been developed and applied to rice paddy fields to increase rice yields [2]. However, long-term or excessive nitrogen fertilization leads to the release of nitrogen loads, i.e., ammonium volatilization, nitrous oxide emission, and nitrate leaching from paddy soils into the natural environment [3]. This contributes to increased environmental problems such as global warming, water eutrophication, and nitrate pollution of groundwater [4; 5]. Reduced nitrogen fertilizer use is necessary for environmentally friendly and sustainable rice production. In addition to nitrogen fertilizers, biological nitrogen fixation (BNF) can incorporate nitrogen into soil under rice paddy field conditions [6]. Flooded paddy soils possess higher nitrogen-fixing activity than upland agricultural soils, and this is important for sustainable soil nitrogen fertility and rice growth [7; 8; 9]. Therefore, diazotrophic bacterial communities in paddy soils have been studied for a long time. In particular, photosynthetic bacteria and cyanobacteria have been extensively studied as important diazotrophs [10]. The application of these diazotrophs as biofertilizers was also investigated with the aim of lowering the amount of nitrogen fertilizer used and reducing nitrogen pollution [11; 12; 13]. Our recent shotgun sequencing analysis (metagenome/metatranscriptome) of paddy soil DNA/RNA suggested that _Anaeromyxobacter_ and _Geobacter_, predominant bacteria in paddy soil, are the major drivers of nitrogen fixation in paddy soils [14]. _Anaeromyxobacter_ and _Geobacter_ are known as metal-reducing bacteria in general [15; 16] and active iron-reducing bacteria in paddy soil [17]. We confirmed the nitrogen-fixing ability of _Anaeromyxobacter_ and _Geobacter_ strains isolated from paddy soils by cultivation studies. They utilized acetic acid as a carbon and energy source (electron donor) and ferric iron as an electron acceptor for anaerobic respiration [18; 19; 20; 21; 22; 23; 24]. Based on whole-genome analysis of the isolates, the taxonomic position of bacteria belonging to the family _Geobacteraceae_ was revised, and the conventional genus _Geobacter_ was divided into the genera _Geobacter_, _Geomonas_, _Oryzomonas_, _Geotalea_, _Geomobilimonas_, _Geomobilincter_, and _Geomanerobacter_[20]. It was strongly suggested that in paddy soils, _Anaeromyxobacter_ and _Geobacteraceae_ (conventional _Geobacter_) grow by utilizing rice straw-derived acetic acid as an electron donor, ferric iron compounds as an electron acceptor, and nitrogen fixation. We hypothesized that the addition of ferric iron compounds and rice straw could enhance the nitrogen-fixing activity of _Anaeromyxobacter_ and _Geobacteraceae_ in paddy soils. In fact, nitrogen-fixing activity of soil and _nifD_ transcripts of _Anaeromyxobacter_ and _Geobacteraceae_ significantly increased after the addition of ferrihydrite or Fe\({}_{2}\)O\({}_{3}\) together with rice straw to laboratory soil microcosms [25]. In our field experiment, a significant increase in soil nitrogen-fixing activity was also observed after the application of iron powder to straw-returned paddy field soil [25]. Another field experiment in China demonstrated that a combination of iron application with 60-80% of the conventional nitrogen fertilization rate could maintain rice yields similar to those of the conventional nitrogen fertilization rate while reducing critical nitrogen losses in flooded paddy fields [26]. These studies strongly suggested that iron application to paddy soil increased the amount of nitrogen fixed in the soil by enhancing nitrogen fixation by iron-reducing bacteria. However, these claims, i.e., the amount of nitrogen incorporated into soil and the incorporation of nitrogen into iron-reducing bacteria, must be demonstrated in direct ways.