Abundance and community succession of nitrogen-fixing bacteria in ferrihydrite enriched cultures of paddy soils is closely related to Fe(III)-reduction.

In flooded paddy soils, some metal reducers are also capable of nitrogen (N) fixation, which is essential in ensuring a reliable N-supply for rice growth. Microbial iron [Fe(III)] reduction is an important biogeochemical process that can be stimulated by ferrihydrite amendment to paddy soil. Therefore, this study aimed to investigate the abundance and succession of the N-fixing bacterial community in ferrihydrite enriched paddy soils collected from Hunan (HN) and Sichuan (SC) provinces, China. The relationship between the N-fixing bacterial community and Fe(III) reduction was also assessed. When compared with the control treatment, ferrihydrite enrichment significantly enhanced nitrogenase (nifH) gene abundance by 8.05 × 10 to 4.45 × 10 copies g soil during the 40-day flooding of HN soil, while nifH gene abundance in SC soil was remarkably increased by 5.90 × 10 to 9.56 × 10 copies g soil during day 1 to 5 in response to ferrihydrite amendment. The relative abundance of N-fixing bacteria peaked on day 5 (21.5% in HN soil and 5.4% in SC soil) and gradually decreased to a stable abundance after day 20. Remarkable increases in relative abundance of N-fixing bacteria during the first 10 days of flooding were detected in both soils with ferrihydrite enrichment, whereas little difference was found after day 10 of flooding. During the early stage of flooding, the Shannon and Simpson indexes of N-fixing bacteria with ferrihydrite enrichment were significantly decreased, and the community structure changed greatly. Most N-fixing bacteria in ferrihydrite enriched paddy soils were phylogenetically related to the order Clostridiales, with some of those potentially capable of Fe(III) reduction. The community succession of N-fixing bacteria closely correlated with Fe(III) reduction. Thus, improving N-fixation via stimulation of Fe(III) reduction might aid in the reduction of N-fertilizer application to paddy field.