Coupling between nitrogen-fixing and iron(III)-reducing bacteria as revealed by the metabolically active bacterial community in flooded paddy soils amended with glucose.

Biological nitrogen fixation can contribute to maintaining the nitrogen balance and reducing the risk of environmental pollution caused by nitrogen fertilizer application in flooded paddy soils. Microorganisms associated with microbial iron [Fe(III)] reduction are prevalent and presumed to be closely linked with biological nitrogen fixation in flooded paddy soils. The relationship between the nitrogen-fixing bacteria (NFB) and Fe(III)-reducing bacteria (FeRB) and their responses to organic carbon addition were investigated based on the metabolically active bacterial community in flooded paddy soils amended with/without glucose (CK: 0 mol C kg soil; OC: 0.1 mol C kg soil). Both putative NFBs and FeRBs were affiliated to the phyla Firmicutes and Proteobacteria, which were the two most abundant phyla in the metabolically active bacterial community. Glucose addition remarkably altered the community structures of the putative NFBs and FeRBs during a 40-day incubation, and the relative abundances of putative NFBs and FeRBs in the OC treatment increased by 0.21%-1.62% and 2.22%-14.82% relative to the CK treatment, respectively, during the later stage of incubation. The putative FeRBs co-occurred with NFBs and hydrogen-oxidizing bacteria, and the relative abundances of NFBs and hydrogen-oxidizing bacteria showed significant positive correlation with that of respiratory FeRBs. Some FeRBs could also be capable of nitrogen fixation and/or hydrogen oxidation. Thus, it might be feasible to enhance biological nitrogen fixation efficiency by promoting the metabolic activities of FeRBs (such as by adding glucose), which contribute directly to biological nitrogen fixation associated with nitrogen-fixing Fe(III) reducers and indirectly by reducing the suppression of hydrogen on nitrogen fixation associated with hydrogen-dependent Fe(III) reducers.