Changes in the composition and diversity of microbial communities during anaerobic nitrate reduction and Fe(II) oxidation at circumneutral pH in paddy soil

Kinetics of nitrate (NO3−) reduction and ferrous iron (Fe(II)) oxidation in paddy soil were investigated under anoxic conditions at circumneutral pH using three different treatments (i.e., Lactate + Fe(II), Lactate + NO3−, and Lactate + NO3− + Fe(II)). The results revealed that NO3− could be rapidly reduced to nitrite (NO2−) within two days in treatments of Lactate + NO3− and Lactate + NO3− + Fe(II), and the presence of Fe(II) facilitated the NO2− reduction. Whereas no obvious Fe(II) oxidation was observed in treatment of Lactate + Fe(II), Fe(II) oxidation took place only when NO3− was added. Illumina high-throughput sequencing used to profile the diversity and abundance of microbial communities showed that the phyla of Proteobacteria and Firmicutes had a dominant presence in all three treatments with lactate. Acidaminobacter, Proteiniclasticum, Alkaliphilus, and Natronincola_Anaerovirgula were found to be the dominant genera during NO3− reduction without Fe(II) after addition of lactate, and all were seldom reported to be associated with NO3− reduction. Azospira, Zoogloea, and Dechloromonas dominated during NO3− reduction in the presence of Fe(II), and all are betaproteobacterial NO3−-reducing bacteria that do not produce ammonium as end products of NO3− reduction. Whereas Azospira, Zoogloea, and Dechloromonas have been isolated or identified from NO3−-reducing Fe(II) oxidation culture previously, the NO2− produced by these NO3− reducing bacteria can also oxidize Fe(II) abiotically, resulting in facilitated NO2− disappearance in the treatment of Lactate + NO3− + Fe(II). These findings increase our understanding of the processes of NO3− reduction in the absence and presence of Fe(II) in anoxic paddy soil at circumneutral pH and extend our knowledge of the microbial communities involved in these processes.