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. (C) 2015 Elsevier Ltd. All rights reserved.