Enhancement of nitrogen fixation and diazotrophs by long-term polychlorinated biphenyl contamination in paddy soil.

Biological nitrogen fixation (BNF) driven by diazotrophs is a major means of increasing available nitrogen (N) in paddy soil, in addition to anthropogenic fertilization. However, the influence of long-term polychlorinated biphenyl (PCB) contamination on the diazotrophic community and nitrogen fixation in paddy soil is poorly understood. In this study, samples were collected from paddy soil subjected to > 30 years of PCB contamination, and the soil diazotrophic community and N fixation rate were evaluated by Illumina MiSeq sequencing and acetylene reduction assays, respectively. The results indicated that high PCB contamination increased diazotrophic abundance and the N fixation rate, and altered diazotrophic community structure in the paddy soil. The random forest model demonstrated that the β-diversity of the diazotrophic community was the most significant predictor of the N fixation rate. Structure equation modeling identified a specialized keystone diazotrophic ecological cluster, predominated by Bradyrhizobium, Desulfomonile, and Cyanobacteria, as the key driver of N fixation. Overall, our findings indicated that long-term PCB contamination enhanced the N fixation rate by altering diazotrophic community abundance and structure, which may deepen our understanding of the ecological function of diazotrophs in organic-contaminated soil.