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 N2 fixation rate were evaluated by Illumina MiSeq sequencing and acetylene reduction assays, respectively. The results indicated that high PCB contamination increased diazo-trophic abundance and the N2 fixation rate, and altered diazotrophic community structure in the paddy soil. The random forest model demonstrated that the beta-diversity of the diazotrophic community was the most significant predictor of the N2 fixation rate. Structure equation modeling identified a specialized keystone diazotrophic ecological cluster, predominated by Bradyrhizobium, Desulfomonile, and Cyanobacteria, as the key driver of N2 fixation. Overall, our findings indicated that long-term PCB contamination enhanced the N2 fixation rate by altering diazotrophic community abundance and structure, which may deepen our understanding of the ecological function of diazotrophs in organic-contaminated soil.