IntroductionThe diversity, nitrogen-fixing capacity and heavy metal tolerance of culturable rhizobia in symbiotic relationship withPongamia pinnatasurviving in vanadium (V) - titanium (Ti) magnetite (VTM) tailings is still unknown, and the rhizobia isolates from the extreme barren VTM tailings contaminated with a variety of metals would provide available rhizobia resources for bioremediation.MethodsP. pinnataplants were cultivated in pots containing the VTM tailings until root nodules formed, and then culturable rhizobia were isolated from root nodules. The diversity, nitrogen-fixing capacity and heavy metal tolerance of rhizobia were performed.ResultsAmong 57 rhizobia isolated from these nodules, only twenty strains showed different levels of tolerance to copper (Cu), nickel (Ni), manganese (Mn) and zinc (Zn), especially strains PP1 and PP76 showing high tolerance against these four heavy metals. Based on the phylogenetic analysis of 16S rRNA and four house-keeping genes (atpD,recA,rpoB,glnII), twelve isolates were identified asBradyrhizobium pachyrhizi, four asOchrobactrum anthropic, three asRhizobium selenitireducensand one asRhizobium pisi. Some rhizobia isolates showed a high nitrogen-fixing capacity and promotedP. pinnatagrowth by increasing nitrogen content by 10%-145% in aboveground plant part and 13%-79% in the root.R. pachyrhiziPP1 showed the strongest capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals, which provided effective rhizobia strains for bioremediation of VTM tailings or other contaminated soils. This study demonstrated that there are at least three genera of culturable rhizobia in symbiosis withP. pinnatain VTM tailings.DiscussionAbundant culturable rhizobia with the capacity of nitrogen fixation, plant growth promotion and resistance to heavy metals survived in VTM tailings, indicating more valuable functional microbes could be isolated from extreme soil environments such as VTM tailings.