Mine tailings are usually fine particles that discarded as wastes during mining processes. Although these materials are considered low quality for extractions, they contain a considerable amount of metal(loid)s. It has been reported that there are more than 8 billion tons of tailings in U.S., 4 billion tons in former Soviet Union, and more than 6 billion in China. Thus, the storage of these hazardous wastes is a serious environmental concern and received extensive attentions globally. To reduces its risk, bioremediation was emerged as a promising method for re-extraction or stabilization of the metals. One of the obstacles, however, is that tailings are usually characterized as low organic content, low nutrient, and high metal concentrations. Indigenous microbial community, however, was adapted to the extreme environmental condition by adopting survival strategies, such as nitrogen fixation. Nitrogen fixation, which converts nitrogen gas to ammonium, is a essential natural process that provide nitrogen source to the ecosystems. As an marconutrient for all life forms, the bio-available nitrogen content limits the tailing restoration and reclamation. Thus, understanding the nitrogen fixation potential is necessary for bioremediation process. Our knowledge regarding how does the heavy metal(loid)s impact the nitrogen fixation process in tailings is still sparse.

In the current study, tailing and soil samples from five representative mining areas across the southern China were collected and analyzed. The geochemical analysis revealed significantly elevated metal concentrations, along with reduced organic carbon and nitrogen contents in tailings. Multivariate analysis suggested that both As and Sb contents were the key factors in shaping both total and nitrogen fixing microbial community compositions, and thus the community structure were distinct between tailings and soils. Cluster I nitrogen fixers, including members of Burkholderiales, Rhizobiales, and Xanothomonadales, were enriched and identified as the core microbiome in tailings. Despite the higher relative nitrogenase gene (nifH) abundance, nitrogen fixation potential in tailings were significantly lower than that in soils. The result of the current study suggested that metal(loid)s contamination, along with the oligotrophic conditions, posit strong suppression on microbial activity and impairs key environmental processes, such as nitrogen fixation.