Assessing the diazotrophic community in a vertical soil profile with antimony and arsenic co-contamination

Nitrogen (N) deficiency usually hinders the bioremediation of mining-derived habitats such as mining-contaminated regions. Diazotrophs can provide N for the growth of plants and microorganisms in these environments. However, diazotrophic communities in mining areas have been less studied and understood than those in other environments, such as agricultural soils or legumes. The current study compares the differences in depth-resolved diazotrophic community compositions and interactions in two contrasting sites (from the surface down to 2 m), including one contaminated site (CS) and one uncontaminated site (UCS). A higher nifH abundance was detected in the UCS rather than in the CS. The nifH abundance was significantly positively correlated with the soil depth. The antimony (Sb) and arsenic (As) co-contamination induced an alpha diversity decline, a loosely connected biotic interaction, and soil deep selection on the diazotrophic communities. Multiple lines of evidence, including the enrichment of diazotrophic taxa in the CS, microbe-microbe interactions, environment-microbe interactions, and the machine learning approach (random forests regression), demonstrated that Rhizobium was the keystone taxon in the vertical profile of contaminated soil and was resistant to the Sb and As contaminant fractions. All of these observations suggest that the diazotroph, Rhizobium, may have played an important role in the nitrogen fixation in the examined contaminated site.