Soil properties and heavy metal concentrations affect the composition and diversity of the diazotrophs communities associated with different land use types in a mining area

Biological nitrogen fixed by diazotrophs is important because it is the largest natural source of nitrogen to support primary productivity and maintain ecosystem sustainability when anthropogenic nitrogen inputs are lacking, especially in mining areas. The present study selected six different land use types (a corn field, a vegetable field, an area of shrubs, a mine tailing, a forest and a citrus orchard) as variables related to the mining activities around the Siding mining area, Liuzhou, Guangxi Province, China. A 0–10 cm soil layer was sampled, and the heavy metal concentrations and soil physicochemical properties were measured. Illumina MiSeq sequencing technology was used to analyze the abundance and structural diversity of the diazotroph community. The distribution of diazotroph assemblages among the different land use types and the main soil environmental properties that drive diazotroph community compositions were determined. The results indicated that the phylum Cyanobacteria (85.89%) was the most dominant taxon in the diazotrophic community in the citrus orchard. However, in the soils of the five other land use types, Proteobacteria was the most dominant taxon in the diazotrophic community, with a high abundance that ranged from 60.08% to 99.96%. Spearman's correlation analysis and redundancy analysis results indicated that the soil organic matter (OM) content in the citrus orchard was significantly higher than that in other areas, and the difference in soil OM content might be the main reason for the difference in the diazotrophic communities. The diazotrophic communities and their associated diversity under different land use types were driven by soil heavy metal and soil N (including NH₄⁺-N, NO₃⁻-N and total N (TN)) contents. Specifically, TN was highly correlated with the NMDS1 score of the soil diazotrophic community, which indicated that the soil TN content was the main reason for the high similarity among the diazotrophic communities in the corn field, vegetable field and shrub soils. Overall, our study offers a detailed description of the characteristics of the diazotroph community structure under different land use types around a mining area.