Phytoremediation of Tungsten Tailings under Conditions of Adding Clean Soil: Microbiological Research by Metagenomic Analysis

Vegetation coverage of metal tailings is an important method for environmental governance. Colonization of plants on some nutrient-poor tailings is difficult. Therefore, the addition of clean soil (CSA) is needed to support plant growth. However, the promotion of plant growth by CSA has been widely reported, and there is a lack of reports on the effects of CSA on soil microbial communities and nutrient cycling-related genes. In this study, using ryegrass as the selected plant, the phytoremediation of tungsten tailings was conducted under conditions of CSA. The research focused on investigating the variation in the microbial community’s structure and elucidating variations in the metabolic pathways and relative abundance of nutrient cycling genes. The results suggest that CSA and planting ryegrass increased the microbial richness in tailings. CSA had a negative impact on the microbial community’s evenness (Shannon index) and richness (Simpson index). In all treatments, the relative abundance of Pseudomonadota ranged from 64.4% to 75.2% and dominated the microbial community. High levels of CSA (T3) reduced the relative abundance of Pseudomonadota by 10–13%, and a higher relative abundance of Ascomycota was observed after planting ryegrass. At the genus level, the growth of ryegrass benefitted from a decrease in the abundance of Pseudomonas, Phenobacterum, and Sphingobium after CSA. Cultivation of ryegrass increased the relative abundance of the nitrogen-fixing bacterium Bradyrhizobium (0.9%), which is beneficial for the sustainability of soil remediation in tailings. Metabolism was the primary activity process of microorganisms in tailing soil, with a relative abundance of 71.3% to 72.7%. Generally, the changes in the microbial community’s composition indicated that CSA and cultivation of ryegrass were beneficial for tailings. Still, the negative effects of CSA on microbial evenness (Shannon index) and richness (Simpson index) need attention.