Co-occurrence network of microbial communities affected by application of anaerobic fermentation residues during phytoremediation of ionic rare earth tailings area

The long-term exploitation of ionic rare earth elements (REEs) in southern China has produced a large-scale of abandoned tailings area. While the application of anaerobic fermentation residues to cultivate economically valuable remediation plants (e.g. energy plant) has become a hotspot due to their merits in low-cost and sustainability in recent years, the succession and co-occurrence patterns of these microbial communities remain unclear. In this study, soil samples were collected from the sustainable restoration area, natural restoration area and tailings area. The composition and diversity of bacterial and fungal communities on five soil samples were evaluated using high-throughput sequencing technology. The results shown that the phytoremediation with anaerobic fermentation residues could significantly improve the physicochemical properties (especially for soil nutrients) and microbial diversity of soil within 3 years, while these parameters in natural restoration area were lower. The nonmetric multidimensional scaling (NMDS) ordinations revealed the shifts of microbial communities depending on soil physicochemical properties and plant species, and soil nutrients were the main factors affecting the microbial variation explained by the variation partition analysis (VPA). The soil nutrient accumulation obviously changed the proportion of oligotrophic and copiotrophic groups, among which the copiotrophic groups were significantly increased, such as Proteobacteria, Bacteroidetes, Gemmatimonadetes and Glomeromycota. The microbial co-occurrence network analysis indicated that application of anaerobic fermentation residues could significantly improve the topological properties and the stability of microbial network. The copiotrophic groups (e.g. Proteobacteria, Ascomycota) became the key to assemble stable network structure. Moreover, herbaceous plants could increase the proportion of fungi (e.g. Ascomycota) in microbial network, which improved the topological properties with bacteria synergistically. Therefore, the soil environment of REEs tailings area was effec-tively optimized by anaerobic fermentation residues and herbaceous plants, which furthered understanding of co-occurrence pattern and mutualistic relationships of microbial communities during sustainable restoration.