Accelerated microbiome reconstruction and soil-like formation in graphite tailings by composite microbial agent CMA

Given the limited global availability of arable land, the conversion of solid waste into functional soil resources is of great importance. In this study, a multifunctional microbial inoculant (CMA) with phosphorus-solubilizing, potassium-mobilizing, and nitrogen-fixing capabilities was developed by combining Bacillus velezensis RL-1, Variovorax boronicumulans JK-2, and Xanthobacter autotrophicus GN-3. The results demonstrated that the application of the composite microbial inoculant significantly improved the physicochemical properties of graphite tailings. The contents of available phosphorus, ammonium nitrogen, available potassium, and organic matter increased by 9.1-fold, 3.5-fold, 3.0-fold, and 1.97-fold, respectively. In addition, the growth of ryegrass was significantly promoted, with /chlorophyll a and chlorophyll b contents increasing to 4.8 mg/ g DW and 2.7 mg /g DW, respectively. Scanning electron microscopy (SEM) analysis revealed that the composite microbial inoculant enhanced the aggregation of tailings particles, thereby improving the structural stability of the tailings. Furthermore, after inoculant application, the relative abundances of the phyla Proteobacteria and Acidobacteriota in the tailings increased significantly. A more stable microbial community structure was established, with phosphorus-solubilizing, nitrogen-fixing, and potassium-mobilizing bacteria as the dominant functional groups. Integrated analyses of UPGMA clustering, principal coordinate analysis (PCoA), and functional pathway prediction indicated that the composite microbial inoculant could directionally reshape the microbial community structure. Overall, CMA effectively improved the physicochemical properties and microbial community structure of graphite tailings, promoted the pedogenesis process of tailings toward soil formation, and provided novel functional microbial resources and experimental support for the ecological restoration of graphite tailings.