Synergistic Effects of Reduced Phosphorus Fertilization and Phosphate-Solubilizing Bacteria on Available Phosphorus...

Tunnel excavation generates large amounts of oligotrophic waste slag, posing ecological and environmental challenges. Enhancing its fertility is critical for sustainable utilization. This study evaluated a fertilization strategy combining phosphate-solubilizing bacteria (PSB), Bacillus subtilis 2 C (NCBI GenBank accession no. PX700567), with a reduced phosphorus fertilizer rate in oligotrophic tunnel waste slag. Lolium multiflorum Lam. was consecutively cultivated for three years with four treatments: sufficient phosphorus fertilizer (PF), reduced phosphorus plus PSB (PSBF), PSB alone, and a control without amendments. Plant growth and nutrient dynamics were monitored, while bacterial communities and phosphorus-related functional genes (phoD and gcd) were analyzed using high-throughput sequencing and quantitative PCR, respectively. Results showed that both PF and PSBF treatments significantly improved plant biomass compared with the control, and fresh weight under PSBF exceeded PF by the third year. Available phosphorus in slag increased with consecutive plantings, reaching the highest level under PSBF, despite overall declines in total nitrogen and phosphorus. Bacterial community analysis revealed that available phosphorus and total phosphorus were the main drivers of community composition. The abundance of phoD was strongly and positively correlated with AP (R² = 0.92, P < 0.01), while gcd abundance correlated with total nitrogen. Taxa such as Bacteroidota, Cyanobacteria, and the genus Devosia were closely associated with phosphorus transformation. These findings demonstrate that halving phosphorus fertilizer input combined with PSB inoculation sustains plant growth and enhances phosphorus availability through bacterially mediated processes. This strategy offers a practical approach for improving the ecological function of tunnel waste slag while reducing fertilizer dependence.