Soil acidification severely constrains soil fertility and crop quality in tobacco-growing regions of southern China. Although biochar and beneficial microorganisms are widely used as soil amendments, their synergistic effects and underlying mechanisms remain insufficiently understood. In this field study, five treatments were established in acidified red soil: a control without biochar or microbial fertilizer (CK), rice husk biochar alone at 1500 kg·ha−1 (BF0), and biochar combined with Bacillus