Biochar, as a new type of soil amendment, has attracted the interest of many researchers in promoting soil organic carbon (SOC) accumulation. However, its dose-dependent microbial regulation mechanisms in highly acidic soils (pH=3.92) remain poorly quantified. This eight-year field study systematically investigated how biochar application rates (0, 2.5, 5, 10, 20, 40 t ha -1) modulate SOC dynamics through microbial restructuring in a subtropical tea plantation ecosystem. Soil samples were collected from plots and the soil chemical properties, content of SOC, DOC, microbial biomass carbon (MBC) and soil bacterial community were analyzed. The results indicated that compared with CK, with the increase of biochar amounts, soil pH, total nitrogen (TN), total phosphorus (TP) and C/N increased by 0.1-0.4, -10.7-39.9 g kg -1, -21.1-17.8 g kg -1 and 4.1% -35.1% respectively, soil electrical conductivity (EC) decreased by 17.7-27.9. SOC and MBC increased by 0.2% -78.9% and 46.8% -115.9%, respectively, while DOC decreased by 0.9% -8.2%. Microbial carbon efficiency: MBC/SOC ratio peaked at BC10 (+108.9%, p<0.05), leaching mitigation: DOC/SOC ratio decreased by 42.75% (BC40, p<0.05). Biochar promoted a significant increase in the Shannon and Chao indices of microorganisms in terms of microbial community. Diversity enhancement: shannon index: +0.32 (BC20, p<0.05), chao richness: +39.91% (BC20, p<0.05). Overall, the application of biochar increased the relative abundance of Subgroup_2, Xanthomonaceae, JG30-KF-AS9, KF-JG30-C25, Acidimicrobiia, Bryobacter, and Bradyrhizobiu genera, while decreased the relative abundance of Acidothermus, Elsterales, Bacillus, Gaiellales, WPS-2, Mycobacterium, Acidibacter and Conexibacter genera. Structural equation modeling revealed: direct biochar effects (β=3.0) > indirect mediation (β=1.6), positive feedback dominance (3.32:1 ratio). Biochar establishes a self-reinforcing carbon sequestration loop in acidic soils through: pH-mediated suppression of acidophilic decomposers (-Δ21.43% Acidothermus), selective enrichment of oligotrophic strategists (+Δ17.6% Bryobacter), microbial carbon pump efficiency (MBC/SOC) increased 2.0-fold.