Synergistic effects of biochar and Bacillus subtilis on soil fertility and tobacco leaf quality in acidified soil: microbial mechanisms and multi-model evaluation

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 subtilis fertilizer at 15, 30, and 60 kg·ha −1 , denoted as BF1, BF2, and BF3, respectively. The effects of these treatments on soil physicochemical properties, microbial communities, and tobacco leaf quality were systematically evaluated. High-throughput 16S rRNA sequencing revealed that biochar–microbe co-application significantly reshaped soil bacterial community structure and enhanced microbial diversity. Functional prediction indicated that moderate microbial input strengthened nitrogen cycling potential. These microbial responses were accompanied by improved soil physicochemical properties and enhanced activities of key enzymes involved in carbon and nitrogen transformation. A comprehensive evaluation based on principal component analysis showed that both BF1 and BF2 achieved the highest soil fertility levels, with no significant difference between them. Tobacco leaf quality was assessed using entropy-weighted grey relational analysis (GRA) and supported by TOPSIS, which consistently indicated that BF2 and BF3 performed best, also without significant difference. Considering both soil fertility and tobacco leaf quality, BF2 was identified as the optimal treatment, achieving a balanced improvement in soil conditions and leaf chemical composition. Partial least squares path modeling (PLS-PM) further revealed that biochar–microbe co-application enhanced soil fertility primarily through enzyme-mediated pathways, with enzyme activity exerting strong direct effects on soil fertility and indirect effects via microbial functional shifts. Improved soil fertility subsequently promoted tobacco growth, optimized leaf chemical composition, and ultimately enhanced overall leaf quality. Overall, these findings demonstrate that combined application of biochar and B. subtilis improves soil fertility and tobacco leaf quality through coupled physicochemical–microbial mechanisms, with moderate microbial input representing a balanced and efficient management strategy for acidified soils.