Soil Microbiome Restructuring Associated with Potassium Conversion Following Compound Bacillus Biofertilizer Application Under Continuous Cropping of Tomato

Greenhouse tomato continuous cropping obstacles stem primarily from soil microecological imbalance and potassium fixation. While K-solubilizing bacteria are widely studied, how compound Bacillus biofertilizer drives cascading K bioactivation and absolute microbial community shifts under long-term protected tomato cropping remains unclear. Using spike-in absolute quantitative sequencing, this study offers novel insights into the linkage between compound Bacillus biofertilizer-driven microbial restructuring and soil K availability. A field trial was conducted in a 27-year continuous tomato cropping system with three fertilizer doses (0, 300, and 500 L/mu). We measured soil physicochemical properties, microbial community composition and metabolism, and root architecture and fruit yield and explored hypothesized path relationships to associate yield- and fruit-quality traits with soil K availability and root traits. The biofertilizer was associated with increased available K consistent with bioactivation rather than expanding the total K pool. At 500 L/mu, available K and microbial biomass K increased by 1383.96 mg/kg and 127.17 mg/kg, respectively, with unchanged total K, confirming that this pattern is consistent with activation of native potassium reserves. Soil macroaggregate content and stability improved with dose; the 500 L/mu treatment increased estimated bacterial 16S rRNA gene abundance by 21.6%, enriched putative beneficial taxa (Sphingomonas, Paenibacillus) and reduced Fusarium abundance. Potassium-solubilizing taxa were coupled with TCA cycle and organic acid synthesis. Single-fruit weight rose from 140.00 to 272.00 g. Structural equation modeling supported a hypothesized indirect yield-associated cascade: soil improvement → K transformation → root growth → yield. Compound Bacillus biofertilizer is consistent with a multi-level cascading regulation framework, providing a promising green strategy for mitigating continuous cropping obstacles.