Edaphic-microbial coupling and rhizosphere network modularity characterize geographic variation in saikosaponin accumulation in cultivated Bupleurum

Geographic divergence of saikosaponins remains a major challenge for standardized cultivation of Bupleurum . To identify the ecological factors associated with this variation, we integrated HPLC-based saponin quantification, soil biochemical assays, and 16S rRNA sequencing across 15 Chinese sampling sites. Total saikosaponins exhibited approximately 6.4-fold spatial variation, with substantial heterogeneity in both SSa and SSd. Soil properties accounted for the largest unique explanatory fraction (45.16%), while the highest-saikosaponin site combined moderate nutrient availability with relatively low urease and sucrase activities, indicating a distinct resource-constrained rhizosphere state. High-saikosaponin habitats also showed low bacterial alpha diversity but pronounced microbial network organization (RM = 4.87). Random Forest analysis identified Sphingomonas , Rhizobium , and Gemmatimonas among the major discriminatory genera, and soil–microbiome variables shared 11.66% of the explained variation. These results reveal that geographic variation in saikosaponin accumulation is more strongly associated with edaphic conditions and rhizosphere community organization than with bacterial richness alone, providing a mechanistically informed ecological basis for microbiome-guided precision cultivation of Bupleurum .