Optimized phosphorus management enhances steroidal alkaloid biosynthesis in Fritillaria cirrhosa via rhizosphere microbiome modulation

Fritillaria cirrhosa is an endangered medicinal plant valued for its steroid alkaloids. Phosphorus (P) has been shown to enhance alkaloid biosynthesis in plants, making P management essential for conserving these valuable resources. However, optimal P rate and its regulatory mechanisms for F. cirrhosa alkaloid accumulation remain unclear. This study conducted a field experiment with six P application levels (P0–P5) to investigate their effects on alkaloid accumulation. The results showed that the P4 treatment (36 g/m²) enhanced the levels of peiminine and sipeimine by 25.5% and 24.5%, respectively, in comparison to the P0. This optimal P rate also significantly enhanced the activity of soil enzymes, including alkaline phosphatase, β-glucosidase, and peroxidase (p < 0.05). Furthermore, P addition prompted a structural reorganization of the microbial community. Co-occurrence network analysis revealed that the P4 treatment increased the complexity and connectivity of both bacterial and fungal communities. Crucially, the relative abundance of putative pathogenic fungi was minimized under P4 (23.4%), while beneficial bacterial taxa (e.g., Pseudomonadaceae, Enterobacteriaceae) and fungal taxa (e.g., Diversispora, Quaeritorhiza haematococci) were enriched. These beneficial microbes showed positive correlations with alkaloid levels (r > 0.6, p < 0.05). Structural equation modeling further validated that appropriate P application modulates the rhizosphere microbiome and potentiates soil enzyme activities, ultimately culminating in improved quality of F. cirrhosa. This study offers theoretical insights and management strategies aimed at improving the productivity and quality of F. cirrhosa, as well as other endangered alkaloid-producing medicinal plants.