Co-application of biochar and organic fertiliser enhances the yield and quality of understory Fritillaria thunbergii Miq. by optimising soil and rhizosphere microecological structure

Fritillaria thunbergii Miq. is an economically important medicinal plant. Understory cultivation helps maintain its medicinal quality; however, poor soil fertility under forest stands severely limits its yield. Biochar and organic fertiliser, as sustainable soil amendment strategies, are expected to improve soil and product quality. However, the combined effects of these strategies on understory cultivation of F. thunbergii have not been determined. In this study, a field experiment was conducted under forest conditions to evaluate the effects of no fertilisation (CK), biochar (BF), organic fertiliser (OF), and both biochar and organic fertiliser (BOF) on F. thunbergii growth, soil properties, rhizosphere microbial communities and soil metabolites. All fertilisation treatments increased the yield and total alkaloid content of F. thunbergii significantly, with BOF producing the best results. BOF optimised soil conditions by increasing available nutrients and soil organic matter. It also improved the rhizosphere microbial community, increasing beneficial bacteria (e.g. Terrabacter) , decreasing pathogenic fungi, and enhancing microbial diversity and network complexity. Furthermore, BOF increased the abundance of genes involved in C, N, P, and S cycling and enriched key metabolic pathways (e.g., glycerophospholipid metabolism and amino acid metabolism). PLS-SEM indicated that BOF-induced improvements in soil quality and microbial community were the core drivers of increased plant yield and quality. In conclusion, the combined application of biochar and organic fertiliser effectively alleviates understory soil limitations on F. thunbergii production in this region, representing a sustainable cultivation strategy for improving yield and quality as well as soil health. • Biochar-organic fertiliser (BOF) improve understory F. thunbergii yield/ actives. • BOF elevated AP/AN/AK/organic matter, optimizing understory soil fertility. • BOF promoted microbial richness and C/N/P/S cycling genes upregulated in soil. • BOF reshaped understory soil metabolic and soil-microbe-metabolite network.