Mechanism allowing biochar to aid in arbuscular mycorrhizal colonization in Panax quinquefolius L. roots and improve secondary metabolite production.

Panax quinquefolius L, a medicinal plant of the family Araliaceae, has been used in China for more than 300 years. The quality of its medicinal materials is a significant concern. Our previous studies have shown that arbuscular mycorrhizal fungi (AMF) promote the growth of P. quinquefolius and facilitate the accumulation of the active ingredient ginsenosides. However, these beneficial effects are limited by the low AMF colonization rate in production settings, requiring interventions to improve the colonization rate. Biochar is considered an effective soil amendment. Our preliminary experiments indicate that biochar can enhance the inter-root microecology of P. quinquefolius, as well as increase the AMF colonization rate, but the mechanism was not clear. Therefore, we propose using biochar to increase the AMF colonization rate. In this study, we explore the use of biochar to promote the AMF infestation rate of P. quinquefolius and its potential mechanisms. The mechanism was explored by setting up eight treatments. The colonization rate and intensity of AMF in P. quinquefolius roots were assessed using a Trypan Blue solution. Rhizosphere soil microorganisms were analyzed by 16S and ITS sequencing, and secondary metabolites were identified via non-targeted metabolomics. The results showed that the AMF and 2% biochar combined (AMF + BC2) treatment significantly increased both the colonization rate and colonization intensity of AMF, which were 53.58% and 195.95% higher than that of AMF, respectively. The colonization and rhizosphere AMF data indicate that the application of biochar promotes AMF colonization from outside to inside the root. In addition, biochar attracted potentially beneficial microorganisms such as Sphingobium, Sphingomonas, and Novosphingobium, which are positively correlated with AMF and promote AMF colonization. These microorganisms are closely linked with active secondary metabolites, such as Sphingobium, which is positively correlated with L-malic acid. In conclusion, biochar can improve the quality of P. quinquefolius by promoting the formation of mycorrhizae. This finding provides a theoretical basis for the observed effect of the co-application of biochar and AMF on the growth and active ingredient accumulation of P. quinquefolius.