Biochar increases Panax notoginseng's survival under continuous cropping by improving soil properties and microbial diversity

Replant problem is widespread in agricultural production and causes serious economic losses, which has limited sustainable cultivation of Panax notoginseng (PN), a well-known medicinal plant in Asia. Here we conducted a field experiment to investigate the effectiveness and possible mechanisms of biochar to improve its survival under continuous cropping. Biochar from tobacco stems was applied at 4 rates of 9.0, 12, 15, and 18 t/ha to a soil where PN has been continuously cultivated for 10 years. After 18 months, soil properties, 5 allelochemicals, including p-hydroxybenzoic acid, vanillic acid, syringic acid, p-coumaric acid, and ferulic acid, key pathogen Fusarium oxysporum, microbial community, and PN survival rate were investigated. Our results show that 10 years' continuous PN cropping led to soil acidification, accumulation of NH4+-N and F. oxysporum, and low PN survival rate. However, biochar increased its survival rate from 6.0% in the control to 69.5% under 15 t/ha treatment. Moreover, soil pH, available P and K, organic matter content, and microbial diversity were increased while NH4+-N and allelochemicals vanillic acid and syringic acid contents were decreased under biochar treatment (P<0.05). Soil available K increased from 177 to 283 mg·kg-1 while NH4+-N decreased from 6.73 to 4.79 mg·kg-1 under 15 t/ha treatment. Further, soil pH, available P and K, and microbial diversity (bacteria and fungi) were positively correlated with PN survival rate, however, NH4+-N content was negatively correlated (P<0.05). Our study indicates that biochar effectively increased the survival rate of Panax notoginseng under continuous cropping by improving soil properties and microbial diversity.