Biochar Application: A Viable and Pyrolysis Temperature Dependent Option for Enhancing Leaf Secondary Metabolites of Cyclocarya paliurus

While numerous studies have suggested that pyrolysis of the forestry residues to produce biochar and return it back for forestry use is an effective way for the utilization of forestry wastes, there have been few reports addressing the effect of pyrolysis temperature on its agronomic functions, especially the secondary metabolism of the medicinal tree. In this study, two kinds of biochar were pyrolyzed using poplar sawdust at 350 °C (C350) and 600 °C (C600), respectively, then applied as auxiliary substrate material for the cultivation of Cyclocarya paliurus seedlings in a greenhouse to study their effects on soil properties, seedling growth and the accumulation of main secondary metabolites in C. paliurus leaves. The results showed that biochar application definitely ameliorated most of the soil properties, and this improvement was further enhanced as the pyrolysis temperature of the biochar increased. Compared with the basic substrate (CK), the growth of C. paliurus seedlings increased under C600 and decreased under C350, but the application of two biochars had no significant effect on the content of the main secondary metabolites in the leaves. In contrast, the seedling growth with the direct application of poplar sawdust (C0) was significantly inhibited, while the content of secondary metabolites in the leaves increased significantly. In terms of the yield of leaf secondary metabolites, the application of biochar with high pyrolysis temperature (C600) had the highest yield, while the direct application of poplar sawdust had the lowest yield. In addition, the contents of several metabolite monomers were higher under C0 than these of the other three treatments. The results indicated a certain trade-off between the primary growth and the synthesis of leaf secondary metabolites in C. paliurus affected by different biochars and poplar sawdust application, while the accumulation of leaf secondary metabolites was generally dependent on leaf biomass.