The result of PCA shows that five principal components may be present in the spectra of biochars. After TT by using the χ(k) k3-spectra of a series of standard Zn compounds (Supplementary Fig. S5), we found six Zn species (i.e. Zn(NO3)2 aqueous (Zn(NO3)2 aq), Zn(OH)2, willemite (Zn2SiO4), smithsonite (ZnCO3), hemimorphite (Zn4(H2O)(Si2O7)(OH)2), and hydrozincite (Zn5(CO3)2(OH)6)) may be contained in these biochars. The number of Zn species obtained by TT was higher than that obtained by PCA, this is due to the similarity between the spectra of some standard species414243. The PCA results suggested that the Zn precipitates/minerals could take up a large fraction of Zn species in biochar.