In plants, some alkali and alkaline earth metal (AAEM), e.g. K, Ca, and Mg, are associated with organic molecules46. During pyrolysis, with the decomposition of the organic matters and reconstruction of the remaining parts, biomass transformed into char (not the ultimate biochar), and AAEMs were then bonded to the char in the form of C–O–Met+ or C–O–Met2+–O–C (Met+ represents K+, and Met2+ represents Ca2+ and Mg2+)47. Once the bonds between AAEMs and char were attacked by free radicals which were formed in pyrolysis, elemental AAEMs were formed47484950. As these species are not stable, they would be easily oxidized to form metal oxides and react with H2O, which was released from biomass during pyrolysis, to form AAEM hydroxides (see Equation 1, M represents elemental AAEMs, such as K, Ca, or Mg). These AAEM hydroxides could be the source of released OH− in the solution. The decomposition of cellulose and lignin1851 lead to the generation of CO2. CO2 may react with H2O and newly formed AAEM hydroxides to form various carbonates (see Equations 2 and 3). Thus, in addition to the carbonates contained in the raw biomass, the newly formed carbonates during pyrolysis could also be the source of CO32− in the solution. The Si species are much more stable than other inorganic species in biomass1852. It is reported that the main forms of Si in wheat biomass are amorphous silica and silicate esters5354, the forms of these Si species would change when the water in biomass evaporated and the organic matters decomposed during pyrolysis, some of the Si species would transform into quartz (see Fig. 2) and dehydroxylated silicates18, some of them would transformed into alkali silicates (such as K2SiO3)52. Without the protection of organic matters, the exposed Si species can be easily released to the solution. Among the Si species formed on biochar, alkali silicates are soluble compounds, the solutions of which contain a wide variety of polysilicate ions55 (consist with the ATR-FTIR results of raw biochar filtrates). Thus alkali silicates are probably the Si species released into water or Zn solution.