Comparison study on silicon forms and leaching characteristics of pyrolysis biochar and hydrochar derived from cow dung digestate

Silicon in soil is critical for crop growth and tolerance to both biotic and abiotic stresses. Si-rich biochar can be used as soil supplements for silicon improvement. In order to investigate the application potential of silicon in pyrolysis biochar (PC) and hydrochar (HC) derived from cow dung digestate (CD), they were compared at different temperatures to explore silicon forms and leaching characteristics. The results indicated that the form and content of silicon changed obviously with variations in pyrolysis temperature. The form of silicon content followed the order of amorphous silicon >> available silicon > active silicon > dissolved silicon in PC. For HC, the form of silicon content followed the order of amorphous silicon >> dissolved silicon > active silicon > available silicon. Compared to HC, PC had a richer pore-layered structure but fewer surface functional groups. The study of dissolving kinetics of silicon indicated that PC550 released the most silicon, which was 3 times more than HC220. The long-term silicon leaching found that PC550 increased rapidly with reaction time, whose dissolving silicon increased to 33.02 mg/g, which was much higher than the recommended silicon content of soil in China (0.095 mg/g soil). This study offers a novel point of view for anaerobic digestion residue utilization as a fertilizer.