Influence of pyrolysis temperature and holding time on properties of biochar derived from medicinal herb (radix isatidis) residue and its effect on soil CO2 emission

• Boiled radix isatidis residue was used for biochar production.
• Pyrolysis temperature and holding time affect biochar properties.
• Biochar properties affect soil CO2 emission.
• Increasing temperature may restrain soil CO2 emission.
• Prolonging holding time may restrain soil CO2 emission.

Radix isatidis is a wide-spread medicinal herb in China. The boiled radix isatidis residue contains considerable quantities of cellulose, hemicellulose, and lignin that could be used as feedstock for biochar production. This study aims to investigate the influence of pyrolysis temperature and holding time on biochar properties as well as the effect of biochar on soil CO2 emission in southern China. The biochar was prepared using pyrolysis over a temperature range of 300–700 °C and a holding time span of 10–180 min. The incubation experiment of soil incorporated with biochar lasted for 46 days. Increasing temperature and prolonging holding time were found to reduce the volatile matter content, strengthen the carbon enrichment, and grow aromatic structure, producing alkaline biochar that has weak surface polarity, high specific surface area and extensively porous structures. The effect of biochar on soil CO2 emission in southern China lays in the combined actions of biochar properties, especially of volatile matter, porous structure, and pH. Increasing temperature and extending holding time may convert the positive effect into a negative effect and reduce soil CO2 emission.