Biological calcium carbonate with a unique organic-inorganic composite structure to enhance biochar stability.

Biochar stability is a key factor affecting the efficiency of soil carbon sequestration. Mineral calcium carbonate (M-CaCO) can enhance the stability of biochar, and the mechanism has been extensively studied; however, similar studies on biological calcium carbonate (Bio-CaCO), another natural form of calcium carbonate, are lacking. In this work, Bio-CaCO was used as an additive to explore the mechanism by which it enhances the stability of biochar. The results showed that Bio-CaCO improved the stability of biochar at pyrolysis temperatures ranging from 250 to 700 °C, and the enhancement effects increased upon increasing the pyrolysis temperature. The enhancement effects of M-CaCO were better at lower temperatures (250 and 400 °C) while Bio-CaCO was better at higher temperatures (550 and 700 °C). Mechanistic studies showed that the enhanced stability of Bio-CaCO at 250 °C was due to the fact that the inorganic component in Bio-CaCO promoted the deoxidation of the carbon matrix and the aromatization of aliphatic carbon at 400 °C. The reasons for the increased stability using Bio-CaCO at high temperatures included two mechanisms. One is that the inorganic components in Bio-CaCO promoted the aromatization of the carbon matrix. The other is that the unique organic nitrogen-containing functional groups in Bio-CaCO underwent dimerization and cyclization with the organic carbon components in biomass to form a more stable pyridinic-N structure. This work provides novel ideas for enhancing biochar stability using Bio-CaCO.