Minerals: A missing role for enhanced biochar carbon sequestration from the thermal conversion of biomass to the application in soil

Turning biomass into biochar via pyrolysis followed by its amendment into soil is regarded as a prospective strategy of sequestrating atmospheric carbon and simultaneously aiding both climate change mitigation and adaptation. Biochar's capability of sequestering carbon depends on the solid carbon retention in biochar after pyrolysis and the stability of this carbon. On average, 50% of the carbon in biomass is retained in biochar, of which 5–10% is estimated to be released in the first 100 years after application to soil. This review firstly focuses on roles of minerals in regulating carbon retention and stability, taking both processes of biochar formation and soil carbon sink into consideration. Carbon retention in biochar can be dramatically increased by adding exogenous minerals prior to pyrolysis, especially P, Si, Ca, Mg, and K containing materials, due to the chemical bonding (P-O-C, C-P, C-Si-C, etc.), the physical encapsulation, and the absorption of CO2 and CH4 by CaO or MgO. Soil minerals also interact with biochar‑carbon, suppressing physical disintegration, abiotic oxidation, and biotic mineralization, with soil clays, Ca2+ bridging, and poorly crystalline Fe/Al being the dominating factors. Future research should concentrate on systematic evaluation of the roles of minerals in the whole biochar carbon cycle process, i.e., pyrolysis conversion and biochar applications, from the perspective of biomass-energy-climate.