Substantial CO₂ uptake by biomass ashes under natural condition in China

A considerable amount of biomass ashes, resulting from agricultural waste field burning, wildfire, and solid biofuel incineration, is typically discarded in field or stored in dumps, where the alkaline oxides (CaO, MgO) they contain undergo carbonation and weathering-erosion processes over extended periods, continuously absorbing CO₂ from the atmosphere and soil. However, their CO₂ absorption behavior under natural conditions remains insufficiently explored in China. Using life cycle assessment (LCA) and material flow analysis (MFA) methods, this study developed a CO₂ absorption analysis model for biomass ashes under natural conditions. We estimated the CO₂ absorption of 9 different types of biomass ash from 1950 to 2022 through Monte Carlo uncertainty simulation. The results show that biomass ashes in China absorbed approximately 24.17Mt/year (95 % CI, 11.10-43.56) of CO₂ under nature conditions, with the annual average CO₂ uptake showing a steady increase from 1950 to 2022. The total CO₂ uptake reached 856.85Mt (95 % CI, 368.73-1526.01) over these decades, mainly due to the significant contribution of biomass ash produced by domestic straw burning and fuelwood combustion, which accounted for 51.97 % and 22.08 %, respectively. Our findings highlight the substantial carbon sink benefits of biomass ash, providing valuable insights for further studies on carbon cycles in natural ecosystems and the potential integration of biomass ash in Carbon Capture, Utilization, and Storage (CCUS) technologies.