Direct evidence of CO2 drawdown through enhanced weathering in soils

The ability of engineered enhanced weathering to impact atmospheric CO₂ has been challenging to demonstrate due to the many processes occurring in soils and the short time span of current projects. Here we report the carbon balance in an Icelandic Histic/Gleyic Andosol that has received large quantities of basaltic dust over 3300 years, providing opportunity to quantify the rates and long term consequences of enhanced weathering. The added basaltic dust has dissolved continuously since its deposition. The alkalinity of the soil waters is more than 10 times higher than in equivalent basalt dust-free soils. After accounting for oxidation and degassing when the soil waters are exposed to the atmosphere, the annual CO₂ drawdown due to alkalinity generation is 0.17 t C ha⁻¹ yr⁻¹. This study validates the ability of fine grained mafic mineral addition to soils to attenuate increasing atmospheric CO₂ by alkalinity export. Induced changes in soil organic carbon storage, however, likely dominate the net CO₂ drawdown of enhanced weathering efforts.