Assessing CO2 fluxes during enhanced weathering from soils through a mesocosm lens

It is becoming increasingly accepted that annual gigatonne-scale CO2 removal, in conjunction with rapid decarbonization, is necessary to meet international climate goals and limit global warming below 2°C. This is going to require the development and rapid scaling of new forms of carbon management. When developing new CDR techniques, it is essential to ensure that there is complete accounting of how the process affects greenhouse gas fluxes. Enhanced weathering (EW), the spreading of finely ground alkaline minerals to soils, has the potential to sequester significant amounts of CO2 while improving soil health. However, its effects on soil organic carbon (SOC) decomposition and CO2 efflux from soils remain debated. It has been proposed that increasing soil pH can lead enhanced SOC remineralization. To move forward this debate, we present CO2 flux and soil carbon pool data from a greenhouse study in large mesocosms. We focused on mildly acidic soil in which the majority of cations from weathering would move into the exchangeable fraction in soils. Therefore, gas fluxes changes should be largely linked to changes in SOC stores. We find no significant correlation between CO2 fluxes and soil pH and no significant correlation between CO2 fluxes and basalt application. Although this does not rule out a link between soil pH and SOC remineralization rates, the effect is small relative to other factors, like temperature and soil moisture. Although minor increases in total inorganic carbon were observed in basalt-amended soils, these increases did not support a direct link between soil pH and increased CO2 emissions. We observed a small increase in soil total organic carbon stocks, but this change was also not significant enough to drive a shift in observed soil CO2 fluxes.