Key limitations in enhanced weathering to remove carbon dioxide across agricultural soils

Enhanced rock weathering (ERW) – the introduction of finely crushed alkaline minerals into agricultural soils – could in principle remove billions of tonnes of carbon dioxide annually at the global scale. However, questions remain over processes leading to the formation and persistence of soil inorganic carbon via ERW, especially amidst the complexity of conditions across Earth’s cropland soil. Here we present a new model, a COUpled Soil Inorganic-organic carbon model for eNhanced wEathering (COUSINE), which mechanistically simulates carbon dioxide removal (CDR) via ERW across diverse climate and soil conditions. COUSINE considers the dynamics of 20 chemical species in the soil system that are driven by soil CO2 dynamics, parent material, soil cation exchange, secondary mineral formation, strong and weak acid weathering, plant and microbial activity, and leaching of elements from the soil system. Principles of mass and charge conservation are maintained across all reactions. We applied the model to various climate and soil conditions – from fertile temperate Alfisols to highly and extremely weathering subtropical Ultisols and tropical Oxisols – to examine the key controls over weathering rates and CDR rates. Our simulations reveal three key limitations in regulating the timing and potential of carbon sequestration under ERW. First, organic acids and clay colloids in fertile soils retain cations in environments with low base saturation and relatively high CEC, creating strong cation sinks, thus delaying increased pore water alkalinity in response to alkaline mineral additions. This lag can be substantial, lasting for over 80 years in Alfisols with high CEC capacity to less than 20 years in Oxisols, which lack cation exchanging organic matter and minerals. Second, competition between carbonic acid and other sources of protons can limit the efficacy of CDR. This is apparent in net nitrogen acidity from nitrogen fertilizer applications, which results in strong acid weathering. Third, climate conditions related to excess moisture and soil temperature control reaction kinetics, which affects the rate at which cations are released into solution and can thus participate in bicarbonate formation. COUSINE informs matrix simulations across soil properties, climates, and application rates, thereby elucidating optimal conditions for maximizing soil carbon sequestration via ERW, providing a new tool for CDR verification