Beyond Inorganic C: Soil Organic C as a Key Pathway for Carbon Sequestration in Enhanced Weathering

Enhanced weathering, a promising CO₂ removal technique, captures CO₂ via two inorganic pathways: pedogenic carbonate formation and leaching of dissolved weathering products. Here, we look beyond those two pathways, identifying other CO₂ sinks and sources relevant for enhanced weathering. Although processes such as clay formation or organic matter decomposition could reduce the efficiency of enhanced weathering, organic matter stabilization could contribute to C storage. In a 15-month mesocosm experiment including two different types of silicates (50 t/ha basalt and 5 t/ha steel slag), the realized inorganic CO₂ removal remained negligible (below 0.12 t CO₂/ha). The majority of released base cations was sorbed to the exchangeable complex or bound in secondary minerals such as (hydr)oxides and/or aluminosilicate clays, thus, not requiring the dissolution of CO₂ for charge balance. Only a negligible minority of base cations was found in pedogenic carbonates or leachates. In comparison to the relatively low inorganic C fluxes, organic C fluxes were several orders of magnitude larger. Increases in soil CO₂ efflux due to SOM decomposition were approximately 25 times higher than the realized inorganic CO₂ removal of enhanced weathering (basalt +0.9 and slag +1.1 t CO₂/ha released over 15 months). Yet, plant C inputs likely increased in silicate-amended treatments, offsetting organic C losses. Although soil organic C stocks remained unaffected by silicate amendment, the distribution of C shifted towards more stable pools. Soil organic C was stabilized via the formation of aggregates and mineral association. Given the increased organic C inputs and the transfer of organic C to more stable soil sinks, long-term studies will be essential to quantify changes in soil organic C stocks and therefore in CO₂ removal.