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.