Beyond inorganic carbon: Soil organic carbon as key pathway for carbon sequestration in Enhanced Weathering

Abstract Enhanced weathering captures CO2 via two pathways: Carbonate formation and leaching of weathering products. Here, we look beyond those two pathways, identifying other CO2 sinks and sources. While processes such as clay formation or organic matter decomposition reduce the efficiency of enhanced weathering, organic matter stabilisation could contribute to C storage. In a 15 month mesocosm experiment including two different types of silicates (basalt and steel slag) inorganic CO2 sequestration indeed remained negligible (below 0.1 t CO2/ha) due to clay formation. Also organic matter decomposition increased in silicate amended treatments (basalt +0.9 and slag +1.1 t CO2/ha released), further lowering the CO2 removal efficiency of enhanced weathering. Other organic C pathways could however contribute substantially to C storage. Aggregate formation and the storage of C within them was promoted in silicate amended treatments (basalt +106 and slag +73 % organic C stored in aggregates >250μm). Next to that, the association of organic C to minerals was determined another possible organic C sink. These results underline the urge for reliable ways to quantify CO2 removal not only including inorganic but also organic carbon dynamics.