Constraints on Enhanced Weathering and related carbon sequestration – a cropland mesocosm approach

Abstract. The weathering of silicates is a major control on atmospheric CO 2 at geologic time scales. It was proposed to enhance this process to actively remove CO 2 from the atmosphere. While there are some studies that propose and theoretically analyze the application of rock powder on agricultural land, results from field experiments are still scarce. In order to evaluate the efficiency and side effects of Enhanced Weathering, a mesocosm experiment was set up and agricultural soil from Belgium was amended with olivine-bearing dunite ground to two different grain sizes, while distinguishing setups with and without crops. Based on measurements of Mg, Si, pH, and DIC, the additional weathering effect of olivine could be confirmed. Calculated weathering rates are up to three orders of magnitude lower than found in other studies. The calculated CO 2 consumption by weathering was comparably low with 2.3 to 4.9 CO 2 t km −2 a −1 . One identified cause was preferential flow leading to a low water-rock interaction time for a significant water volume in the setup, not addressed in previous Enhanced Weathering experiments for CO 2 consumption. Correction for preferential flow leads to fluxes about a magnitude higher, confirming that this process and surface runoff in the field must be included in assessments for the CO 2 consumption potential of Enhanced Weathering in general. Pore water Mg / Si molar ratios suggest that dissolved Si from the added minerals stays in the system over the observation period, because a cation depleted Si layer forms on the reactive mineral surface of freshly ground rocks. This layer has not reached equilibrium thickness within the first two years. The release of potentially harmful trace elements is an acknowledged side effect of Enhanced Weathering. Primarily Ni and Cr are elevated in soil solution, while Ni concentrations exceed the limits of drinking water quality. The use of olivine, rich in Ni and Cr, is not recommend and alternative rock sources are suggested for the application.