Enhanced Weathering and related element fluxes – a cropland mesocosm approach

<p>The weathering of silicates is a major control on atmospheric <span class="inline-formula">CO₂</span> at geologic timescales. It was proposed to enhance this process to actively remove <span class="inline-formula">CO₂</span> from the atmosphere. While there are some studies that propose and theoretically analyze the application of rock powder to agricultural land, results from field experiments are still scarce.</p> <p>In order to evaluate the efficiency and side effects of Enhanced Weathering (EW), 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.</p> <p>Based on measurements of Mg, Si, pH, and DIC, the additional weathering effect of olivine could be confirmed. Calculated weathering rates are up to 3 orders of magnitude lower than found in other studies. The calculated <span class="inline-formula">CO₂</span> consumption by weathering based on the outlet water of the mesocosm systems was low with 2.3–4.9&thinsp;<span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow class="unit"><mi mathvariant="normal">t</mi><mspace width="0.125em" linebreak="nobreak"/><msub><mi mathvariant="normal">CO</mi><mn mathvariant="normal">2</mn></msub><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">km</mi><mrow><mo>-</mo><mn mathvariant="normal">2</mn></mrow></msup><mspace linebreak="nobreak" width="0.125em"/><msup><mi mathvariant="normal">a</mi><mrow><mo>-</mo><mn mathvariant="normal">1</mn></mrow></msup></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="72pt" height="16pt" class="svg-formula" dspmath="mathimg" md5hash="ef5dfbca643d1c7c552e0008bc662989"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-17-103-2020-ie00001.svg" width="72pt" height="16pt" src="bg-17-103-2020-ie00001.png"/></svg:svg></span></span> if compared with previous theoretical estimates. Suspected causes were the removal or dilution of Mg as a weathering product by processes like adsorption, mineralization, plant uptake, evapotranspiration, and/or preferential flow, not specifically addressed in previous EW experiments for <span class="inline-formula">CO₂</span> consumption. The observation that Mg concentrations in the upper soil layers were about 1 order of magnitude higher than in the outlet water indicates that a careful tracking of weathering indicators like Mg in the field is essential for a precise estimate of the <span class="inline-formula">CO₂</span> consumption potential of EW, specifically under global deployment scenarios with a high diversity of ecosystem settings. Porewater <span class="inline-formula">Mg∕Si</span> molar ratios suggest that dissolved Si is reprecipitating, forming a cation-depleted Si layer on the reactive mineral surface of freshly ground rocks.</p> <p>The release of potentially harmful trace elements is an acknowledged side effect of EW. Primarily Ni and Cr are elevated in the soil solution, while Ni concentrations exceed the limits of drinking water quality. The use of olivine, rich in Ni and Cr, is not recommended, and alternative rock sources are suggested for the application.</p>