Assessing mineral dissolution and secondary mineral formation during enhanced weathering on agricultural sites

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Enhanced rock weathering (ERW) has the potential of being a comparatively cheap carbon dioxide removal method, as most infrastructure already exists. However, questions still remain on both dissolution rates of the feedstock added to agricultural fields, as well as processes that reduce the efficiency of the CO2 drawdown reaction.Such significant process are the soil’s cation exchange capacity (CEC) and secondary mineral formation. These take up elements from the dissolving feedstock, and retain them on timescales varying from several years (CEC) to thousands of years (secondary minerals).In this study we have sampled 10 fields in Germany 27 months years after they were amended with Eifelgold basalt by the Carbon Drawdown Initiative. These samples, combined with pre-amendment and control samples, allow us to examine feedstock dissolution and secondary mineral formation on fields that subsequently underwent standard agricultural use. Amendment amounts vary between 12 and 48 t/ha, and the fields also have a range in cation exchange capacity and soil pH.We use a combination of SOMBA (soil mass balance approach – i.e. cation/Ti ratios determined by isotope dilution (Suhrhoff et al., 2025)), sequential leaching of the soils to separate different secondary phases, and lithium isotope ratios. The latter are fractionated by secondary mineral formation, and provide a highly sensitive method to estimate secondary mineral neoformation (Pogge von Strandmann et al., 2025). Based on these methods, between 25 and 45% of the feedstock dissolved during the 27 months of reaction. Of this dissolved material, on average 40% was taken into the CEC, 15% into carbonates, 4% into oxides and 20% of the total feedstock actually remained in solution after secondary mineral formation, with an additional 3 to 16% of the total feedstock temporarily retained by cation exchange. These values align with those predicted for German climate from weathering experiments and natural basalt samples. Pogge von Strandmann P. A. E., He X., Zhou Y. and Wilson D. J. (2025) Comparing open versus closed system weathering experiments using lithium isotopes. Applied Geochemistry 189, 106458.Suhrhoff T. J., Reershemius T., Jordan J. S., Li S., Zhang S., Milliken E., Kalderon-Asael B., Ebert Y., Nyateka R., Thompson J. T., Reinhard C. T. and Planavsky N. J. (2025) An Updated Framework and Signal-to-Noise Analysis of Soil Mass Balance Approaches for Quantifying Enhanced Weathering on Managed Lands. Environmental Science & Technology 59, 26440-26453.

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