Carrying capacity as constraint for maximum efficient CDR in agricultural soils

Enhanced Rock Weathering (ERW), in which crushed basaltic rocks are spread on croplands, has emerged as a promising carbon dioxide removal (CDR) approach to mitigate climate change impacts. Important known constraints on weathering rates include temperature, humidity, and feedstock grain size. However, the quantitative prediction and optimization of CDR is currently limited by uncertainty in the processes and rates governing weathering and export. Here, we propose to evaluate the product of effective groundwater recharge and dissolved inorganic carbon (DIC) concentrations as a measure of CDR. Since maximum DIC concentrations in pore water are controlled by soil gas PCO ₂ and achievable Ca and Mg concentrations from weathering, we define this CDR flux as the “carrying capacity”. We consider the onset of precipitation of secondary Ca-carbonate minerals in solution due to the accumulation of ERW reaction products in the shallow soil pore water as the upper limit for the effective generation of CDR. Our results therefore present values of “maximum efficient CDR”, yielding CDR export to groundwater in the absence of carbonate mineral precipitation, as a function of effective groundwater recharge. Extending the carrying capacity concept to global croplands highlights the potential importance of groundwater recharge in determining regions with highest ERW potential. Given the simplifying assumptions in our assessment, we estimate a global CDR potential of 0.15 and 0.85 Gt CO ₂ yr ^− 1 . Our results indicate that regions with high groundwater recharge and feedstocks rich in leachable Mg provide the highest potential for efficient CDR generation, assuming feedstock dissolution is not limiting. Our analysis does not account for CDR losses in the near field or far-field zone, for example due to nitrification or the release of stored acidity, but also omits soil exchange and other reactions that may restrict calcite precipitation and thus lead to higher maximum efficient CDR.