Novel extraction method designed to estimate the topsoil pore water reservoir of carbon dioxide removal through enhanced weathering of silicate minerals

Enhanced weathering (EW) of silicate minerals on agricultural fields is a promising natural carbon dioxide removal (CDR) method that has potential co-benefits for soil health and crop safety. However, the scalability of EW is suffering from labor-intense requirements of soil pore water extraction for monitoring, reporting and verification (MRV) of carbon credits. Furthermore, in-field extraction of soil pore water for MRV can be challenging as existing methods, such as rhizon samplers or ceramic suction cups, may lose vacuum or be ineffective at low moisture levels, leading to insufficient volumes of sample obtained for chemical analysis. Critically, the volume of soil from which pore water is drawn using these conventional methods will differ depending on soil moisture content and pore connectivity, making it difficult to determine the precise volume of soil sampled and consequently adding further uncertainty to CDR estimates. Here, a novel method for extracting weathering products from soil for determination of EW and CDR (termed “SAT-C”) is presented. SAT-C attempts to alleviate some of the limitations of traditional soil pore water extraction by obtaining weathering products from a known soil volume. In the SAT-C approach, a soil sample is saturated using de-ionized water and subsequently centrifuged in order to separate the sample into aqueous and solid phases, both of which are later analyzed for weathering products. In this study, SAT-C was applied to cores extracted from the upper 5-10 cm of the soil profile at two different EW deployment sites, where conventional rhizon and ceramic suction cups are also installed at 10 and 5-10 cm depths, respectively. Base cation and anion concentrations for all three methods were in the same order of magnitude. Estimated bicarbonate from the charge balance of major cations and anions correlated well with measured alkalinity across the two different soil types. In addition to comparable pore water chemistry, the SAT-C method offers the quantitative estimation of water filled porosity (which is needed for a direct measure for the field wide pore water reservoir of CDR at that point in time) that conventional methods lack. Furthermore, the method is not inhibited by low levels of field moisture during the crop growing season.