Evidence for temporal decoupling of olivine weathering and inorganic carbon removal in unsaturated soil columns

Introduction Enhanced Rock Weathering (ERW) is a nature-based carbon dioxide (CO 2 ) removal technology that speeds the natural weathering process of rocks by applying finely ground silicate rocks over the land surface. It has been proposed that ERW can remove between 0.5 and 2 Gt CO 2 year −1 when deployed at scale. To investigate how soil porewater responds to the dissolution of silicate dust during enhanced rock weathering, we conducted unsaturated soil column experiments amended with olivine (Mg 2 SiO 4 ). Methods Since the main principle of ERW-based carbon removal is the reaction of atmospheric CO 2 dissolved in rainwater with silicate rocks, leading to release of base cations and bicarbonate ions, we measured cations, anions, pH, alkalinity, dissolved inorganic carbon (DIC) and dissolved organic carbon (DOC) in the leached effluent. Complementary solid-phase soil measurements of exchangeable cations, pH, total inorganic carbon (TIC) and total organic carbon (TOC) were performed to improve understanding of weathering product transport and retention, while aqueous speciation calculations provided saturation state information of relevant secondary mineral phases. Results We demonstrate that measurable olivine weathering can occur without proportional short-term export of bicarbonate or DIC, and estimates of potential CO 2 removal based on weathering-derived cation release exceeded realized inorganic CO 2 removal quantified from bicarbonate and DIC export. Furthermore, the observed increase in DOC export and decrease in surface-soil organic carbon suggest that enhanced weathering may alter soil organic carbon dynamics. Discussion The discrepancy between weathering-derived cation release and inorganic carbon export is consistent with temporal decoupling between weathering and realized carbon removal, likely caused by retention of weathering-derived products within the soil. Although the mechanisms underlying the observed changes in organic carbon dynamics require further investigation, these results indicate that ERW may affect soil carbon pools beyond inorganic carbon alone. Although alkalinity export was lower than would be predicted from weathering rates alone, quantification of silicate dissolution remains an important upper-bound indicator of future carbon removal potential. These findings highlight the importance of considering soil retention processes, organic carbon dynamics, and transport timescales when interpreting ERW monitoring data and quantifying carbon removal in agricultural soils.