Surface Proton Reservoirs Constrain Alkalinity Export from Enhanced Weathering

The addition of silicate and carbonate materials to soils (enhanced weathering) is a proposed carbon dioxide removal (CDR) strategy with potential agronomic co-benefits, especially for soil pH management. However, alkalinity from mineral dissolution interacts with the surface proton reservoir (SPR), consuming alkalinity prior to export and complicating quantification. Because durable CDR requires transmission of alkalinity to marine storage, soil export constitutes a mechanistically definable upper bound on EW performance. Here, we develop a framework that partitions operational cation exchange measurements into electrostatic and pH-dependent coordinative surface reservoirs and embed this representation within a reactive transport model to explicitly couple the depth-dependent SPR, cation exchange, alkalinity generation, and CO2(g) exchange. We apply this approach to contrasting silicate (olivine) and carbonate (calcite) amendments across representative hydrologic conditions. Using distance-based generalized sensitivity analysis, we identify the multivariate control space involving SPR magnitude, baseline soil pH, soil moisture, and amendment rate that governs alkalinity export efficiency. Simulations show that amendment-derived alkalinity is largely neutralized during early dissolution, with substantial impairment occurring below the shallow (>25 cm) soil zone. As a result, proxy-based estimates of alkalinity export derived from dissolved inorganic carbon or base cations exhibit systematic and depth-dependent bias. These findings highlight the SPR as a first-order limitation on EW alkalinity export and suggest appropriate screening and monitoring strategies for CDR quantification.