Integrating Experiments and Models To Unravel Interactions between Soil Organic Matter and Enhanced Weathering

Key uncertainties remain in predicting carbon sequestration through enhanced weathering (EW), particularly regarding secondary mineral formation and interactions with the organic matter. We compared a coupled inorganic-organic geochemical model (PHREEQCENTURY) with soil measurements and CO₂ efflux from a 389-day mesocosm experiment using soils amended with varying alkaline manure, basalt, and dunite inputs. Silicate amendments did not enhance dissolved inorganic C leaching or significantly increase pedogenic carbonate accumulation. PHREEQC simulations indicated that base cations preferentially precipitated as secondary clays rather than carbonates, inhibiting CO₂ removal. Sequential extractions suggested that Al, Fe and base cations were mainly retained via adsorption to (hydr)oxides and organic matter, with additional retention in secondary clays indicated by models. Higher organic matter addition did not increase element release from basalt─and reduced leached K and Fe─while decreasing the reactive surface area of basalt, indicating a counteractive effect of manure on rock weathering. Alkaline manure addition also likely decreased weathering of pyroxene and olivine minerals. Cumulative soil CO₂ efflux did not differ significantly among treatments, consistent with PHREEQCENTURY simulations, predicting limited treatment effects due to minor changes in soil pH, moisture and organic C stabilization as mineral-associated organic matter in this soil.