Integrating experiments and models to unravel interactions between soil organic matter and enhanced weathering

Enhanced weathering (EW) aims to accelerate rock dissolution to sequester atmospheric CO2. Here, we investigated key uncertainties in modelling C sequestration through EW by comparing a coupled inorganic-organic geochemical model against soil measurements and soil CO₂ efflux from a 389-day mesocosm experiment with soils amended with varying manure, basalt and dunite inputs. Silicate amendments did not enhance dissolved inorganic C leaching or significantly increase pedogenic carbonate accumulation. PHREEQC simulations indicate that base cations preferentially precipitated as secondary clays instead of carbonates, thereby inhibiting inorganic CO2 removal. Sequential extractions suggest that Al, Fe and base cations were retained in soils through cation exchange, adsorption to (hydr)oxides and organic matter, with model results indicating additional retention in secondary clays. Cumulative soil CO2 efflux and was not significantly altered by rock amendments. Higher organic matter addition did not increase the release of elements from basalt (and even reduced leached K and Fe), while decreasing the reactive surface area of basalt, indicating a counteractive effect of organic matter on rock weathering. Cumulative soil CO₂ efflux did not differ significantly among silicate-amended treatments, consistent with PHREEQCENTURY simulations. The model predicted limited treatment effects on CO₂ efflux in this soil due to minor changes in soil pH and moisture and limited stabilization of organic C that may correspond to MAOM formation.