CO2 diffusion into pore spaces limits weathering rate of an experimental basalt landscape

Basalt weathering is a key control over the global carbon cycle, though in situ measurements of carbon cycling are lacking. In an experimental, vegetation-free hillslope containing 330 m³ of ground basalt scoria, we measured real-time inorganic carbon dynamics within the porous media and seepage flow. The hillslope carbon flux (0.6–5.1 mg C m^–2 h^–1) matched weathering rates of natural basalt landscapes (0.4–8.8 mg C m^–2 h^–1) despite lacking the expected field-based impediments to weathering. After rainfall, a decrease in CO₂ concentration ([CO₂]) in pore spaces into solution suggested rapid carbon sequestration but slow reactant supply. Persistent low soil [CO₂] implied that diffusion limited CO₂ supply, while when sufficiently dry, reaction product concentrations limited further weathering. Strong influence of diffusion could cause spatial heterogeneity of weathering even in natural settings, implying that modeling studies need to include variable soil [CO₂] to improve carbon cycling estimates associated with potential carbon sequestration methods.