Swapping carbonate for silicate in agricultural enhanced rock weathering

Enhanced rock weathering with crushed silicates is often considered as an alternative to agricultural liming for soil pH management and carbon dioxide removal. But swapping carbonates for silicates does not guarantee better carbon removal outcomes. Carbonates weather rapidly, and recent work has found that they can remove more carbon than fast-reacting silicates in some environments. On the other hand, carbonate dissolution can mobilize fossil carbon and potentially lead to carbon emissions, depending on the spatial and temporal boundaries of the system. Here, we use a one-dimensional reactive transport model, SCEPTER, to analyze the conditions where carbonate weathering breaks even with basalt — a common silicate rock used in enhanced weathering — from an end-to-end carbon removal perspective. We show that current liming practices can remove more carbon than basalt enhanced weathering projects, especially in less acidic conditions and at lower rock application fluxes. However, the methods for increasing silicate weathering carbon removal — including adding more and finer rock — are generally less effective for carbonates. We also show how the carbonate-silicate break-even line changes when we consider the effects of downstream losses, upstream emissions, and carbon accounting decisions. Our results emphasize how existing agricultural practices can be optimized for carbon removal, and present a series of key questions that will be critical for attempts to navigate the carbonate-silicate swap on managed lands.