Synergistic Effects of a Microbial Amendment and Crushed Basalt: Soil Geochemical and Microbial Responses

Over geologic timescales, the natural weathering of silicate minerals in soils and regolith regulates atmospheric CO₂. Although this process is slow relative to anthropogenic emissions, several strategies have been proposed to accelerate this process for climate mitigation. These include the application of finely-ground silicate rock to increase mineral surface area (enhanced weathering, EW) and the use of microbes that catalyze mineral dissolution and CO₂ biomineralization (microbial carbon dioxide mineralization, MCM). While both approaches show promise, their combined application has rarely been tested. Here, we examined how soil chemistry and bacterial communities respond to a basalt feedstock rich in silicate minerals, a Bacillus subtilis strain (MP1) previously shown to enhance weathering, and their combination. In a 91-day soybean mesocosm experiment with slightly acidic soil (pH 6.6), MP1 persisted where applied, indicating successful inoculation via seed treatment. Basalt amendments had the strongest effect on soil bacterial community composition, whereas inoculation with MP1 exerted a smaller but detectable influence. Biogeochemical indices of weathering indicated that co-application of basalt and MP1 enhanced carbonate alkalinity beyond basalt alone. Soil carbonate alkalinity increased with MP1 treatment both with and without basalt, while soil pH and cation exchange capacity (CEC) increased with basalt in both MP1 and non-MP1 treatments. Total carbon was highest in the combined MP1 + basalt treatment, suggesting that MP1 may mitigate short-term organic carbon losses associated with basalt-driven priming. Overall, these results provide new insights into interactions between biological and mineral-based carbon dioxide removal (CDR) strategies, suggesting that co-application of MP1 with basalt in slightly acidic soil may enhance carbonate alkalinity while reducing organic carbon losses relative to basalt alone. Thus, pairing B. subtilis MP1 with enhanced weathering deployments emerges as a promising strategy to improve CDR efficiency.