From silicates to soil carbonates: Tracing the cation budget of microbially-accelerated weathering

Microbial carbon dioxide mineralization (MCM) is a promising strategy for soil-based carbon dioxide removal (CDR) that leverages beneficial soil microbes applied in agricultural fields to promote native silicate mineral dissolution and carbon drawdown. Unlike enhanced weathering, MCM avoids addition of a mineral feedstock, greatly reducing the carbon footprint from mining, grinding, transporting, and applying the mineral to land. MCM also fits seamlessly into existing farming practices, and the microbial inoculum can be delivered through traditional seed treatments. A key obstacle to measurement, reporting, and verification (MRV) for MCM is ensuring that weathering products are sourced from new silicate dissolution rather than redistribution of pre-existing cations from the exchangeable, oxidizable, or reducible soil pools. To address this, we conducted a 63-day mesocosm study with soybean, applying soil sequential extractions to track the buildup and distribution of weathering products in soil columns inoculated with Bacillus subtilis strain MP1. Our results indicated that MP1-treated soils yield a net increase in base cations, corresponding to 52.8 mEq column-1 relative to the untreated control soils. The majority of base cation increases from silicate weathering were partitioned between the carbonate and exchangeable soil pools, with significant increases of carbonate in the MP1-treated soils. We also observed a significant accumulation of silicon and magnesium in the reducible fraction, suggesting secondary clay mineral formation. However, these changes were small relative to the carbonate and exchangeable fractions. We estimate that 37% to 67% of the dissolution-derived cations formed carbonates, achieving a realized CDR of 0.20 to 0.36 g CO2 kg-1 soil. Overall, our findings support that Bacillus subtilis MP1 couples native soil silicate mineral dissolution with carbonate precipitation, confirming MCM as a viable CDR strategy and suggesting soil inorganic carbon measurements as a suitable approach for its MRV.