Synergistic effects of enhanced rock weathering and organic inputs on soil carbon accrual.

Enhanced Rock Weathering (ERW)—the application of crushed silicate minerals to croplands—holds promise for drawing down excess atmospheric carbon dioxide through accelerated biological and chemical weathering. Silicate rock amendments, such as olivine and basalt, are increasingly recognized for increasing inorganic carbon (C) drawdown by enhancing the flux of soil alkalinity. However, their effects on soil organic carbon (SOC)—Earth’s largest terrestrial C reservoir and a key driver of soil fertility regulated by microbial processes—remain underexplored, especially when combined with organic amendments commonly used in agriculture to promote organic C accrual and soil health. In a soil microcosm study, we assessed the impact of adding a readily available C source in combination with olivine inputs on soil C and microbial dynamics over a 55-week incubation period. Additionally, we used ¹³C-glucose tracers to track C flow across early and late incubation stages, and NanoSIMS imaging to directly visualize organic matter-mineral interactions. Co-application of olivine and glucose led to higher total C, greater microbial biomass, lower microbial respiration per unit biomass, and increased mineral-associated organic matter (MAOM) formation with greater incorporation of new C—surpassing the effects of either amendment alone. NanoSIMS imaging and SEM-EDS revealed a microscale distribution of glucose-derived ¹³C across mineral-dominated and OM-dominated patches that were not directly co-located with olivine particles. Treatments without an organic input showed limited changes in microbial C use strategies or lower SOC. Our findings provide some of the first direct evidence for synergistic effects of crushed rock and organic inputs on soil C accrual, through microbial processes that enhance both organic and inorganic C storage. Integrating ERW with organic inputs enhances biologically mediated C retention, strengthening both climate mitigation potential and soil health, especially in agroecosystems where microbial functioning is vital yet vulnerable.