Microbially mediated interactions between enhanced weathering and carbon inputs shift soil organic carbon dynamics

Enhanced weathering, adding crushed silicate minerals to soils, is being explored as a way to draw down atmospheric carbon dioxide, but its effects on soil organic carbon remain uncertain. We show that olivine amendments change soil carbon cycling depending on time and on labile carbon availability. In a 55-week soil microcosm experiment, adding olivine alone reduces total soil organic carbon. Adding olivine with glucose initially suppresses microbial carbon uptake, but by week 55 instead boosts microbial biomass and mineral-associated organic matter more than glucose alone, while lowering respiratory carbon loss. These late-stage gains grow stronger alongside geochemical signs of mineral weathering. Nanoscale imaging shows that newly formed soil organic carbon accumulates within pre-existing organic and mineral microsites rather than directly on olivine or newly formed clay surfaces. Our findings suggest that when labile carbon is available, enhanced weathering can increase soil carbon storage primarily through microbial processing, rather than by creating new mineral surfaces for carbon to bind to.