The enhanced rock weathering stages determine the fluxes and interactions of soil inorganic and organic carbon pools

Enhanced rock weathering (ERW) has emerged as a promising strategy for atmospheric CO₂ removal via promoting inorganic carbon (IC) sequestration. Despite its impact on IC accrual that has been extensively studied and modelled, the lack of understanding of its impact on the largest terrestrial C stock - organic carbon (OC), and the overall C fluxes throughout the weathering stages impede the long-term assessment of ERW in C sequestration. Here, we conducted a 6-month microcosm study using fresh basalt (fine size) and weathered basalt (coarse and fine size) to simulate the impacts of basalt on C fluxes with weathering progressing in a temperate cropland topsoil. We also incorporated ¹³C-labeled straw to understand their effects on the turnover of new straw-derived organic matter (OM) and the native OM. Our findings show that both fresh and weathered basalt treatments increase IC through the release of exchangeable cations, with the fresh basalt contributing more exchangeable Mg and the weathered basalt shifting toward exchangeable Ca dominance as olivine minerals deplete. The fresh basalt treatments lead to a significant loss of soil OC, driven by soil alkalinity. Nevertheless, they concurrently reduce CO₂ emissions by promoting IC accrual in soils and the leaching of dissolved organic carbon (DOC) and dissolved inorganic carbon (DIC). With the progress of weathering, the alkalinity effect diminishes. The weathered basalt (fine size) treatments demonstrate improved OC retention for both native soil organic matter (SOM) and straw-derived OM. This is accompanied by the reduced DOC and DIC leaching, attributed to increased specific surface area (SSA), low pH and SOM stabilization through Ca. However, the IC accrual can be labile to fresh biomass inputs, which enhance CO₂ emissions and deplete the accumulated IC in bulk soils and DIC leaching. These findings suggest that in soils with continuous biomass inputs, the benefits of ERW (e.g. basalt) throughout the weathering stages lie in reducing OC loss driven by weathered ERW materials rather than sustaining IC accumulation, which can be easily lost by environmental fluctuations in temperate zones.