Evaluating trace-metal responses to field deployment of enhanced rock weathering across agricultural and riparian soils

Enhanced rock weathering (ERW) is proposed for carbon dioxide removal, but responsible field deployment requires careful evaluation of potential trace-metal risks. We applied finely crushed, Fe–Al-rich basalt (20 t ha⁻¹) to the soil surface of hayfield and pasture soils in a working Vermont dairy agroecosystem and monitored an adjacent riparian corridor hydrologically connected to the treated fields. Agricultural surface soils (0–15 cm) were sampled twice before application (fall 2022 and spring 2023) and four times afterward (fall 2023; spring 2024; fall 2024; spring 2025), and riparian soils were sampled once before application and during four post-treatment campaigns. Forage biomass and tissue chemistry were measured at first cut in spring 2023 (pre-treatment) and spring 2024 (one year post-treatment). Extractable soil metals were quantified using the Modified Morgan extractant. In agricultural plots, extractable Ni, Cr, Pb, Al, Mn, Zn, Cu, and Cd did not increase relative to controls, and forage yields and tissue metal concentrations did not differ between treatments. Across plots, extractable Ni, Al, and Pb covaried most strongly with soil pH. Downslope riparian soils exhibited large declines (60–80%) in extractable Ni, Al, and Pb, coincident with a transient rise in extractable Ca:Al ratios. These coupled soil and groundwater patterns are consistent with reduced metal lability in hydrologically connected riparian soils where basalt-derived solutes accumulated. Together, these results indicate that agronomic-rate applications of Fe–Al-rich basalt pose minimal trace-metal risk in treated fields and promote conditions consistent with trace-metal sequestration in zones of weathering product accumulation. Effective ERW monitoring should consider hydrologic connectivity and track soil pH and Ca:Al ratios as practical indicators of evolving metal availability.