Ecosystem responses determine the effectiveness of enhanced rock weathering for climate mitigation

The effectiveness of enhanced rock weathering (ERW) for carbon dioxide removal (CDR) in open systems, such as forests, remains poorly quantified. Although ERW has been deployed predominantly in agricultural systems, its performance in forestry contexts remains underexplored despite its substantial mitigation potential. Forests offer the potential for both inorganic and both above- and belowground organic CDR following silicate feedstock amendment, but field-scale evidence remains limited. We report findings from a four-year ERW-reforestation experiment in South Wales, UK (11.5 ha; N = 64 plots), designed to capture ecosystem-level responses. The experiment used a fully factorial design that manipulated forest type (native broadleaf versus coniferous), feedstock amendment (amended versus unamended), and incorporated measurements of feedstock dissolution, pore water chemistry, soil CO₂ efflux, soil carbon pools, and aboveground tree biomass. Inorganic CDR was detectable but small: alkalinity export from upper soil layers corresponded to -0.19 ± 0.21 t CO₂eq ha^-1 yr^-1, with most weathering products remaining in the soil column. Organic pathways dominated cumulative system responses. Tree growth accelerated following metabasalt amendment, yielding an estimated -0.34 ± 0.07 t CO₂eq ha^-1 yr^-1 of additional aboveground CDR. By contrast, soil CO₂ efflux increased by 2.54 ± 4.04 t CO₂eq ha^-1 yr^-1, but with substantial variability across time and space. When integrated, these fluxes produced a net ecosystem carbon emission of 2.01 ± 4.05 t CO₂eq ha^-1 yr^-1 over the study period. Although belowground plant biomass was not directly quantified, plausible upper‑bound estimates (≈0.3–0.4 t CO₂eq ha⁻¹ yr⁻¹) do not alter the magnitude or direction of this net flux. Overall, ERW influenced CDR primarily through organic pathways, underscoring the need to better constrain plant-soil feedbacks before large-scale deployment.