Nearly pure carbon dioxide (CO₂) is diffusing through the flanks of a dormant volcano, serving as a laboratory for exploring effects of unintended releases from carbon capture and sequestration operations. Tracts of healthy forest reaching 15 ha in size began dying following the 1989 proliferation of high-frequency low-magnitude seismic events (“volcanic earthquake swarms”) beneath Mammoth Mountain, situated on the western rim of Long Valley Caldera in Mono County, California. Volumetric concentrations of CO₂ in soil gas routinely exceed 20–70% in kill zones, where instantaneous CO₂ fluxes have exceeded 10 kg m^–2 d^–1. Carbon dioxide emissions have fluctuated over time, generally decreasing since volcano-seismic unrest breached an inferred magma-derived CO₂ reservoir at depth. Despite shrinkage of the highest-CO₂ zones, revegetation is sparse. To assess the impacts of long-term CO₂ emissions on fertility, soils were sampled along transects from healthy forest into the largest kill zone in 1995 and again in 2010. Sampled soils were analyzed for exchangeable cations and solid-phase composition. Compared to unaffected forest, essential base-metal cations (K, Ca, Mg, Na) on the exchange complexes of kill-zone soils had dropped 13–44% below healthy levels by 1995. Base-metal cations declined further by 2010, in some cases to levels below one third of initial healthy-forest values. Mechanisms responsible for the loss of fertility include long-term leaching involving direct acidification of the exchange complex and sterilization effects involving reduced vegetative biocycling and reduced mineral weathering by microbial symbionts. The potentially destructive effects of long-term CO₂ emission on soils and ecosystems should factor into risk analyses for sequestration activities.