Comparison of the impacts of elevated CO2 soil gas concentrations on selected European terrestrial environments

Selected European studies have illustrated the impacts of elevated CO2 concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO2 thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO2 storage. The sites include those where CO2 is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO2 seepage, where CO2 has been injected into the unsaturated soil horizon. Whilst soil gas concentrations will be influenced by flux rates and other factors, the results suggest that a concentration of between 10% and 15% CO2 soil gas at 20 cm depth, which is within the root zone, is an important threshold level for observing changes in plant coverage. Site-specific plant 'indicators' are also observed for CO2 concentrations at ≥35%. Microbiological changes are seen where CO2 soil gas concentrations are between 15% and 40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken.