Carbon dioxide capture and storage is one option for reducing man-made greenhouse gas emissions to the atmosphere. However, the possibility of leakage from the reservoir, and any subsequent effects on the environment, need to be considered. Consequently, the impact of elevated CO2 soil gas concentrations on near-surface ecosystems (to a depth of ~1 m) has been an area of active research for a number of years, particularly in the context of terrestrial systems, and is currently being addressed as part of the FP7 RISCS (Research into Impacts and Safety in CO2 storage) project. In Europe terrestrial studies have targeted natural sites which are being used as pasture, where CO2 is being released to the atmosphere from deep geological formations (e.g. Florina, Greece (Ziogou et al, in press); Laacher See, Germany (Krüger et al, 2011); Latera, Italy; (Beaubien et al, 2008)); and pristine non-adapted sites where CO2 is being injected into the soil zone (ASGARD, England) to ascertain impacts on pasture and crop plants (West et al, 2008; Smith et al, in press). For the natural sites, botanical, soil gas and gas flux, chemical and microbiological surveys were carried out across CO2 gas vents. For the controlled injection site, similar surveys were conducted across 2.5 x 2.5 m plots where CO2 was being injected into the plot centre and the effects were compared with those from control plots. Full details of the sites, methodologies and results are given elsewhere (Beaubien et al, 2008; Krüger et al, 2011; Smith et al, in press; West et al, 2008; Ziogou et al, in press)

Each of these studies is yielding considerable information on the impacts of elevated CO2 soil gas concentrations on a range of parameters, including botany and soil microbiology, for pasture in different climatic conditions. A comparison of these data can now be made to better understand how local conditions can potentially modify the impact of any CO2 leak. Broadly, this shows:

  1. The impact of CO2 gas vents on the surface ecosystem at natural sites appears to be highly localised and is dependent on the prevailing extreme CO2 regime. The spatial variations of soil CO2 concentration and fluxes are generally controlled by a complex interplay of factors such as the migration pathway at depth, the physical and chemical properties of the vadose zone, the features and hydrogeology of the near surface deposits etc. Thus, soil gas analyses and flux measurements will be required for baseline characterisation of migration pathways for soil gases and for the design of any near-surface gas geochemistry monitoring program of CO2 geological storage sites.
  2. Results from botanical studies at all the natural sites, where plants have been exposed to high CO2 soil gas concentrations (up to 80% at 20 cm depth at Florina) for many years, indicate species-specific response to high CO2 concentrations depending on soil properties, mineralogical composition and temporal and spatial soil CO2 concentration and flux patterns. Site specific plant ‘bioindicators’ are also observed for concentrations at ≥35%, such as Polygonatum arenastrum at Laacher See. At Florina and Latera, monocotyledonous plants (grasses) become increasingly dominant above 10% CO2 soil gas concentrations at 20 cm depth and this continues until CO2 soil gas concentrations reach ~30-40%. Interestingly, this was also observed at ASGARD after the pasture plot had been exposed to elevated CO2 soil gas concentrations for 24 months. This grass predominance persisted once gassing had stopped.
  3. Microbiological analyses at the natural CO2 vent sites suggest significant changes in the soil microbial community caused by high CO2 levels in the soil gas. For example, there is a shift in the microbial community composition towards anaerobic and acid tolerant microorganisms as well as an ecosystem adaption to the CO2 induced soil biogeochemistry. There are also significant effects on metabolic rates with increasing CO2 soil gas concentrations although this is not reflected in overall bacterial and archaeal cell numbers. However, at ASGARD, after 24 months gassing, total microbial numbers increased when CO2 soil gas concentrations were between 20-50%. Additionally, at ASGARD where CO2 soil gas concentrations are <8%, there is little impact on the microbial ecosystem. These differences may be related to long-term adaptation of the microbial ecosystems at the natural sites.