CO2 capture and storage (CCS) in deep geological formations is one option currently evaluated to reduce greenhouse gas emissions. Consequently, the impact of a possible CO2 leakage from a storage site into surface environments has to be evaluated. During such a hypothetical leakage event, the CO2 migrates upwards along fractures entering surface soils, a scenario similar to naturally occurring CO2 vents. Therefore, such a natural analogue site at the Laacher See was chosen for an ecosystem study on the effects of high CO2 concentrations on soil chemistry and microbiology. The microbial activities revealed differences in their spatial distribution and temporal variability for CO2 -rich and reference soils. Furthermore, the abundance of several functional and group-specific gene markers revealed further differences, for example, a decrease in Geobacteraceae and an increase in sulphate-reducing prokaryotes in the vent centre. Molecular-biological fingerprinting of the microbial communities with DGGE indicated a shift in the environmental conditions within the Laacher See soil column leading to anaerobic and potentially acidic microenvironments. Furthermore, the distribution and phylogenetic affiliation of the archaeal 16S rRNA genes, the presence of ammonia-oxidizing Archaea and the biomarker analysis revealed a predominance of Thaumarchaeota as possible indicator organisms for elevated CO2 concentrations in soils.

The RISCS (Research into Impacts and Safety in Carbon Storage) project is investigating potential environmental impacts of CO2 leakage. At ASGARD (Artificial Soil Gassing and Response Detection), a fully-replicated facility for controlled injection of CO2 into soil, investigations have been carried out to determine the effects of elevated soil CO2 on crops, soil microbiology, soil flux and soil CO2 concentration

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Selected European studies have illustrated the impacts of elevated CO 2 concentrations in shallow soils on pasture. For the first time, general unified conclusions can be made, providing CO 2 thresholds where effects on plants and soil microbiology are observed and making recommendations on how this information can be used when planning projects for CO 2 storage. The sites include those where CO 2 is being naturally released to the atmosphere from deep geological formations; and a non-adapted site, with no previous history of CO 2 seepage, where CO 2 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 10-15% CO 2 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 CO 2 concentrations at ≥35%. Microbiological changes are seen where CO 2 soil gas concentrations are between 15-40%. As part of site characterisation, an evaluation of the risks of leakage and their potential environmental impacts should be undertaken.

Impacts of long-term CO 2 exposure on environmental processes and microbial populations of near surface soils are poorly understood. This near-surface long-term CO 2 injection study demonstrated that soil microbiology and geochemistry is influenced more by seasonal parameters than elevated CO 2 . Soil samples were taken during a three-year field experiment including sampling campaigns before, during and after 24 months of continuous CO 2 injection. CO 2 concentrations within CO 2 -injected plots increased up to 23% during the injection period. No CO 2 impacts on geochemistry were detected over time. In addition, CO 2 -exposed samples did not show significant changes in microbial CO 2 and CH 4 turnover rates compared to reference samples. Likewise, no significant CO 2 -induced variations were detected for the abundance of Bacteria, Archaea (16S rDNA) and gene copy numbers of the mcrA gene, Crenarchaeota and amoA gene. The majority (75-95%) of the bacterial sequences were assigned into five phyla: Firmicutes, Proteobacteria, Actinobacteria, Acidobacteria and Bacteroidetes. The majority of the archaeal sequences (85-100%) were assigned to the thaumarchaeotal cluster I.1b (soil group). Univariate and multivariate statistical as well as principal component analyses (PCA) showed no significant CO 2 -induced variation. Instead, seasonal impacts especially temperature and precipitation were detected.3