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Recent research into C02 geological storage has shown that it has potential to be a safe and effective way to rapidly decrease short-term anthropogenic C02 emissions. Despite this progress, stakeholders must be convinced that the scientific community has studied all possible scenarios, including a potential leak into the biosphere. To better understand the potential impact of such an event, a detailed geochemical and biological study was conducted during two different seasons on a naturally occurring gas vent located within a Mediterranean pasture ecosystem (Latera geothermal field, central Italy). Results from botanical, soil gas, and gas flux surveys, and from chemical and biological analyses of shallow soil samples (0-20 cm depth), show that a significant impact is only observed in the 6 In wide centre of the vent, where C02 flux rates exceed 2000-3000 g M-2 d-1. In this "vent core" there is no vegetation, pH is low (minimum 3.5), and small changes are observed in mineralogy and bulk chemistry. in addition, microbial activities and populations are regulated in this interval by near-anoxic conditions, and by elevated soil gas C02 (>95%) and trace reduced gases (CH4, H2S, and 142). An approximately 20 In wide halo surrounding the core forms a transition zone, over which there is a gradual decrease in C02 concentrations, a rapid decrease in C02 fluxes, and the absence of reactive gas species. In this transition zone grasses dominate near the vent core, but these are progressively replaced by clover and a greater plant diversity moving away from the vent centre. Physical parameters (e.g. pH, bulk chemistry, mineralogy) and microbial systems also gradually return to background values across this transition zone. Results indicate that, even at this anomalous high-flux site, the effects ofthe gas vent are spatially limited and that the ecosystem appears to have adapted to the different conditions through species substitution or adaptation. (c) 2008 Elsevier Ltd. All rights reserved

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