The impact of a naturally-occurring CO2 gas vent on the shallow ecosystem and soil chemistry of a Mediterranean pasture (Latera, Italy)
Résumé
Recent research into CO2 geological storage has shown that it has potential to be a safe and effective way to rapidly decrease short-term anthropogenic CO2 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 m wide centre of the vent, where CO2 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 CO2 (>95%) and trace reduced gases (CH4, H2S, and H2). An approximately 20 m wide halo surrounding the core forms a transition zone, over which there is a gradual decrease in CO2 concentrations, a rapid decrease in CO2 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 of the gas vent are spatially limited and that the ecosystem appears to have adapted to the different conditions through species substitution or adaptation.