Carbon isotopic signature of interstitial soil gases reveals the potential role of ecosystems in mitigating geogenic greenhouse gas emissions: Case studies from hydrothermal systems in Italy.

Volcanic and hydrothermal areas largely contribute to the natural emission of greenhouse gases to the atmosphere, although large uncertainties in estimating their global output still remain. Nevertheless, CO and CH discharged from hydrothermal fluid reservoirs may support active soil microbial communities. Such secondary processes can control and reduce the flux of these gases to the atmosphere. In order to evaluate the effects deriving from the presence of microbial activity, chemical and carbon (in CO and CH) isotopic composition of interstitial soil gases, as well as diffuse CO fluxes, of three hydrothermal systems from Italy were investigated, i.e. (i) Solfatara crater (Campi Flegrei), (ii) Monterotondo Marittimo (Larderello geothermal field) and (iii) Baia di Levante in Vulcano Island (Aeolian Archipelago), where soil CO fluxes up to 2400, 1920 and 346 g m day were measured, respectively. Despite the large supply of hydrothermal fluids, CO enrichments were observed in interstitial soil gases with respect to the fumarolic gas discharges, pointing to the occurrence of autotrophic CO fixation processes during the migration of deep-sourced fluids towards the soil-air interface. On the other hand, (i) the δC-CH values (up to ~48‰ vs. V-PDB higher than those measured at the fumarolic emissions) of the interstitial soil gases and (ii) the comparison of the CO/CH ratios between soil gases and fumarolic emissions suggested that the deep-sourced CH was partly consumed by methanotrophic activity, as supported by isotope fractionation modeling. These findings confirmed the key role that methanotrophs play in mitigating the release of geogenic greenhouse gases from volcanic and hydrothermal environments.