Effects of microbial activity on C-bearing volatile compounds in interstitial soil gases from the Vicano-Cimino volcanic system (central Italy)

Carbon-bearing volatiles emitted from both anthropogenic and natural sources have a strong environmental impact on air quality, global climate and human health. Notwithstanding, a reliable evaluation of the input rates to the atmosphere of these gases is still a challenge. Secondary processes occurring in soils play a fundamental role for the release of gases produced under reducing conditions, such as those released from volcanic and geothermal fluid reservoirs. In this study the effects of microbial activity on the behavior of CO2, CH4 and volatile organic compounds (VOCs) originating from the Vicano-Cimino hydrothermal system (Central Italy) were investigated. The database consisted of the chemical composition and d13C-CO2 and d13C-CH4 values of interstitial soil gases from a 150×100 m wide area, located in the surroundings of Viterbo and characterized by intense degassing of deep-originated fluids. Gas samples were collected from 5 vertical profiles (from 10 cm down to 50 cm depth) at regular depth intervals (5 cm), and from 20 sites at 40 cm depth. The vertical trends of the d13C-CH4 values, as well as the vertical gradients of CH4, H2S, and O2 concentrations, consistently indicated that CH4 strongly depends on microbial activity occurring at < 50 cm depth, where free O2 is available. On the contrary, according to the d13C-CO2 values, CO2 was not significantly affected by biodegradation, except for those gases sampled from sites affected by significant air contamination. Microbial activity efficiently degraded hydrothermal alkanes, alkenes, cyclics, and hydrogenated halocarbons, whereas benzene behaved as recalcitrant species. Oxygenated compounds from hydrocarbon degradation consisted of alcohols, with minor aldehydes, ketones and carboxylic acids. A predominance of alcohols at a high rate of degassing flux, corresponding to a short residence time of hydrothermal gases within the soil, indicated incomplete oxidation. N-bearing compounds were also detected and likely produced by humic substances in the soil and/or related to contamination of pesticides. Thus, these species were not related to biodegradation processes of geogenic gases. Similarly, a-pinene was found to trace air entering the soil. These results showed that the microbial communities in the soil of the study area were able to strongly mitigate the impact on air quality of hydrothermal C-bearing gases, especially CH4, released through diffuse degassing, a mechanism that in Central Italy significantly contributes to the discharge of CO2-rich gases from deep sources.