Soils as natural filters for GHG: an imbalance between the expected CH4 fluxes and the direct measurements

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The final composition of the atmosphere results from several processes and exchanges between all the Earth’sspheres. Some of these are widely known and others, such as the methane degassing from hydrothermal areas,are still understudied. Methane plays a key role in climate change being an efficient greenhouse gas. Although itwould be crucial, the total CH4 output from geogenic sources is still not well defined; limitations in CH4 outputestimations are due to many factors concerning a scarce dataset availability, difficulty in direct measurements,and interaction with methane-consuming microbiota in the soil. Often, the CH4 flux estimation was obtainedindirectly, e.g. cross-correlating CO2/CH4 or H2O/CH4 values of the main gaseous manifestations with measuredCO2 or H2O fluxes (Etiope et al., 2007). These methods are commonly used but, although acceptable in principle,may lead to strong over- or under-estimated values. This is particularly true when referred to hydrothermal systemslike Nisyros Island (South Aegean Active Volcanic Arc, Greece) showing a wide range in the fumarolic CO2/CH4ratios.Nisyros’ Island total CH4 output has been previously estimated indirectly in 54 tons/year (Etiope et al., 2007). In2013, a gas prospection was carried at Lakki Plain, the main exhalative area. Methane flux values were measuredat 130 sites ranging from –3.4 to 1420 mg/m2 day. Data were processed by sGs approach and the total flux wasestimated in 1 ton/year. Soils were sampled at 10 sites with different T (27 to 70 C), pH (1.4 to 3.7) and gascomposition (e.g. H2S from 0.3 to 3.6%). They were used for incubation experiments carried on an atmosphereenriched in methane. Consumption was detected in a range from 5 to 40 ng CH4/gDW hour, with higher values insamples with milder environmental conditions (lower temperature and H2S contents and higher pH).The present study indicates that the previous estimate at Nisyros Island, made by cross-correlating CO2 outputdata with the CO2/CH4 ratios of its gaseous manifestations, has been excessively large.This result is not a surprise, over-estimation of the CH4 output has been evidenced also for Pantelleria Island (Italy)and Sousaki (Greece). Also in these cases high methanotrophic activity in the soil has been confirmed, indicating agood efficiency of the soil to act as natural filter for GHG (D’Alessandro et al., 2009; 2011; Gagliano et al., 2014).Notwithstanding, the hydrothermal areas are a significant source of CH4 but probably their contribution has beenoverestimated. Differences in the estimations derive from both disregarding methanotrophic activity within thesoils and from an incorrect, and sometimes forced by the data availability, mean CO2/CH4 ratio of the fumarolicemissions used for the indirect estimation.A lot has still to be done to assess the global hydrothermal CH4 burden, but the importance of direct CH4 fluxmeasurements has to be underscored.
Lingua originaleEnglish
Numero di pagine1
Stato di pubblicazionePublished - 2018

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