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

Abstract

The final composition of the atmosphere results from several processes and exchanges between all the Earth’s spheres. 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 it would be crucial, the total CH4 output from geogenic sources is still not well defined; limitations in CH4 output estimations 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 obtained indirectly, e.g. cross-correlating CO2/CH4 or H2O/CH4 values of the main gaseous manifestations with measured CO2 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 systems like Nisyros Island (South Aegean Active Volcanic Arc, Greece) showing a wide range in the fumarolic CO2/CH4 ratios. Nisyros’ Island total CH4 output has been previously estimated indirectly in 54 tons/year (Etiope et al., 2007). In 2013, a gas prospection was carried at Lakki Plain, the main exhalative area. Methane flux values were measured at 130 sites ranging from –3.4 to 1420 mg/m2 day. Data were processed by sGs approach and the total flux was estimated in 1 ton/year. Soils were sampled at 10 sites with different T (27 to 70 C), pH (1.4 to 3.7) and gas composition (e.g. H2S from 0.3 to 3.6%). They were used for incubation experiments carried on an atmosphere enriched in methane. Consumption was detected in a range from 5 to 40 ng CH4/gDW hour, with higher values in samples 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 output data 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 a good 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 been overestimated. Differences in the estimations derive from both disregarding methanotrophic activity within the soils and from an incorrect, and sometimes forced by the data availability, mean CO2/CH4 ratio of the fumarolic emissions 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 flux measurements has to be underscored.