Quantifying Microbial Methane Oxidation Efficiencies in Two Experimental Landfill Biocovers Using Stable Isotopes

Abstract

Stable isotope analyses were performed on gas samples collected within two instrumented biocovers, with the goal of evaluating CH 4 oxidation efficiencies (f 0 ). In each of the biocovers, gas probes were installed at four locations and at several depths. One of the biocovers was fed with biogas directly from the waste mass, whereas the other was fed through a gas distribution system that allowed monitoring of biogas fluxes. While the f 0 values obtained at a depth of 0.1 m were low (between 0.0% and 25.2%) for profiles with poor aeration, they were high for profiles with better aeration, reaching 89.7%.Several interrelated factors affecting aeration seem to be influencing f 0 , including the degree of water saturation, the magnitude of the biogas flux and the temperature within the substrate. Low f 0 values do not mean necessarily that little CH 4 was oxidized. In fact, in certain cases where the CH 4 loading was high, the absolute amount of CH 4 oxidized was quite high and comparable to the rate of CH 4 oxidation for cases with low CH 4 loading and high f 0 . For the experimental biocover for which the CH 4 loading was known, the oxidation efficiency obtained using stable isotopes (f 0 =55.67% for samples taken inside flux chambers) was compared to the value obtained by mass balance (f 0 =70.0%). Several factors can explain this discrepancy, including the high sensitivity of f 0 to slight changes in the isotopic fractionation factor for bacterial oxidation, α ox , uncertainties related to mass flow metre readings and to the static chamber method.

Improved quantification of microbial CH<sub>4</sub> oxidation efficiency in arctic wetland soils using carbon isotope fractionation

“…The higher diffusion coefficient in the porous medium (5.54 × 10 −6 m 2 s −1 ) could facilitate isotopic fractionation by diffusion. Measurements in this study showed no correlation between diffusion coefficients and α diff , but might show a different relationship with higher concentration gradients as found in landfill habitats where CH 4 concentrations up to 60 % are found at the bottom of cover soils (Cabral et al, 2010).…”

“…GPPTs are not easily applicable at sites with low oxidation rates and high water saturation (Gomez et al, 2008;Urmann et al, 2007), such as tundra wetlands, and have only been successfully applied in near-surface soils with a cylinder driven 50 cm into the soil (Nauer and Schroth, 2010). Furthermore, mass balance calculations using loading and surface flux measurements to determine the fraction of oxidized CH 4 , e.g., in biofilters or landfill cover soils (Powelson et al, 2007;Cabral et al, 2010;Gebert et al, 2003), are difficult to apply in wetlands since loading rates cannot be quantified in these open systems.…”

“…While for the microbial oxidation process isotopic fractionation factors ranging between 1.003 and 1.049 have been reported (Cabral et al, 2010;Templeton et al, 2006;Reeburgh et al, 1997), fractionation factors for gas transport are scarce and calculations of CH 4 oxidation efficiencies for landfill cover soils predominantly have assumed α trans = 1, supposing that gas transport of CH 4 is dominated by advection (Liptay et al, 1998). To our knowledge, the isotopic fractionation factor for diffusion has so far not been determined for soils, but only for a glass bead (diameter 2-3 mm) porous medium with α diff = 1.0178 ± 0.001 (De Visscher et al, 2004).…”

Aerobic degradation processes in active landfill cells

“…The value of δ 13 C-CO 2 in the CO 2 that arises from CH 4 oxidation is primarily affected by fractionation during the CH 4 oxidation process, and less dependent on the relative 13 C abundance in the source CH 4 . The CO 2 arising from CH 4 oxidation is highly depleted in 13 C and therefore typically exhibits a highly negative value of δ 13 C-CO 2 (Börjesson et al, 2001;Cabral et al, 2010;Chanton et al, 2011a;Liptay et al, 1998a;Widory et al, 2012a). …”

Aerobic processes in landfill: an Australian field trial

“…Stable carbon isotopes have proven to be one of the most popular methods to estimate CH4 oxidation efficiency in cover soil systems over the last two decades (Bergmaschi and Harris, 1995, Chanton and Liptay, 2000, Liptay et al, 1998, Cabral and Capanema, 2012, Cabral et al, 2010, Widory et al, 2012. Liptay et al (1998) proposed that a landfill was an open system, as migrating gas reaches a branch point, where CH4 can be oxidised by methanotrophs or emitted to the atmosphere via gas transport, this can be expressed as:…”