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Coupling Stable Isotope Analysis with Gas Push‐Pull Tests to Derive In Situ Values for the Fractionation Factor α ox Associated with the Microbial Oxidation of Methane in Soils
Author(s) -
Gebert Julia,
Streese-Kleeberg Jan
Publication year - 2017
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj2016.11.0387
Subject(s) - isotopologue , fractionation , anaerobic oxidation of methane , soil water , methane , isotope fractionation , stable isotope ratio , chemistry , environmental chemistry , isotope , leachate , soil test , soil science , environmental science , chromatography , organic chemistry , quantum mechanics , molecule , physics
Core Ideas: Methane CH 4 isotopologue fraction was measured in situ for the first time. α ox varied strongly with oxidation rate. α ox varied between landfills and between individual points on the same landfill. α ox varied with spatial and temporal variability of soil properties. Our findings limit application of stable isotopes for CH 4 oxidation quantification. A prerequisite for the application of stable isotope fractionation for the quantification of the methane (CH 4 ) oxidation efficiency of landfill covers is that the fractionation factor α ox is known or can be estimated with adequate accuracy. So far, α ox has only been determined in laboratory experiments. In this study, α ox was determined under in situ conditions in the field by coupling two independent methods, gas push‐pull tests and stable isotope analysis, to assess biological fractionation of CH 4 isotopologues in landfill cover soils. On six landfills with nine points of investigation, 22 measurements were performed, covering a wide range of environmental conditions, such as soil temperature and moisture and observed oxidation rates. Values for α ox varied between near 1, indicating little fractionation, and 1.151. Correlation of α ox with the CH 4 oxidation rate found by gas push‐pull tests revealed a clear asymptotic relationship, with low rates being associated with high values for α ox and high rates resulting in little fractionation. Values for α ox varied between the different landfills and between the individual points of investigation on the same landfill. The latter is assumed to reflect the spatial variability of methanotrophic activity due to spatial variability in soil moisture and hence air‐filled porosity as well as the spatial variability of gas fluxes. Significant variation of α ox was observed for the same sampling point, presumably reflecting the temporal variability of factors influencing methanotrophic activity. These effects could include seasonally changing environmental conditions (e.g., soil temperature and moisture) but also the temporal variability of gas fluxes through the landfill soil cover, changing exposure of methanotrophs to CH 4 and oxygen and hence their activity. The quantification of the CH 4 oxidation efficiency using fractionation of stable isotopes is very sensitive to α ox . Assuming a value constant in time and space and transferring this value from laboratory experiments to field settings entails significant uncertainty regarding the quantification of CH 4 oxidation.