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Quantification of Microbial Methane Oxidation in an Alpine Peat Bog

Institute of Biogeochemistry and Pollutant Dynamics, ETH Zurich, Universitätsstrasse 16, 8092 Zurich, Switzerland

View larger version (17K): [in this window] [in a new window]   FIG. 1. Gas-phase CH4 concentration profiles at three hummocks (H1, H2, and H3) before gas push-pull tests. Note the logarithmic scale of the x axis. The depth of the water table measured from the peat surface in each hummock is indicated by a dashed line.

View larger version (34K): [in this window] [in a new window]   FIG. 2. Breakthrough curves of He, Ne, and CH4 at the injection–extraction point during three pairs of gas push-pull tests (GPPTs); "a" designates curves from active GPPTs, and "i" designates curves from inactive tests. The part of each GPPT test pair where CH4 consumption was discernible is shown in more detail. Note the different scales of both axes. Noble gases could not be measured for the entire extraction period in GPPT 3 as the analytical method available at the time was less sensitive. In GPPT 1 He dropped below the quantification limit after 1.4 h, and Ne after 2.1 h.

View larger version (17K): [in this window] [in a new window]   FIG. 3. Plot for the determination of apparent first-order rate constants k for CH4 oxidation from three pairs of gas push-pull tests (GPPTs). Residence time is the total time the gas stayed in the subsurface (see "Materials and Methods"). Slopes from linear regressions (solid lines) are apparent first-order rate constants k. C*(CH4)a and C*(CH4)i are relative CH4 concentrations from an active and the corresponding inactive GPPT, respectively. For better readability, early data from GPPT 1 and 2 are not shown. First-order rate constants k are given with 95% confidence intervals.

View larger version (18K): [in this window] [in a new window]   FIG. 4. Stable carbon isotope ratios in CH4 over time during three pairs of gas push-pull tests (GPPTs) at the injection–extraction point; "a" designates active GPPTs, "i" designates inactive GPPTs.

View larger version (21K): [in this window] [in a new window]   FIG. 5. Measured and simulated breakthrough curves at the injection–extraction point during gas push-pull test 3i. Symbols represent measured data, lines represent simulations (van Genuchten water-retention parameters n = 4, = 0.06 cm–1, and residual saturation 0.10).

View larger version (14K): [in this window] [in a new window]   FIG. 6. Simulated CH4 and C2H2 concentrations at a 30-cm radius from the injection point over the entire duration of gas push-pull test 3i. Arrows indicate the time when each gas reached a concentration of 0.01 mL L–1.