Hydrogen Concentration and Stable Isotopic Composition of Methane in Bubble Gas Observed in a Natural Wetland

Bubble gas samples were collected at three different vegetation sites and two different depths (surface and 40 cm) in a natural wetland, Mizorogaike in Kyoto city, to investigate hydrogen concentration and δD and δ¹³C values of CH₄. Hydrogen concentration in bubble gas varied from 1 to 205 ppm, and that collected during summer was higher than that during winter. Bubble samples collected at 40 cm at sphagnum site usually showed the lowest H₂ concentration among the samples collected at the three sites and two depths on the same day. The lowest H₂ concentration observed at 40 cm at sphagnum site was similar to that expected for environmental water in which H₂ producer and consumer need to assemble for free energy requirement. Low δ¹³C and high δD (relatively small hydrogen fractionation; ‰) were observed in CH₄ collected at a deeper (40 cm) layer of sphagnum site during winter, when H₂ concentration was low (typically 2–4 ppm). On the other hand, CH₄ in the bubble samples collected during summer showed high δ¹³C and low δD (relatively large hydrogen fractionation; ‰), when H₂ concentration was high. Carbon and hydrogen isotope fractionation during CH₄ production were variable, possibly depending on the H₂ concentration and the production rate. Difference in enzymatic reaction and magnitude of hydrogen isotope exchange among water, CH₄, and H₂ may cause the variation in isotope fractionation during CH₄ production.