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

Abstract

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; \(\Delta\delta\hbox{D}_{{\rm water}-{\rm CH}_4}\cong 220\)‰) 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; \(\Delta\delta\hbox{D}_{{\rm water}-{\rm CH}_4}\cong300\)‰), 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.