Lansdown, J.M., 1992, The carbon and hydrogen stable isotope composition of methane released from natural wetlands and ruminants: University of Washington, Seattle, Ph.D. dissertation, 226 p., illust., maps.
The [delta]13rmC of CH4 emitted from the tropical Amazon river floodplain, temperate peat bogs in Washington and Minnesota, and the arctic Alaskan tundra was 59, 73, 66, and 65‰, respectively. The [delta]D of CH4 from these sites was 294, 308, 339, and 391‰, respectively, and a linear relationship was observed between the [delta]D of CH4 and soil water. A 13C balance between CH4, CO2, and soil organic matter indicated a higher percentage of CH4 production via methyl conversion at the Amazon floodplain than at the other wetland sites and that the anoxic CO2 flux was 1.5 to 2.0 times the CH4 flux. The 13C balance provided greater constraint on the anoxic CO2 flux than calculations based on soil water gradients. An in situ value of 0.774 for the hydrogen kinetic isotope effect during microbial CH4 oxidation was estimated from the increase in the [delta]13rmC of CH4 and [delta]D of CH4 in flux samples from the Amazon site. The average [delta]13rmC of CH4 released from an acidic peat bog in Washington state (pH = 3.5) was 73‰, lower than previously measured at freshwater wetland sites. Soil incubations with 14C-labeled CO2 and acetate substrates showed that CO2 reduction accounted for essentially all methane production in the bog. An in situ value of 0.933 for the carbon kinetic isotope effect for CO2 reduction was calculated from the [delta]13rmC of the CH4 flux and soil water CO2 The [delta]13rmC and [delta]D of CH4 emitted from ruminants was measured and averaged 63 and 404‰, respectively. CO2 reduction accounted for ~70% of rumen CH4 production based on the change in the [delta]13rmC and [delta]D of rumen CH4 vs. time during normal conditions and after the addition of deuterated water to the rumen. These results contrast the dogma in the literature that CO2 reduction accounts for essentially all CH4 production in the rumen. A global budget for the [delta]D of CH4 was constructed, based on the above wetland and ruminant measurements. The global average [delta]D of the wetland and ruminant CH4 sources were calculated to be 322 and 316‰, respectively. The average [delta]D of the global CH4 source was estimated at 258‰. Coexisting [alpha]rmC is an isotopic tracer of CO2 reduction but not methyl conversion pathway. The [delta]13rmC of CH4 is controlled by the CH4 production pathway; the proportion of the CH4 production pathways can be estimated from the [delta]13rmC of CH4 in conjunction with the [delta]13rmC of CO2 and soil organic matter and in situ values for the kinetic isotope effect for CO2 reduction and methyl conversion. The [delta]D of CH4 is controlled by the [delta]D of formational water; secondary controls on the[delta]D of CH4 are the CH4 production pathway and possibly H2 concentration.
Theses and Dissertations