Stable Isotope Evidence for the Effectiveness of Methanotrophy and the Importance of Plant-Mediated Transport of Methane in Blanket Bogs

Stable carbon isotope values and rates of methane flux were determined monthly at a blanket bog in Elan Valley, Wales UK during the summer of 2003. The rate and δ13C value of methane flux varied between months as a result of changes in soil methanogenesis and methanotrophy caused by shifts in soil temperature and water-table levels. During a period of low water-table levels in July 2003, decreases in flux rates at both Sphagnum- and sedge-rich areas were accompanied by 3 to 4 ‰ increases in δ13C values of methane emissions. The flux rate from Sphagnum-dominated surfaces consistently was an order of magnitude less than that from sedge-rich areas. Despite differences in emission rates, the δ13C values of methane flux were remarkably similar from both types of vegetated surfaces during each sampling period. Methane flux had very low δ13C values (-90.1 to -82.2 ‰) during all months, suggesting that the smaller quantity of methane released from the Sphagnum-dominated surfaces was via the few vascular plants that were present rather than by diffusion through the soil profile where methanotrophy would have caused 13C-enrichment of the residual escaping methane. The unusually negative δ13C values of methane emissions from the blanket bog appear to be caused by the combined effects of a predominance of methanogenesis via CO2 reduction linked to H2 oxidation in soils and stable isotope fractionation during transport through vascular flora. Comparison of δ13C values of methane in flux and within pore water at the top of the anaerobic zone indicates that a fractionation of approximately 8 to 12 ‰ occurs during plant-mediated transport at this particular site. The results of this study suggest that the δ13C value (-60 ± 5 ‰) currently assumed for methane emissions from peatlands in models of the global methane cycle may be too positive, possibly resulting in the underestimation of methane flux from 13C-enriched sources.