Methane stable isotope distribution at a Carex dominated fen in north central Alberta

The methane stable isotope distribution was characterized at a Carer domina ted fen in boreal Alberta, Canada, over three growing seasons to examine me thane production, oxidation, and transport to the atmosphere; processes whi ch are strongly tied to emergent vegetation and the influence of the rhizos phere (upper 20 cm of peat in this system]. At times when standing floodwat er was present, delta(13)C values of emitted methane averaged -63.6 +/- 2.3 , -66.3 +/- 1.6, and -65.4 +/- 1.3 parts per thousand for the 1994, 1995, a nd 1996 seasons, respectively. These emissions were significantly C-13 depl eted relative to the belowground methane dissolved in rhizospheric pore wat ers, indicating that gas transport in Carer is dominated by passive diffusi on. The rhizosphere was (CH4)-C-13 enriched relative to depths below the rh izosphere, consistent with the occurrence of root associated methane oxidat ion, preferential mobilization of (CH4)-C-13, and a relatively greater role of acetate fermentation type methane production. Dual isotope tracers, del ta(13)C and delta D, help qualify the role of each of these processes and a id in describing the distribution of production pathways, CO2 reduction, an d acetate fermentation. Inverse trends in delta(13)C-CH4 and delta D-CH4 de pth profiles are consistent with an interpretation suggesting an evolution toward methane production by CO2 reduction with increasing depth. A shift i n production mechanisms appears to be the dominate process affecting the st able isotope distribution below 10 cm in the peat column, while oxidation a nd transport isotope effects are dominant above 10 cm. To test several hypo theses regarding the effects of transport, oxidation, and production on met hane isotope distributions, we also present measurements from sites fertili zed and sites devegetated (continually clipped) over the 3 year period. Rem oval of vegetation quickly halted rhizospheric methane oxidation and gas tr ansport while gradually increasing the relative role of CO2 reduction in ne t methane production as labile substrate was used up. The fertilizer treatm ent increased above ground biomass and primary productivity but had little effect on the stable isotope distribution. A mass balance calculation indic ates that methane emissions are attenuated 0-34% by methane oxidation in th e rhizosphere. Results showed little seasonal variability other than during a period when floodwater levels dropped below the peat surface resulting i n the (CH4)-C-13 enrichment of methane emissions.