Methane emission rates from an ombrotrophic mire show marked seasonality which is independent of nitrogen supply and soil temperature

Methane fluxes were measured in an area of ombrotrophic mire at the Migneint in North Wales. Measurements were made at approximately monthly intervals between August 1993 and August 1995. A marked seasonal change in emission rates was noted. Peaks of methane emission were observed in August 1993 (6.2 mg m−2 d−1), September 1994 (6.0 mg m−2 d−1 and June 1995 (5.5mg m−2 d−1. Minimum emission rates were recorded in March 1994 (0.8 mg m−2 d−1 and March 1995 (1.4 mg m−2 d−1. Nitrogen, in the form of ammonium and/or nitrate, was applied to plots on the mire surface. Between August 1993 and July 1994 application rates were equivalent to a 50 or 100% increase in N deposition. From August 1994 to August 1995 the application rates were equivalent to a 500 or 1000% increase in N deposition. These applications of nitrogen had no effect on the methane emission rates at any date, with the exception of the measurement from November 1994. No correlation was found between methane flux and either soil temperature or water table. We suggest that the state of the vegetation at the site may be important, either through the role of the plants in transport of methane, or as the source of the organic carbon that is the substrate for methanogenesis.