Characterisation of methane net ecosystem exchange from a Scottish peatland.

Characterisation of methane net ecosystem exchange from a Scottish peatland. Carole Helfter, Julia Drewer, Pete Levy, Ian Leith, Sarah Leeson, Rebecca McKenzie, Mark Sutton, Eiko Nemitz and Ute Skiba. Centre for Ecology and Hydrology, Bush Estate, Penicuik, EH26 0QB, Scotland, UK. Continuous monitoring of net ecosystem exchange (NEE) of methane (CH4) by the eddy-covariance (EC) technique began in August 2010 at Auchencorth Moss (55° 47' N, 3° 12‘ E), a low-lying, drained acid peatland 20 km South-West of Edinburgh, Scotland, UK. The site is an open moorland with extensive uniform fetch of blanket bog to the south, west and north. The vegetation present within the flux measurement footprint comprises mixed grass species, heather and substantial areas of moss species (Sphagnum spp. and Polytrichum spp.). Over the course of the first year of measurements, the site was found to be a net source of methane with emissions amounting to 2.3 g m-2 y-1. This corresponds to a global warming potential of 58 g CO2-eq m-2 y-1, approximately half the measured net sequestration of CO2 at the site. Emissions were recorded even during a prolonged period of complete snow cover at the site. CH4 fluxes at Auchencorth Moss appear to be turbulence-driven with a strong correlation to friction velocity (u*) and, to a lesser extent, to soil temperature and water table depth. This dependence on u* was observed for all wind directions and net deposition was found primarily for u* < 0.1 m s-1. Eddy-covariance data differed from static chamber measurements taken at the site, albeit outside the estimated footprint of the EC system, by up to two orders of magnitude. These discrepancies could be explained by spatial heterogeneity and turbulence-driven venting of CH4 accumulated within the undergrowth, which would not be captured by the chamber technique. Whilst further work needs to be carried out to strengthen the interpretation of these results, it seems plausible that the eddy-covariance approach might reflect the site’s net exchange of methane but not necessarily capture its low-level source/ sink dynamics