CH4 emission from a hollow-ridge complex in a raised bog:: The role of CH4 production and oxidation

The aim of this study was to correlate magnitude and controls of CH4 fluxes with the microtopography and the vegetation in a hollow-ridge complex of a raised bog. High CH4 emission rates were measured from hollows and mud-bottom hollows, while hummocks consumed atmospheric CH4 at a low rate. The highest emissions were measured from plots with Eriophorum vaginatum and Scheuchzeria palustris. CH4 emission ceased after Scheuchzeria had been clipped below the water table, indicating the importance of this aerenchymatic plant as a conduit for CH4. Peat in the upper catotelm of hollows was younger and less decomposed than in hummocks. Potential CH4 production in vitro was higher and the methanogenic association was better adapted to higher temperatures in hollow than in hummock peat. Higher temperatures in hollows resulted in a stronger CH4 source in hollows than in hummocks. Negative fluxes from hummocks indicated that even in wetlands methanotrophic bacteria exist that are able to oxidize CH4 at atmospheric mixing ratios, and that oxidation controls CH4 emission completely. The CH4 mixing ratio was low in the acrotelm, but it increased within the catotelm. Comparing fluxes measured in static chambers with fluxes calculated from the porewater CH4 profiles it was deduced that the zone of methane oxidation was located close to the water table. In hollows, CH4 production at in situ temperature was far higher than emission into the atmosphere, corresponding to an oxidation rate of nearly 99%. The CH4 flux between the catotelm and the acrotelm of hollows was also higher than the emission, indicating the importance of CH4 oxidation in the aerobic acrotelm, too. CH4 microprofiles showed that CH4 oxidation in mud-bottom hollows was confined to the topmost 2 mm, and that in Sphagnum-covered hollows CH4 oxidation occurred at the lower edge of green Sphagnum-parts.