Trace gas exchange in a high-arctic valley 1. Variations in CO2 and CH4 flux between tundra vegetation types

Ecosystem exchanges of CO2 and CH4 were studied by chamber techniques in fi ve different vegetation types in a high arctic valley at Zackenberg, NE Gre enland. The vegetation types were categorized as Cassiope heath, hummocky f en, continuous fen, grass land and Salix arctica snowbed. Integrated daytim e fluxes for the different vegetation types of the valley showed that the f en areas and the grassland, were significant sources of CH4 with a mean eff lux of 6.3 mg CH4 m(-2) h(-1) and sinks for CO2, with almost -170 mg CO2 m( -2) hr(-1). The heath and snowbed areas had much lower carbon sequestration rates of about -25 mg CO2 m(-2) hr(-1) and were also sinks for CH4. Methan e emissions from the valley dominated in the hummocky fens. Computation of area integrated mean daytime flux values across all vegetation types of the entire valley bottom revealed that it was a sink of CO2 in the order of -9 6+/-33 mg CO2 m-2 hr-1 and a source of 1.9+/-0.7 m(-2) CH4 m(-2) hr(-1). Th ese values were in accordance with eddy correlation measurements reported e lsewhere in this issue and reflect a high-carbon exchange despite the high arctic location. In the fens, where the water table was at or above the soi l surface, methane emissions increased with net ecosystem CO2 flux. In plac es with the water table below the soil surface, such as particularly in the hummocky parts of the fen, oxidation tended to become the dominant control ling factor on methane efflux.