Methane dynamics in the subarctic tundra: combining stable isotope analyses,plot- and ecosystem-scale flux measurements

[摘要] Methane (CH₄) fluxes were investigated in a subarctic Russian tundrasite in a multi-approach study combining plot-scale data, ecosystem-scaleeddy covariance (EC) measurements, and a fine-resolution land coverclassification scheme for regional upscaling. The flux data as measured bythe two independent techniques resulted in a seasonal (May–October 2008)cumulative CH₄ emission of 2.4 (EC) and 3.7 g CH₄ m⁻² (manualchambers) for the source area representative of the footprint of the ECinstruments. Upon upscaling for the entire study region of 98.6 km²,the chamber measured flux data yielded a regional flux estimate of 6.7 g CH₄ m⁻² yr⁻¹. Our upscaling efforts accounted for the largespatial variability in the distribution of the various land cover types(LCTs) predominant at our study site. Wetlands with emissions ranging from34 to 53 g CH₄ m⁻² yr⁻¹ were the most dominantCH₄-emitting surfaces. Emissions from thermokarst lakes were an orderof magnitude lower, while the rest of the landscape (mineral tundra) was aweak sink for atmospheric methane. Vascular plant cover was a key factor inexplaining the spatial variability of CH₄ emissions among wetlandtypes, as indicated by the positive correlation of emissions with the leafarea index (LAI). As elucidated through a stable isotope analysis, thedominant CH₄ release pathway from wetlands to the atmosphere wasplant-mediated diffusion through aerenchyma, a process that discriminatesagainst ¹³C-CH₄. The CH₄ released to the atmosphere waslighter than that in the surface porewater, and δ¹³C in theemitted CH₄ correlated negatively with the vascular plant cover (LAI).The mean value of δ¹³C obtained here for the emitted CH₄,−68.2 ± 2.0 ‰, is within the range of values fromother wetlands, thus reinforcing the use of inverse modelling tools to betterconstrain the CH₄ budget. Based on the IPCC A1B emission scenario, atemperature increase of 6.1 °C relative to the present day hasbeen predicted for the European Russian tundra by the end of the 21stCentury. A regional warming of this magnitude will have profound effects onthe permafrost distribution leading to considerable changes in the regionallandscape with a potential for an increase in the areal extent ofCH₄-emitting wet surfaces.

[学科分类] 地球化学与岩石