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

• Published in: Biogeosciences
• Main Authors: Marushchak, M. E., Friborg, T., Biasi, C., Herbst, M., Johansson, T., Kiepe, I., Liimatainen, M., Lind, S. E., Martikainen, P. J., Virtanen, T., Soegaard, H., Shurpali, N. J.
• Format: Text
• Language: English
• Published: 2018
• Subjects: permafrost; Subarctic; Thermokarst; Tundra
• Online Access: https://doi.org/10.5194/bg-13-597-2016; https://www.biogeosciences.net/13/597/2016/

Methane (CH 4 ) fluxes were investigated in a subarctic Russian tundra site in a multi-approach study combining plot-scale data, ecosystem-scale eddy covariance (EC) measurements, and a fine-resolution land cover classification scheme for regional upscaling. The flux data as measured by the two independent techniques resulted in a seasonal (May–October 2008) cumulative CH 4 emission of 2.4 (EC) and 3.7 g CH 4 m −2 (manual chambers) for the source area representative of the footprint of the EC instruments. Upon upscaling for the entire study region of 98.6 km 2 , the chamber measured flux data yielded a regional flux estimate of 6.7 g CH 4 m −2 yr −1 . Our upscaling efforts accounted for the large spatial variability in the distribution of the various land cover types (LCTs) predominant at our study site. Wetlands with emissions ranging from 34 to 53 g CH 4 m −2 yr −1 were the most dominant CH 4 -emitting surfaces. Emissions from thermokarst lakes were an order of magnitude lower, while the rest of the landscape (mineral tundra) was a weak sink for atmospheric methane. Vascular plant cover was a key factor in explaining the spatial variability of CH 4 emissions among wetland types, as indicated by the positive correlation of emissions with the leaf area index (LAI). As elucidated through a stable isotope analysis, the dominant CH 4 release pathway from wetlands to the atmosphere was plant-mediated diffusion through aerenchyma, a process that discriminates against 13 C-CH 4 . The CH 4 released to the atmosphere was lighter than that in the surface porewater, and δ 13 C in the emitted CH 4 correlated negatively with the vascular plant cover (LAI). The mean value of δ 13 C obtained here for the emitted CH 4 , −68.2 ± 2.0 ‰, is within the range of values from other wetlands, thus reinforcing the use of inverse modelling tools to better constrain the CH 4 budget. Based on the IPCC A1B emission scenario, a temperature increase of 6.1 °C relative to the present day has been predicted for the European Russian tundra by the end of the 21st Century. A regional warming of this magnitude will have profound effects on the permafrost distribution leading to considerable changes in the regional landscape with a potential for an increase in the areal extent of CH 4 -emitting wet surfaces.