Greenhoue gas flux over a 50-year post permafrost thaw gradient. Decomposition of soil organic carbon from the Swedish tussock tundra.

Abstract

As the globe is warming the extent of permafrost drastically decreases in the high latitudes. Upon thaw the stored soil organic carbon (SOC) undergoes rapid decomposition and is partly released as trace gases to the atmosphere. There are few studies that focus exclusively on post permafrost sites even though we estimate that up to 81 % of the present permafrost will thaw before the end of the century. It is of utmost importance to understand how the permafrost will respond to the future warming for us to be able to predict climate change with any precision. This thesis aims to highlight the importance of studying post permafrost regions and to discuss similarities and differences between three tussock tundra communities in northern Sweden. Soil samples were collected from three sites along a 50-year post permafrost gradient located in the Swedish tussock tundra near Abisko. Soil were collected from three soil pits at each site and separated based on which soil horizon they were collected from. The samples were then further divided into four treatment groups (cold aerobic, cold anaerobic, warm aerobic and warm anaerobic) and incubated for 121 days at constant temperatures (5˚ C and 15 ˚ C). Gas samples were collected from the headspace of the incubated soil jars and analyzed by gas chromatography (GC). My results provide evidence that GHG flux increases over time in post permafrost tussock tundra sites. The site where permafrost thawed the longest time ago have almost an order of magnitude higher greenhouse gas (GHG) flux compared to the other two sites. Although soil properties were similar the flux was significantly higher in the site were permafrost thawed first. This thesis also provides further evidence that microbial decomposition is most effective in warm aerobic conditions. The GHG flux decreased with both time and depth for all sites and horizons. However, methane (CH4) flux increased rapidly towards the end of the incubation period in the organic rich A-horizons of the warm temperature treatment. The results can be used as an indicator of the complexity of SOC decomposition. The effect of warming in the high latitudes will likely lead to increased GHG flux. As the thawing process are complex and interacts with several other factors, in reality the GHG flux increases might be offset by other processes not fully disclosed in this thesis. This thesis thoroughly examines how GHG flux responds to experimental warming under fixed conditions.