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

University essay from Göteborgs universitet/Institutionen för geovetenskaper

Author: Johan Martinelli; [2020-02-11]

Keywords: greenhouse gas; permafrost;

Abstract: As the globe is warming the extent of permafrost drastically decreases in the high latitudes. Upon thawthe stored soil organic carbon (SOC) undergoes rapid decomposition and is partly released as trace gasesto the atmosphere. There are few studies that focus exclusively on post permafrost sites even though weestimate that up to 81 % of the present permafrost will thaw before the end of the century. It is ofutmost importance to understand how the permafrost will respond to the future warming for us to beable to predict climate change with any precision. This thesis aims to highlight the importance ofstudying post permafrost regions and to discuss similarities and differences between three tussocktundra communities in northern Sweden. Soil samples were collected from three sites along a 50-yearpost permafrost gradient located in the Swedish tussock tundra near Abisko. Soil were collected fromthree soil pits at each site and separated based on which soil horizon they were collected from. Thesamples were then further divided into four treatment groups (cold aerobic, cold anaerobic, warmaerobic and warm anaerobic) and incubated for 121 days at constant temperatures (5˚ C and 15 ˚ C). Gassamples were collected from the headspace of the incubated soil jars and analyzed by gaschromatography (GC). My results provide evidence that GHG flux increases over time in post permafrosttussock tundra sites. The site where permafrost thawed the longest time ago have almost an order ofmagnitude higher greenhouse gas (GHG) flux compared to the other two sites. Although soil propertieswere similar the flux was significantly higher in the site were permafrost thawed first. This thesis alsoprovides 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 thewarm temperature treatment. The results can be used as an indicator of the complexity of SOCdecomposition. The effect of warming in the high latitudes will likely lead to increased GHG flux. As thethawing process are complex and interacts with several other factors, in reality the GHG flux increasesmight be offset by other processes not fully disclosed in this thesis. This thesis thoroughly examines howGHG flux responds to experimental warming under fixed conditions.

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