Permafrost Carbon and Climate Feedbacks in a Warmer World.

Approximately 1670 Pg (billion tons) of soil carbon are stored in the northern circumpolar permafrost zone, more than twice as much carbon than currently contained in the atmosphere.  Permafrost thaw, and the microbial decomposition of previously frozen organic carbon, is considered one of the most likely positive feedbacks from terrestrial ecosystems to the atmosphere in a warmer world.  Yet, the rate and form of release is highly uncertain but crucial for predicting the strength and timing of this carbon cycle feedback this century and beyond.  Here we report results from a new ecosystem warming manipulation —the Carbon in Permafrost Experimental Heating Research (CiPEHR) project—where we increased air and soil temperature, and degraded the surface permafrost.  We used snow fences coupled with spring snow removal to increase deep soil temperatures and thaw depth (winter warming) and open top chambers to increase growing season air temperatures (summer warming).  Winter warming increased depth-integrated soil temperature by 1.5^o C, which resulted in a 10% increase in thaw depth that persisted into the following winter.  Surprisingly, the 2 kg C m^-2 contained in the additional thawed soil in the winter warming plots did not result in significant changes in cumulative growing season respiration, which may have been inhibited by soil saturation at the base of the active layer.  However, the limited effect of deep soil warming during the growing season contrasted with the large increase in winter respiration, which in sum doubled the net loss of carbon dioxide to the atmosphere on an annual basis.  While most changes to the abiotic environment at CiPEHR were driven by winter warming, summer warming (mainly air) effects on plant and soil processes resulted in 20 percent increases in both gross primary productivity and growing season ecosystem respiration and significantly altered the age and sources of carbon dioxide respired from this ecosystem.  These results demonstrate the vulnerability of organic C stored in near surface permafrost to increasing temperatures, but also highlight the interactive effect of moisture for determining ecosystem carbon balance in response to climate change.