A comparison of active-layer and permafrost microbial community responses to freeze-thaw stress: Can microbes adapt to changing climates?

Background/Question/Methods Permafrost soils contain a large portion of the world’s terrestrial carbon (C) stocks, which could be vulnerable to decomposition as permafrost thaws in response to climate warming. Permafrost decomposition could be a significant climate-carbon feedback if the C currently sequestered in these soils is emitted as carbon dioxide or methane. However, the accuracy of our C efflux predictions is constrained by our inadequate understanding of the controls on decomposition in thawed permafrost. I propose three mechanisms that might affect decomposition of thawed permafrost by stimulating or hindering microbial activity. First, the rate, length and maximum temperature reached during the thaw may influence microbes’ ability to decompose permafrost C. To date, other permafrost thawing experiments have raised temperatures to values unrealistic even under the most extreme climate warming predictions. Second, nitrogen (N) availability may limit decomposition because microbes need N to produce proteins involved in both survival and decomposition. Finally, decomposition in thawed permafrost may be constrained by the adaptations of decomposer organisms themselves. Viable permafrost microbes have been selected to survive under continuously frozen conditions, but may not have the ability to survive thawing, which induces both osmotic and turgor stress and requires specific adaptations. Permafrost cores and active-layer soils were collected from Caribou Poker Creeks Research Watershed, AK (CPCRW) in July 2008 to capture low nitrogen concentrations in the active layer. We subjected permafrost and active layer soils to multiple freeze-thaw cycles to assess whether the physiology of permafrost microorganisms differs from active-layer microorganisms. We added inorganic N to a subset of cores to investigate microbial responses to freeze/thaw stress. We are analyzing microbial community structure and function using TRFLP to assess microbial responses to these stress events. Results/Conclusions Nitrogen additions doubled microbial respiration rates (p = 0.0088), suggesting that decomposition in thawed permafrost is likely constrained by nutrient availability. N is likely to limit the vulnerability of thawed permafrost soils to decomposition. Our preliminary analyses of the microbial responses indicate that permafrost communities differ in composition from active layer communities, and that microbial taxa respond over different time scales to freeze-thaw stress, consistent with theoretical models suggesting that microbes span a range of r- versus K- selected growth strategies. Despite the importance of arctic soils in the global C cycle, our ability to predict their response to global change is very limited. Our work suggests that several previously unexplored mechanisms will affect this critical climate-carbon feedback.