Tracing the New Carbon Cycle From Plant Inputs to Microbial Outputs Across an Arctic Permafrost Thaw Gradient

Abstract

Arctic systems are experiencing some of the most rapid warming due to climate change, causing permafrost C stocks to thaw and become available for decomposition. Since these systems store approximately 1.7 times the amount of C currently in the atmosphere, its decomposition and release as CO2 and CH4 could have profound effects as a positive feedback to climate change. However, the net effect of permafrost thaw depends not just on decomposition of the old C, but also on the changes taking place in the “new C-cycle” controlled by plant C-uptake and decomposition of their inputs to the soil. In many places, plant communities are expected to become more productive as temperatures warm which may increase C uptake from the atmosphere. Changes in plant community composition may also alter microbial community composition and the decomposability and input rates of litter, resulting changes to C storage versus production of CO2 and CH4. Here we investigate these processes in a thawing permafrost peatland. We found that plant community composition plays an important role in shaping phyllosphere microbial communities but environmental conditions are more important to shaping rhizosphere communities. Plant communities were especially important in shaping methanogenic and methanotrophic communities, which may have important implications for CH4 production. Plant community change also resulted in increased rates of C and nutrient inputs to the soil due to a transition from perennial to annual communities. These litter inputs are decomposed most rapidly in post-thaw areas and drive an increased rate of CH4 production from both the litter itself and the soil organic material already present. Overall, we found that permafrost thaw in peat-dominated systems leads to an increasing rate of C-cycling (larger inputs as well as outputs) driven in large part by changes in plant community composition and their impacts on microbial community and decomposition. Plant community characteristics may be especially important in determining the pathways to CH4 production as well as the timing and total quantities produced. We suggest that shifts in the plant community after permafrost thaw in peat-dominated systems result in major changes to the “new C cycle” which may have as important an impact on climate change feedbacks as decomposition of thawed permafrost itself.