Claudia Czimczik, Associate Professor, Department of Earth System Science, University of California, Irvine
Rapid warming and increases in precipitation in the Arctic may expose vast amounts of ancient, organic carbon (C) stored in permafrost to microbial decomposition and further amplify climate change. While much of our knowledge of Arctic biogeochemistry is based on data collected during the short growing season, autumn and winter are emerging as critical periods for C cycling that are turning the Arctic into a C source.
I describe our recent progress in closing the winter gap in Arctic C cycle studies. I introduce a newly developed sampler for collecting depth-resolved, time-integrated soil CO2 for radiocarbon (14C) analysis. These data allow us to understand the transit time of C in the soil system and quantify contributions from permafrost C that like fossil fuels are strongly depleted in 14C.
We continuously deploy the sampler in a widely distributed graminoid tundra system in northern Alaska for two years to elucidate which C pools fuel microbial activity during the polar night. The data reveal a large seasonal dynamic in the soil C pools accessed by microorganisms, from modern C consumed during the growing season to increasingly older C in autumn and winter. This suggests that non-rhizosphere microbes switch from decomposing dissolved organic C to locally available, older C pool as the active layer freezes. Thus, emissions in fall and winter, which are increasing across the Arctic, transfer soil C pools that have accumulated over millennia to the modern atmosphere and contribute to climate change.
We also study the impact of a changing precipitation regime on permafrost C by taking advantage of a 25-year snow augmentation experiment. Coupling the new sampler with bulk soil analyses, incubations, and soil thaw and land-atmosphere C flux observations demonstrates that additional snow causes active layer deepening, compaction, and subsidence, accelerates nitrogen cycling, and exposes significant amounts of ancient permafrost C to microbial decay.
Together, these studies begin to fill the winter gap in our understanding of the rapidly changing Arctic, highlight the importance of autumn and winter for permafrost C loss, and pave the way for a regional-scale assessment of permafrost C emissions.