Department Seminar: Claudia Czimczik

Title: The face of a permafrost-free Arctic

Abstract: Rapid warming and increases in precipitation in the Arctic may expose vast amounts of ancient, organic carbon (C) frozen in permafrost to microbial decomposition and further amplify climate change. Yet the magnitude and timing of the permafrost C feedback to climate change remains very uncertain. To better understand the changing Arctic, we have been studying the impact of climate change on soil C cycling and greenhouse gas (GHG) emissions at several long-term climate manipulation experiments and across landscape gradients, and expanded measurement capabilities into the polar night. Our year-round work reveals that soil microorganisms access a greater variety of C pools than expected from laboratory incubation experiments and field studies conducted during the growing season. A shift from modern C consumed during the growing season to increasingly older C in autumn and winter suggests that non-rhizosphere microorganisms 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.

Furthermore, combining analyses of deep soil cores with decades of thaw depth measurements suggest that active layer deepening may result in a pulse of GHG emissions that may not be sustained with ongoing thaw and challenge the current representation of permafrost C loss and GHG emissions in biogeochemical models.

Studying climate manipulations experiments in the continuous permafrost zone, we find that soil moisture strongly influences the vulnerability of permafrost C by shifting preservation mechanisms from cold to waterlogged, and increasing vegetation productivity, which is strongly coupled to soil microbial activity and GHG emission and consumption. With further permafrost thaw, however, we expect greater water and C connectivity, and argue that future progress in our understanding of the Arctic urgently requires a landscape approach and comprehensive view of land-water-atmosphere interactions.