Tundra Carbon Losses with Rapid Permafrost Thaw

Nonlinear CO2 flux response to seven years of experimentally induced permafrost thaw.

The Science

Frozen in permafrost soil, northern latitudes store almost twice as much carbon as is currently in the atmosphere. Rapid Arctic warming is expected to expose previously frozen soil carbon to microbial decomposition and increase carbon dioxide (CO2) release to the atmosphere. The impact of permafrost thaw on the CO2 balance is, however, unclear because warmer temperatures and nutrients released from thawing permafrost also increase plant growth and could offset CO2 losses. The scientists used an experimental warming manipulation to distinguish the effect of warmer air temperature from the effect of warmer soil and permafrost thaw on tundra ecosystem CO2 uptake and loss.

The Impact

Models and observations currently disagree over how Arctic warming will affect the CO2 balance of tundra ecosystems, and few studies combine warmer air temperatures and permafrost thaw to evaluate ecosystem CO2 balance. This work demonstrates that tundra CO2 uptake and loss responded much more strongly to permafrost thaw than to warmer air temperatures alone. Rapid permafrost thaw did initially stimulate CO2 uptake during the summer, but the effect leveled off with very deep thaw. In all years of the experiment, summer CO2 uptake was insufficient to offset year-round CO2 losses.

Summary

Seven years of experimental air and soil warming in tundra show that soil warming and permafrost thaw had a much stronger effect on carbon balance than air warming. Permafrost thaw initially stimulated greater summer CO2 uptake than CO2 loss; however, the initial increases were not sustained. As thaw continued to progress, summer CO2 uptake and CO2 loss leveled off. Leveling off CO2 uptake and release could be explained by slowing of plant growth and greater soil saturation as thaw caused the ground surface to collapse. The complex interactions between permafrost thaw, plant growth, and soil moisture could be captured mathematically by a quadratic relationship showing that the effect of thaw on CO2 uptake and loss changed over time. Models and measurements used to estimate CO2 losses during the winter found that the tundra was losing CO2 on an annual basis, even during those summers when thaw stimulated high plant growth and CO2 uptake.

Principal Investigator

Ted Schuur
Northern Arizona University
ted.schuur@nau.edu

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
daniel.stover@science.doe.gov

Funding

This work was supported by the Terrestrial Ecosystem Science program of the Office of Biological and Environmental Research, within the U.S. Department of Energy Office of Science (DE-SC0006982 and DE-SC0014085); National Science Foundation (NSF) CAREER program (#0747195); NSF Bonanza Creek LTER program (#1026415); NSF Office of Polar Programs (#1203777); and National Parks Inventory and Monitoring Program.

Related Links

References

Mauritz, M., R. Brancho, G. Celis, and J. Hutchings, et al. "Nonlinear CO2 flux response to 7 years of experimentally induced permafrost thaw". Global Change Biology 23 (9), 3646–3666  (2017). https://doi.org/10.1111/gcb.13661.