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

• Schuur Lab - Ecosystem Dynamics Research
• Data Interpretation: Carbon balance in an Arctic Warming Manipulation

References