Lower Soil Moisture and Deep Soil Temperatures in Thermokarst Features Increase Old Soil Carbon Loss After Ten Years of Experimental Permafrost Warming

The Science

Waterlogged permafrost soil can decrease old soil carbon decomposition deep in the soil layer, but when terrain dries, old soil carbon loss can increase up to 30 times. Old soil carbon has been stored for hundreds to thousands of years, and its release to the atmosphere has implications for climate change.

The Impact

As a way to separate plant and soil respiration from the carbon dioxide (CO₂) measured at the ecosystem scale, the researchers included environmental data, such as gross primary productivity, soil temperature, and soil moisture, that gave their model more information and helped them to better understand which environmental conditions contribute to higher soil decomposition. Accounting for plant and soil respiration at the ecosystem scale is important because higher soil decomposition in permafrost can increase the release of greenhouse gases to the atmosphere and worsen the impacts of climate change.

Summary

Almost half global terrestrial soil carbon (C) is stored in the northern circumpolar permafrost region, where air temperatures are increasing two times faster than the global average. As climate warms, permafrost thaws and soil organic matter becomes vulnerable to greater microbial decomposition. Long-term soil warming of ice-rich permafrost can result in thermokarst formation that creates variability in environmental conditions. Consequently, plant and microbial proportional contributions to ecosystem respiration may change in response to long-term soil warming. Natural abundance d¹³C and D¹⁴C of above- and belowground plant material and of young and old soil respiration were used to inform a mixing model to partition the contribution of each source to ecosystem respiration fluxes. The researchers employed a hierarchical Bayesian approach that incorporated gross primary productivity and environmental drivers to constrain source contributions. They found that long-term experimental permafrost warming introduced a soil hydrology component that interacted with temperature to affect old soil carbon respiration. Old soil carbon loss was suppressed in plots with warmer deep soil temperatures because they tended to be wetter. When soil volumetric water content significantly decreased in 2018 relative to 2016 and 2017, the dominant respiration sources shifted from plant aboveground and young soil to old soil respiration. The proportion of ecosystem respiration from old soil carbon accounted for up to 39% of ecosystem respiration and represented a 30-fold increase compared to the wet-year average. The study’s findings show that thermokarst formation may act to moderate microbial decomposition of old soil carbon when soil is highly saturated. However, when soil moisture decreases, a higher proportion of old soil carbon is vulnerable to decomposition and can become a large flux to the atmosphere. As permafrost systems continue to change with climate, the thresholds that may propel these systems from a carbon sink to a source must be understood.

Principal Investigator

Edward Schuur
Northern Arizona University
[email protected]

Program Manager

Daniel Stover
U.S. Department of Energy, Biological and Environmental Research (SC-33)
Environmental System Science
[email protected]

Funding

This work was supported by funding from the U.S. Department of Energy, Office of Biological and Environmental Research, Terrestrial Ecosystem Science (TES) Program, award DE-SC0006982 updated with DE-SC0014085 (2015–2018); National Science Foundation CAREER program, award 0747195; National Parks Inventory and Monitoring Program; National Science Foundation Bonanza Creek LTER program, award 1026415; and the Grant-In-Aid of Research from Sigma Xi, The Scientific Research Society.

References