The permafrost region is the part of the Arctic where soils can remain frozen all year round, and this region stores huge amounts of soil organic carbon. The seasonal freezing and thawing of the ground, combined with the effect of permanently frozen ground below is causing the formation of periglacial landforms that only occur in cold environments.

Ice wedge at Herschel Island. Photo: Matthias Siewert

Unique spatial variability

In an article in Global Biogeochemical Cycles, Matthias Siewert from Umeå University and colleagues from Stockholm University, the Alfred-Wegener-Institute in Germany and the University of Vienna in Austria show how the formation of these landforms generates a unique spatial variability in the soils of the region.

The researchers looked at a large number of soils and sampled them to quantify how their variability changes at different spatial scales, from the centimeters to landscape level. The field data was collected on Herschel Island, of the Canadian coast in the Arctic Ocean.

They show that at the fine scale from centimeters to meters, a soil process called cryoturbation causes the formation of small landforms such as hummocks and nonsorted circles. Cryoturbation means that soil material is moving in response to annual freezing and thawing of the soil. This process is slow and it takes decades to move material several centimeters. But over thousands of years, this dramatically mixes and sorts material in the ground.

At the medium or terrain scale, from meters to tens of meters, variability of these soils is caused by a different landform called ice-wedges. Ice-wedges form during cold winters when the ground freezes and contracts, leaving meters deep cracks in the ground that are filled by snow and water. Over time, this accumulates vertically and forms wedge-shaped ice-bodies that form a polygonal pattern visible from a bird perspective. The formation of these ice-wedges contributes to a further increase in spatial variability of the soil organic carbon and ground ice. Finally, soil also changes over hundreds of meters along slopes, when permafrost thaws over large scale or when valleys form.

“The importance of this study is to point out, that permafrost soils are among the most variable soils on Earth. This needs to be considered when mapping and predicting the potential feedback of permafrost regions in the global carbon cycle,” says Matthias Siewert, post doctor at the Department of Ecology and Environmental Sciences at Umeå university.

A large community effort

A second article published in Science Advances represents a large community effort to better understand soil organic carbon variability across the entire permafrost region. The researchers, including Matthias Siewert, combined a large amount of soil profiles from studies over the last years with a spatial modeling approach. This helped to identify key factors controlling the distribution of soil organic carbon.

The study provides a first truly pixel based estimate of soil organic carbon in this area. The team, led by Umakant Mishra from the Sandia National Laboratory in USA, estimated that 1 014 petagram  organic carbon (one petagram is a billion metric tons) is stored in the top three meters of northern hemisphere permafrost-region soils. Although the total amount is slightly lower than earlier estimates, this new assessment suggests more carbon is stored within a meter of the surface and, thus, is more vulnerable to top-down warming.

Help to improve modeling

The finding will help to improve modeling the response of permafrost-affected soils to climate change.

 “Both studies are very complementary. One shows that permafrost soils are tremendously variable looking at centimeter to landscape scale. The second study picks up at the landscape scale and maps the soil organic carbon storage with coarse pixels of 250 meters from satellite data for the entire permafrost region. Together both studies emphasis the variability of soil organic carbon in the region and that it is much more pronounced than in warmer climates,” says Matthias Siewert.

“These studies show the unique properties of soils in the permafrost region. To move further, we need to use this knowledge to better project how thawing permafrost will affect future climate, which means focusing on development of new types of models that account for landforms,” says Gustaf Hugelius, researcher at Bolin Centre for Climate Research and at the Department of Physical Geography, Stockholm University.

Original articles:

1. Siewert, M. B., Lantuit, H., Richter, A., & Hugelius, G. (2021). Permafrost causes unique fine-scale spatial variability across tundra soils. Global Biogeochemical Cycles, 35, e2020GB006659. https://doi.org/10.1029/2020GB006659

2. Mishra, U., Hugelius, G. Shelev, E., Yang, Y., Strauss, J. Lupachev, A., Harden, J.W., Jastrow, J.D., Ping, C.-P., Riley, W. J., Schuur, E. A. G., Matamala, R., Palmtag, J., Kuhry, P., Treat, C.C., Zubrzycki, S., Hoffmanm, F., Elberling, B., Camill, P., Veremeeva, A., Orr, A. (2021). Spatial heterogeneity and environmental predictors of permafrost region soil organic carbon stocks. Science Advances, 7, eaaz5236. DOI: 10.1126/sciadv.aaz5236