Arctic soil, the vast expanse of ground in Earth’s coldest regions, is defined by its frozen state. This perpetually cold ground covers immense territories, shaping unique landscapes across the polar regions. Its most fundamental characteristic is the presence of ice, which dictates its structure, composition, and the life it can support.
The Unique Composition of Arctic Soil
The structure of arctic soil is fundamentally layered, a direct consequence of the extreme cold. These soils, often called Gelisols or Cryosols, are defined by the presence of permafrost—ground that remains at or below 0°C for at least two consecutive years. Above this permanently frozen layer sits the “active layer,” which thaws during the brief summer and refreezes in the winter. The thickness of this active layer can vary from a few centimeters to a few meters.
The boundary between the active layer and the permafrost is a zone of significant physical activity. This repeated freezing and thawing generates a process known as cryoturbation, or frost churning. As water in the soil freezes, it expands and forms ice lenses and wedges, which push soil particles around. This churning action mixes organic material from the surface deep into the soil profile and can force rocks and other coarse materials toward the surface.
This constant movement disrupts typical soil horizons, creating warped, broken, and irregular layers. Cryoturbation is also responsible for the unique patterned ground seen across the tundra, such as circles, polygons, and stripes. The process sorts soil materials, creating distinct geometric shapes on the landscape.
Life Within the Frozen Ground
Despite the harsh conditions, life has found a way to persist in and on arctic soil. The vegetation is dominated by hardy, low-growing plants adapted to the short growing seasons, low temperatures, and shallow active layer. These plants often have shallow root systems to exist entirely within the thin layer of soil that thaws each summer. They include:
- Mosses
- Lichens
- Sedges
- Dwarf shrubs
Below the surface, the soil hosts a diversity of microscopic life. These microbial communities, composed of bacteria, archaea, and fungi, are adapted to the extreme cold. In the active layer, they are responsible for the very slow decomposition of organic matter and nutrient cycling. Different bacterial groups dominate at different depths; for example, some are often prevalent in the active layer, while other groups are more abundant in the deeper permafrost.
These microbes play a role in the arctic ecosystem. Some are capable of nitrogen fixation, a process that converts atmospheric nitrogen into a form usable by plants, which is a limiting nutrient in these environments. As the soil environment changes, these microbial communities can shift, with significant consequences for nutrient cycles.
A Massive Carbon Reservoir
Arctic soils are a massive repository of organic carbon because frigid temperatures dramatically slow decomposition. When plants and animals die, their organic matter is not fully broken down, allowing it to accumulate and become locked in the permafrost. This process has removed vast quantities of carbon from the active global cycle for millennia, making permafrost one of the planet’s largest terrestrial carbon sinks.
Scientific estimates suggest that the soils in the northern circumpolar permafrost region hold between 1,460 and 1,600 billion metric tons of organic carbon. This is roughly double the amount of carbon currently present in Earth’s atmosphere. Much of this carbon is stored within the top three meters of soil, but significant stocks also exist in deeper deposits, such as the vast Yedoma region of Siberia and Alaska, which contains ancient, carbon-rich silt.
The Consequences of Thawing Permafrost
Rising global temperatures are causing arctic permafrost to thaw at an accelerating rate, threatening to unlock its massive carbon stores. As the ground warms, dormant microbes “wake up” and begin to decompose the ancient organic matter that has been frozen for centuries or millennia. This decomposition process releases the stored carbon into the atmosphere in the form of carbon dioxide and methane. Methane is a particularly potent greenhouse gas, with a warming power over 80 times that of carbon dioxide over a 20-year period.
This release of greenhouse gases from thawing permafrost creates a positive feedback loop. The released gases contribute to further global warming, which in turn causes more permafrost to thaw, releasing even more gases. Scientists are concerned that this feedback could significantly accelerate climate change. Some estimates suggest that thawing permafrost could release 110 to 231 billion tons of CO2 equivalents by 2040.
Beyond the climatic implications, thawing permafrost has direct physical consequences. The melting of ground ice causes the land to slump and collapse, a process known as thermokarst. This destabilizes the ground, damaging infrastructure like roads, buildings, and pipelines built upon the once-solid foundation. In some cases, entire communities are at risk of being relocated, and there are concerns that ancient, frozen pathogens could be released as the ground thaws.