How is carbon “stored” in soils of the Arctic and Subarctic?

Carbon is the central element in organic compounds necessary for life. Organic compounds make up the cells and other structures of organisms and carry out life processes. Carbon is the main element in organic compounds, so carbon is essential to life on Earth. Even humans are made of carbon compounds!

Thick sedge and sphagnum peat deposits
Thick sedge and sphagnum peat deposits formed under saturated, anaerobic conditions in a depression. Credit: Mark Clark, NRCS, retired

Why is carbon so basic to life? Carbon is able to form stable bonds with many elements, including itself. This property allows carbon to form a huge variety of very large and complex molecules. In fact, there are nearly 10 million carbon-based compounds in living things!

One only needs to look at a diagram of the carbon cycle to appreciate the role that carbon fulfills supporting life on earth. This element moves readily between atmosphere, oceans, and terrestrial environments in response to natural processes and human activities. Massive amounts of carbon are stored in soil and plants, oceans, and in geologic formations such as coal, oil and gas.

How does carbon get into soil? Well, that can be a very complicated answer. However, the main way is actually through plants. Plants use sunlight and carbon dioxide (CO2) to make their food, in form of sugars (which all have some form of carbon in their chemical name). That’s the first way that carbon is pulled from the atmosphere into a terrestrial plant.

From there, there are other plant processes that change the sugars into other compounds. When a plant dies, that carbon may be recycled into the soil by soil-dwelling insects and microbes. Organic matter provides the basic food for microbes that, in turn, help to cycle nutrients and assist in root metabolic processes essential to plant health. Larger animals that eat the plants also leave their waste products on the soil (think rabbit pellets all the way to bear scat!)

In this way, carbon from the atmosphere moves through plants into the soil as “organic matter.” Not only is this organic matter food for various life forms, it helps soil do its job. Organic matter improves the ability of the soil to hold water by adding pore space. It helps bond mineral soil grains to form aggregates, which loosen the soil. This allows water to be captured, held, and redistributed though soil (for more on this, read How does water move through soil or Soil – the largest reactor on the planet.)  At the same time, improving infiltration lessens the hazard of surface runoff and erosion. This improved condition or “soil tilth” increases porosity of soils allowing roots to extend unimpeded to greater depths to access stored water during periods of drought and nutrients previously unavailable for plant growth.

Fibrous organic layer of moss, twigs and roots
Fibrous organic layer of moss, twigs, and roots formed under cool, acid, aerobic conditions on a convex upland position. Note the dark layer of charcoal at about 30-35 cm. Credit: Chien-Lu Ping, University of Alaska, Fairbanks, retired

Organic matter storage in Subarctic and Arctic regions looks different than it does in the lower 48 and Hawaii. A thick accumulation of fibrous organic material of moss, roots, twigs, and litter covers the soil. Seasonal freezing and thawing generate intense internal pressures. This churns mineral soil and organic matter into a swirled mass of black, brown, and gray often mottled with brighter reds and greens. The high Arctic is the coldest and windiest region on Earth. It does not support substantial plants with roots and other “vascular” systems. Patches of lichens and moss are the only vestige of plant life tenacious enough to survive these inhospitable conditions. Low production equates to very low organic carbon generation or storage in soils.

Thick “organic mats” can form above Alaskan soils. There are two ways they form:

  1. In low areas, water collects during spring melts and other precipitation. These soils get saturated – full of water. There is not a lot of pore space between soil particles to allow for oxygen, a condition called anaerobic. Certain soil microbes can still function in these environments, though. Carbon-rich sedge and sphagnum peat deposits several meters or more thick (see photo 1) build up under these oxygen-deficient soil conditions.
  2. The process of organic mat formation differs considerably in adjacent uplands. The organic mats develop under cold, acid, aerobic conditions. Organic mats commonly range from 10 to 40 centimeters throughout the uplands once late succession stunted conifer woodlands have established. This normally requires a hundred years or more without fire or other surface disturbance (see photo 2). At this point, the organic mat reaches sufficient thickness for permafrost to form and perch water over an impermeable frozen layer and further reduce organic matter turnover as anaerobic conditions are added to the equation.

Soils in the Arctic and Subarctic contain nearly half of the planet’s terrestrial carbon. That amount is double what is held in the entire Earth’s atmosphere! Gelisols, soils with permafrost within two meters of the surface, are responsible for tying up the bulk of this carbon. They entomb carbon within the permanently frozen substrate soils. In addition, the thick organic mats common to these soils store more carbon.

A warming climate is quickly changing landscapes of the Arctic and Subarctic as permafrost melts and soils change from net carbon sinks to atmospheric sources of carbon dioxide. Changing environmental conditions such as fire frequency, thermal erosion, and the encroachment of diseases and invasive species not previously documented are likely to continue as temperatures rise.

To read more about what Alaska’s soils are telling us, read my 2017 blog post What are Alaska’s soils telling us?

Answered by Mark H. Clark, CPSS, former NRCS Alaska Soil Scientist

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