Permafrost soils and carbon cycling

The importance of permafrost soils

Permafrost soils make up 15 % of the global land cover and store more than 822 petagrams of carbon in their upper most three centimetres alone (the weight of 182,000 million adult elephants). When comparing this with the annual carbon dioxide emissions of an average German citizen of approx. 2.4 tons C per year1 it becomes clear that we need to prevent the breaking down of the masses of carbon stored in these soils. Warming of permafrost leads to the release of carbon, making them a source of the greenhouse gasses carbon dioxide and methane.

What are permafrost soils?

Permafrost soils are soils with a year-round frozen layer below their seasonal thawed top layer (image 01). They occur in regions where the annual average temperature is below zero degrees Celsius (32 degrees Fahrenheit), thus large areas of the northern hemisphere located above 60 degrees North, (from Oslo, Norway and further north) are covered with permafrost.

Image 01. Permafrost soil profile with chunks of organic matter mixed into mineral soil at 50 to 80 cm depth and the true permafrost layer below 80 cm depth. Source: Ping et al. 2015.

The shrinkage and expansion of the thawing and freezing top layer leads to strong forces acting on the soil profile and to the formation of unique patterns (Image 02). The lower soil layers are frozen and act as a water barrier and these soils are generally wet and cold, even slowing down the metabolism of bacteria. Most of the plant residues grown during the short summer periods will build up on the top layer of the soil with little degradation taking place. Over the last millions of years these soils have accumulated a huge amount of plant material and carbon, thus are very important for the global carbon cycle.

Image 02. Oil painting like formations resulting from freeze-thaw events. Source: Wikipedia commons.

Typically a permafrost soil has four layers. The true permafrost layer that has been frozen for hundreds to millions of years is on the bottom. Above that is the transient layer which was formed by fluctuations of the true permafrosts depth at a ten years time interval. Then the intermediate layer is characterized by ice-rich structures like ‘underground icicles’. These icicles form when water enters cracks in the soil during the summer months and freezes in winter. The ice expands and pushes the surrounding soil apart making the crack larger and more water can accumulate inside it in summer. Finally the active layer forms the modern soil at the surface. The active layer is ice free in the summer months providing a living space for organisms. In autumn this layer gets frozen from the top down, leading to a sandwich of two ice sheets, on the top the active layer and on the bottom the true permafrost layer and in the middle unfrozen soil. This is the time when strong forces act on the middle unfrozen layer as the two frozen layers start expanding and mixing the soil. Intriguing surface and subsurface pattern are formed in this way (Images 02 and 03). Organic material can be moved down and trapped in under-laying mineral soil (Image 01), fine and coarse material is separated from one another forming sorted stone circles on the surface (Image 04) or the natural layering of the soil profile may be interwoven giving it the appearance of an oil painting (Image 02).

Image 03. Surface structures resulting from freeze-thaw cycles of the active layer of a permafrost soil. Source: Wikipedia commons.

 

Image 04. Sorted stone circles above a permafrost soil resulting from freeze-thaw cycles. Source: Wikipedia commons.

Permafrost soils and climate change

With rising temperatures permafrost soils will thaw and microorganisms that are otherwise limited by the cold, freezing temperatures will become more active and degrade the readily available organic carbon with ease forming carbon dioxide and methane. Many scientists have tried to estimate the carbon losses from permafrost soils but these estimates remain uncertain and predictions range from 7 to 250 petagrams carbon (1,600 million to 55,600 million elephants of pure carbon) by the end of this century, depending on fossil fuel usage. Other uncertainties include the actual amount of carbon stored in the whole soil profiles and how strongly temperature rise will affect plant growth and their ability to take up carbon from the atmosphere. The organic matter may be well mixed into the surrounding soil, making it harder for microorganisms to access and transform it into carbon dioxide or methane, thus reducing the speed of greenhouse gas emissions. The melting of the ice in the upper layers may lead to water logging on top of the still frozen layers. As a consequence less air and oxygen reaches into the soil, which creates ideal conditions for methane production. Methane has 28 times the warming potential of carbon dioxide, thus methane emissions from permafrost soils are critical to limiting temperature rise globally.

Outlook

In recent years many researchers have begun studying permafrost soils due to their importance for the global carbon cycle and their huge implications on climate change. Even though there are still large uncertainties on the speed of carbon release under a changing climate we know that preserving the permafrost is extremely important if we want to limit the effects of climate change. Simple actions may help limiting temperature rise and the thawing of permafrost like reducing air travels to the necessary minimum, reduce meat consumption and increase wast recycling.

Source

Ping, C. L., Jastrow, J. D., Jorgenson, M. T., Michaelson, G. J., and Shur, Y. L.: Permafrost soils and carbon cycling, SOIL, 1, 147-171, https://doi.org/10.5194/soil-1-147-2015, 2015.

References

1World Bank 2017. https://data.worldbank.org/indicator/EN.ATM.CO2E.PC?locations=DE

Featured image: Sebastian Zubrzycki, 2012. Distributed via imaggeo.egu.eu.

Share this:
Hannes Keck

Hannes Keck

I am a PhD student studying greenhouse gas fluxes from agricultural ecosystems at the Swedish University of Agricultural Sciences (SLU). Currently, my research focuses on developing a new measurement technique to make it easier to analyse all relevant greenhouse gas fluxes from terrestrial ecosystems. Before my PhD studies I was working in the field of soil science / physics at several European research institutes and completed my masters at University of Copenhagen, Denmark and SLU, Sweden in environmental sciences (EnvEuro programme).

Leave a Reply

Your email address will not be published. Required fields are marked *