Arctic Soils

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Overview

By Tia Chung-Swanson
Arctic soils contain a large reservoir of stored carbon in the form of permafrost. When permafrost thaws, soil microbes activate and contribute heavily to biogeochemical nutrient cycling. This may have severe implications for Arctic microbial communities and the future of climate change.

Detailed Environmental Description

Global distribution of soils based on thickness of permafrost layer. Dark purple is continuous permafrost. From the International Permafrost Association.

In the first soil classification system, created in the 1890s, there were five natural soil zones: tundra, podzol, chernozem, desert, and laterite.[1] Arctic soils were classified in this system as tundra soils. Even with the limited knowledge from the 19th century, it was clear that cold-climate zones were unique because of the much more relevant layer of permafrost. Permafrost is soil that remains permanently frozen for the entire year, which is topped by an active layer that thaws each summer and then freezes again for the winter.[2] Today, arctic soils are classified as gelisols within the soil taxonomy created by the Natural Resources Conservation Service. A gelisol is defined as a soil that has either “permafrost within 100 cm of the soil surface,” or “gelic materials within 100 cm of the soil surface and permafrost within 200 cm of the soil surface.”[3] Since permafrost is frozen year-round, it stores a large amount of organic matter that does not get degraded. Arctic soils are particularly interesting at the moment due to anthropogenic climate change. When permafrost melts from the rising temperatures, the microbes stored in that soil activate and they can use all the newly accessible nutrients. They release CO2[4] and possibly other greenhouse gases, like CH4, and N2O.[5] This permanently increases the thickness of the active layer, which has implications for the biogeochemical cycling that the Arctic and other gelisols are capable of.

Overview of Microbial Ecology as it is known

Alpha diversity of bacterial phyla, bacterial classes, archaeal phyla, and fungal phyla. From a meta-analysis from Malard and Pearce, 2018.

Arctic soils are dominated by Proteobacteria and Acidobacteria. The next most common phyla are Bacteriodetes and Actinobacteria, but the proportion of these two varies by study site.[6] The proportion of cyanobacteria is very low compared to other nutrient-rich soils like molisols due to the much thinner active layer of Arctic soils.
The archaeal phyla are much less consistent, though there are consistently very few members from the Parvarchaeota phylum. Fungal phyla are dominated by Ascomycota and Basidiomycota. [6]

Contributions to Climate Change

Anthropogenic climate change is ongoing, and it causes temperatures to rise, particularly at high latitudes. This warming contributes to permafrost thaw, which creates a positive feedback loop that releases more greenhouse gases and spurs further climate change.[7] This has been happening for a relatively short time scale, so it is not completely certain how this will affect microbes in the affected areas. However, some experiments have found that soil microbes do not acclimate to the warming trend and continue to accelerate climate change.[8] Microbial communities in Arctic soil also may be permanently altered by the warming trend.[9]


Key Microbial Players

Nitrogen fixing Proteobacteria, including Rhizobiales, Burkholderiales, Xanthomonadales, and Myxococcales dominate and do all of the nitrogen fixing in Arctic soils. These orders belong to Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, and Deltaproteobacteria respectively. Despite the low percentage of Cyanobacteria in these soils, Cyanobacteria are still critically important because they are responsible for most of the CO2 and N2 uptake.[6] Nitrogen fixers are so important that one study found that Arctic microbial communities are actually N-limited, rather than C-limited like most other soil microbial communities.[5]

Conclusion

Arctic soils are diverse in terms of microbial life, despite their extreme climatic conditions. As the active layer gets thicker, additional microbial activity contributes to greenhouse gases that add to further warming in a positive feedback loop. More work is needed in the study of Arctic soils to determine how to slow this warming effect and prevent more harm to our planet.

References



Authored for Earth 373 Microbial Ecology, taught by Magdalena Osburn, 2020, NU Earth Page.