The Arctic is warming at a rate that outstrips any other region of the planet, and the rapidly thawing permafrost houses an estimated 1,400 gigatons of carbon, which is nearly triple the total amount of carbon circulating in the atmosphere. Understanding soil dynamics and their feedbacks is vital to understanding how Earth’s climate will change in the future.
To understand what will happen as permafrost continues to thaw, scientists look to Arctic environments where the soils already undergo regular freeze-thaw cycles. About a quarter of the Arctic is made up of High Arctic polar deserts. In these regions, the soil thaws in the summer as temperatures average in the single digits on the Celsius scale. During this time, polar desert plants—mostly mosses, algae, lichens, and small shrubs—and soil microbes come out of hibernation and perform all the chemistries of life that cycle carbon and nitrogen through the environment.
The freeze-thaw cycle also produces unique physics within the soil itself, creating frost boils, which churn nutrients vertically through the ground. In some instances, this process can create nutrient-rich soil layers called diapirs. Diapirism can occur in viscous soils and is expected to increase with soil moisture caused by permafrost thaw in a warming climate. Scientists have previously observed that some plants, especially Salix arctica, preferentially target diapirs with their roots, making the soils a potential nutrient source for greenhouse gas production. However, the biological processes at play in diapirs are still quite poorly understood.
Here Ota et al. compare the soil in frost boils with diapirs to the soil in frost boils without diapirs. The study took place in two different types of polar deserts near Alexandra Fjord, Ellesmere Island, Nunavut, Canada, during July and August of 2013. Diapirs were 29% richer in nitrogen. The scientists also found that diapirism increased the concentration of low-quality carbon—carbon contained in molecules that are challenging to microbial decomposition. Although the lower-quality carbon reduced microbial activity, the researchers confirmed that Salix arctica does increase its root density in diapirs, a finding they say suggests a mutualistic relationship between the plant and the microbe, with both benefiting from diapirism overall.
The researchers also found an abundance of polysaccharides in the frost boils that contained diapirs. Polysaccharides are sugar molecules common in both plant and microbial chemistry. What’s interesting about the presence of polysaccharides is that they make the soil more viscous, which should make it more prone to diapirism. One hypothesis the researchers have is that the plants or the microbes (or both) are secreting these sugars into the ground to promote diapirism.
This complex interplay between soil physics, plant, and microbes—what the scientists call “geomorphologic-plant-microbe interactions”—might even explain why only 30% of the frost boils in the study region contained diapirs in the first place. Because these nutrient-rich soil layers are a hotbed for Arctic greenhouse gas production, understanding how they form could greatly improve our knowledge of just how quickly the planet warms and the Arctic thaws. (Journal of Geophysical Research: Biogeosciences, https://doi.org/10.1029/2019JG005263, 2020)
—David Shultz, Science Writer