This article is part of the Spring 2022 issue of the Earth Science Matters Newsletter.
Understanding the complexities of carbon loss in Alaskan peatlands after permafrost thaw
Boreal peatlands, found in much of Canada, Alaska, and Siberia, play an important role in the global carbon budget and are estimated to store ~20 % of the global soil carbon stock. Over half of the carbon found in northern peatlands has been protected from decomposition by permafrost, soil that has remained frozen for at least two consecutive years. Over the past few decades air temperatures within the northern high latitudes have warmed at a faster rate than other locations around the globe, increasing the rate of permafrost thaw. At times this thaw results in ecosystems changing from a relatively dry forest to an inundated wetland, such as a bog.
Changes in ecosystem type impact soil carbon storage because rates of both carbon inputs (through plant litter) and losses (through decomposition) are affected. Some studies have found small losses of carbon when permafrost thaw results in the creation of bogs, while other studies have found that over a third of the carbon that was stored in the soil can be released to the atmosphere. To help inform this science, USGS scientists measured the size of post-thaw carbon losses for a series of bogs located near the Tanana River, Interior Alaska (Figure 1).
USGS scientists found soil carbon losses of 34 – 46%; however, since these sites originally contained less carbon than found at other Interior Alaska locations, the absolute amounts of carbon lost (20 – 27 kg C m-2) were smaller than other locations. They also discovered that the size of these thaw-related bogs was not related to their age (a common assumption). Instead, it appears that factors such as the amount of ice in the permafrost and the amount of rainwater or snowmelt entering the bogs from the surrounding forests may control their expansion rates. This information will help modelers represent bog expansion more accurately.
The authors of this study compared their carbon losses to other studies to help determine what factors control the magnitude of these losses. They found that factors related to time (i.e., age since peat initiation, number of years the site had permafrost) are important but don’t fully explain why different areas experience different levels of carbon loss. However, when information about how permafrost formed was included, they saw a trend: sites where permafrost formed much later than peat formation (epigenetic permafrost) had smaller carbon losses than sites where permafrost and peats formed at about the same time (syngenetic permafrost).
Determining how permafrost formed is difficult (Figure 2). Since thaw bogs, fens (another type of wetland), and forested permafrost plateaus all have different vegetation communities, the authors were able to use microscopic plant remains found in the cores to determine how vegetation changed over each core’s history. This vegetation information also provides clues about permafrost history. This study also found that an easier and more common measurement, the ratio of carbon to nitrogen (C/N) of the soil was different in samples with epigenetic versus syngenetic permafrost formation, suggesting that C/N could provide preliminary clues about permafrost formation history.
This study’s results will help modelers and other researchers predict how accelerated rates of permafrost thaw will impact the amount of carbon that remains in Alaskan soils or is released back to the atmosphere. The paper, “Influence of permafrost type and site history on losses of permafrost carbon after thaw”, was published in the Journal of Geophysical Research: Biogeosciences.