Now a team from the US and Sweden has found that as the thaw proceeds, the organic matter becomes more biodegradable and more likely to produce methane than carbon dioxide. Methane’s greenhouse effect is 33 times larger than that of carbon dioxide.

“As permafrost thaws, the ground thaws, collapses and becomes flooded, and this increasing water inundation leads to changes in surface vegetation,” Suzanne Hodgkins of Florida State University told environmentalresearchweb. “Specifically, Sphagnum moss, which grows in partially thawed semi-wet areas, is replaced by sedges, which grow in fully thawed wet areas. These sedges produce organic matter that has lower concentrations of organic acids and other compounds rich in organically bound oxygen than the organic matter produced by Sphagnum.”

Microbes, particularly those that produce methane, prefer to feed on organic matter low in oxygen-rich compounds. This means that sedges are more biodegradable, and more likely to be converted to methane, than sphagnum moss.

As a result, production of both methane and carbon dioxide increases as permafrost thaws, with methane emissions increasing at a faster rate. “These results imply an increasing greenhouse-gas effect from organic matter decay in thawing permafrost wetlands, which could exacerbate the effects of greenhouse warming from human activities,” said Hodgkins.

To come up with the findings, the teams carried out lab tests on samples from Stordalen Mire, a Swedish peatland north of the Arctic Circle. Extensive permafrost thaw has occurred at the site over the past 40 years.

“This thaw occurs at different rates in different parts of the study site, so that some areas are still frozen while others are fully thawed,” said Hodgkins. “To determine how permafrost thaw affects methane production, we looked at several sites along a gradient of increasing time since onset of thaw. This spatial thaw gradient represents a proxy for the progression of thaw over time.”

In the past, climate models have assumed that permafrost emits a constant mix of methane and carbon dioxide as it thaws. But this research revealed that the ratio of methane to carbon dioxide increases as thaw progresses because of changes in plant growth.

“Prior to this study, we knew that permafrost thaw could be a major climate feedback because thawed organic matter decomposes more rapidly than frozen organic matter, and as the ground collapses and becomes inundated, the soil becomes less exposed to oxygen gas, allowing the microbes that produce methane to thrive,” said Hodgkins. “We found that there is yet another mechanism for increasing methane production with permafrost thaw: changing organic matter chemistry, induced by changing surface vegetation, increases the biodegradability and methane production potential of soil organic matter.”

According to Hodgkins, distinguishing the exact controls on organic matter chemistry represents a challenging “chicken-and-egg” question. “Organic matter chemistry affects the speed at which it [organic matter] decays, and this decay in turn affects the organic matter chemistry,” she said. “We will be conducting more detailed studies of organic matter chemistry along the thaw progression, which will help us to distinguish which changes are caused directly by changes…produced by surface vegetation, versus indirectly by changes in decomposition by microbes.”

The team reported their results in PNAS.

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