By Bianca Nogrady
Arctic peatlands may become a substantial source of a greenhouse gas 300 times more potent than carbon dioxide when they thaw, a new study suggests.
The study by a team of Scandinavian scientists indicated that thawing permafrost could release nitrous oxide (N2O) — also known as 'laughing gas' — under increasing temperatures.
Based on an analysis of frozen peat cores exposed to warming conditions in the laboratory, they estimated nitrous oxide emissions could occur from surfaces covering almost one-fourth of the entire Arctic.
The highest emissions, from bare peat samples, were on a par with tropical forest soils — the largest known natural source of nitrous oxide — they reported in Proceedings of the National Academy of Sciences.
Lead author PhD candidate Carolina Voigt said the findings could alter the greenhouse gas budget for the Arctic circle.
She said while research had been done on how much methane and carbon dioxide might be released from thawing Arctic permafrost, few studies had looked at nitrous oxide.
"Usually nitrous oxide emissions from Arctic soils were believed to be negligible, basically because the nitrogen content — the substrate for production of N2O — is probably rather low, or the production rate is rather low due to the cold climate," said Ms Voigt, from the Department of Environmental and Biological Sciences at the University of Eastern Finland.
The study was prompted by the discovery of some nitrous oxide 'hot spots' in the sub-Arctic; regions of bare peatland that seemed to be emitting the gas at a much higher level than the surrounding terrain.
Peat samples from Finnish Lapland
Peat cores samples were collected in Finnish Lapland. (Supplied: Carolina Voigt)
To investigate what impact a warming climate might have on permanently frozen peat deposits, Ms Voight and her colleagues extracted 16 cores of frozen peat — along with whatever plant or lichens grew on their surface — from a site in Finnish Lapland.
The samples were kept frozen for around five months as an incubation period, then warmed in a specially-designed chamber, which allowed researchers to control light levels, humidity, and the water level of the peat.
The study revealed that the thawing of the bare peat permafrost — where there was no vegetation on the peat's surface — led to a five-fold increase in the amount of nitrous oxide release, compared to the amount released when only the uppermost part of the peat thaws, as it does seasonally.
The researchers also showed that a lack of surface vegetation significantly increase the release of nitrous oxide from the thawing peat.
"Plants take up nitrogen from the surface soil so they reduce the nitrogen pool that's available for N2O production in the soil profile, so plants are very effective at reducing N20 emissions," Ms Voigt said.
While the growth of surface plants lowers nitrous oxide emissions by 90 per cent, the researchers said plant-covered peatlands could still make a "small but significant" contribution to total emissions for the region
The study also showed that the wetter the peat, the less nitrous oxide is released.
Commenting on the paper, Professor Peter Grace — Professor of global change at the Queensland University of Technology — said that the study could only add limited information to the understanding of nitrous oxide emissions in the Arctic because it was laboratory-based.
"They also do not present a full budget because of the uncertainties with the actual field based information, [such as] the extent of different peatlands," said Professor Grace, who has written previously about the issue of nitrous oxide release from nitrogen fertilised agricultural soils.
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Image credit: Tarmo Virtanen, University of Helsinki