Nitrous oxide from Tibetan permafrost packs global warming punch

Permafrost, the perennially frozen layer of soil found in cold regions, releases the greenhouse gases carbon dioxide and methane as it thaws—a problem becoming more prevalent with global climate change. But permafrost researchers have often overlooked emissions of nitrous oxide, a gas with 300 times the warming power of CO₂. Now, a new study estimates that thawing areas of Tibetan alpine permafrost are releasing so much N₂O that their emission rates rival those from the largest known terrestrial sources of the gas (Environ. Sci. Technol. 2018, DOI: 10.1021/acs.est.8b02271).

Made famous as laughing gas, N₂O in the atmosphere not only traps heat efficiently, it is also the leading ozone-depleting substance in the stratosphere. The gas is mostly formed when bacteria and fungi in soil or seawater convert the nutrients nitrate and ammonia to N₂O. Even though permafrost soil stores large amounts of organic nitrogen in the form of frozen plant and animal parts, scientists have assumed that the cold temperatures prevent most of it from decomposing into the nitrate and ammonia that fuel N₂O production. In fact, plants growing on top of permafrost greedily compete for the small amounts of nutrients produced in the frigid soil, says Yuanhe Yang, a global change ecologist with the Institute of Botany at the Chinese Academy of Sciences.

A handful of studies have hinted that thawing permafrost could release substantial amounts of nitrogen in the form of N₂O. But these studies focused on so-called lowland permafrost ecosystems, which tend to be waterlogged after thawing and thus slow N₂O production. Because upland permafrost covers nearly 1 million km² worldwide and is drier, its thawing could be a big source of N₂O emissions. Yang and his colleagues decided to directly measure these emissions in the Tibetan Plateau.

The team trekked to a grassy alpine area that included a large gully that has been eroding and collapsing for more than 20 years due to warming of the permafrost. The researchers compared an undisturbed control site to several thawed patches in the gully.

Suspecting that nitrate supplies might be fueling N₂O production, the researchers measured both compounds at each of the sampling sites. The most recently thawed and exposed site had the highest soil nitrate levels, and the scientists measured N₂O emissions of 3.1 mg/m² per day, more than 100 times the amount from the control site. “This matches the nitrous oxide flux from tropical forest soils, which is the largest terrestrial source of nitrous oxide,” Yang says. He calculated that the global warming potential of N₂O releases from the newly thawed gully was significant—equaling nearly 60% of the global warming potential of methane from that same location.

For each site, Yang’s team also tracked the abundance of three microbial genes that control the formation of N₂O. A statistical analysis revealed that the combined effects of environmental factors and gene frequencies accounted for the patterns of N₂O emissions along the gully. Yang explains that the newly warmed and exposed soil had ideal moisture conditions for microbes that produce N₂O and no plant cover to compete for nitrate.

“This is the first field experiment to link the liberation of nitrate and the change in microbial genes to increased nitrous oxide flux,” says Carolina Voigt, a biogeochemist at the University of Montreal.

As the planet warms, more and more upland permafrost will thaw, Yang says. “Based on the results of this study, more nitrous oxide will be released from upland permafrost soils to the atmosphere and become a positive feedback to climate change,” he concludes.

This article has been translated into Spanish by Divulgame.org and can be found here.