Nitrous oxide, often called “laughing gas,” is perhaps most famous for its use as an anesthetic. However, it is also a powerful greenhouse gas that harms the ozone layer; in fact, nitrous oxide contributes more to ozone depletion than any other component of human emissions. But there are many natural sources of the gas as well. In a new study, Gil et al. explore the isotopic composition and potential sources of nitrous oxide emitted by soils in the subarctic tundra. The findings could aid predictions of future trends in atmospheric nitrous oxide.
About 60% of the nitrous oxide found in the atmosphere escapes from the soils of farms and tropical forests. Until recently, scientists assumed that nitrous oxide emission was negligible in colder climates. Then, in 2009, researchers discovered that bare areas of peat in frozen tundra soils emit the odorless gas at rates similar to those of tropical forests.
The team already knew that various microbes can produce the nitrous oxide emitted from soils at multiple points in the nitrogen cycle. Different microbial processes leave distinct isotopic fingerprints on nitrous oxide; thus, they hoped to figure out the relative amounts of nitrous oxide emitted by different nitrogen-processing microbes in the tundra.
The researchers examined nitrogen and oxygen isotopes in samples collected from peat emission sites in northwestern Russia. They also identified the specific isotopes occupying the two different nitrogen sites within the nitrous oxide molecule.
The results suggest that the high tundra emissions might be due to nitrifier denitrification, in which microbes transform ammonia into dinitrogen (N2) in a series of steps, one of which produces nitrous oxide. However, the data were inconclusive, and their technique did not provide enough information to tease out the relative roles of different microbes.
Nonetheless, the findings are valuable because isotopic nitrous oxide data from the Arctic and sub-Arctic are extremely rare. Nitrous oxide from natural sources has a different isotopic fingerprint than nitrous oxide from agricultural sources. In the future, enhanced emissions from subartic tundra might cancel out mitigation actions at lower latitudes. Therefore, a rich set of isotopic nitrous oxide data from around the world could help scientists pinpoint regions that serve as emission sources as climate change progresses. (Global Biogeochemical Cycles, https://doi.org/10.1002/2015GB005370, 2017)
—Sarah Stanley, Freelance Writer