Chemical Tug-of-War Determines Composition of Nitrogen in the Atmosphere

Talk to a group of carbon scientists and they will probably tell you that of all the elements, carbon makes the world go round. But all living things also depend on nitrogen, as well as carbon, to survive. Like carbon, nitrogen has a biogeochemical cycle that shuttles the element between the deep Earth, oceans, and atmosphere, but details of that cycle are poorly understood.

Now, a new study by DCO researchers provides a novel way to track the movement of nitrogen through the environment. Laurence Yeung (Rice University, USA), Edward Young and Issaku Kohl, (both at the University of California, Los Angeles), Tobias Fischer (University of New Mexico, USA), and colleagues, used the new DCO-funded, high-resolution mass spectrometer named Panorama, to make the first accurate measurements of atmospheric 15N15N. These rare molecules of nitrogen gas are slightly heavier than most because each atom in the pair has one extra neutron in the nucleus. This discovery, described in a new paper in Science Advances [1], paves the way for using 15N15N as a natural tracer to monitor nitrogen cycling on Earth and other planetary bodies.

Young and colleagues at UCLA designed Panorama [2] to answer questions about the origins of methane in the deep subsurface, but the instrument has much broader applications. “It’s an ultra-high-resolution instrument that can separate all different types of molecules from each other, molecules that you wouldn’t ordinarily get to separate, and we can quantify them to a very high degree of precision,” said Yeung. Panorama measures isotopologues, which are molecules with different isotopic composition, meaning that at least one atom that has a different number of neutrons in its nucleus and thus a slightly different mass. Tracking isotopologues can reveal information about geochemical processes occurring in an environment and even the age and origin of minerals, gases and fluids. While brainstorming applications for this powerful research tool, Yeung realized it is a perfect instrument for measuring how nitrogen isotopes are bound together in the atmosphere.  

The researchers used Panorama to analyze air from UCLA and Rice University and discovered that it has a surprisingly high amount of 15N15N. At 19 parts per thousand (‰), surface air has a 10 to 20-fold larger enrichment in 15N15N than they had expected. After taking nearly a year to determine that the observation was not a hidden measurement error, they set out to find the cause of the isotopologue enrichments.

Microbes play an important role in the nitrogen cycle, so collaborators at Michigan State University investigated how the activities of several species of bacteria alter 15N15N abundances. They tested microbes that engage in different parts of the cycle, and found that in total, these reactions actually drive down the amount of 15N15N in the atmosphere rather than drive them up.

Next, the researchers looked at the contribution of volcanic gases, collaborating with Fischer. Their analysis showed that volcanic emissions had some 15N15N, but again, not enough to explain the high levels in the atmosphere.

Finally, the researchers turned to atmospheric chemistry to explain the isotope levels. They determined that 15N15N in the air primarily comes from chemical reactions occurring in the upper reaches of the atmosphere in a layer called the thermosphere, where the International Space Station orbits Earth. They combined different concentrations of oxygen and nitrogen gas at low pressures and added electricity to mimic the conditions in the thermosphere. The resulting mixtures drove 15N15N levels up to 23‰.

The chemical reactions that cause this enrichment in 15N15N are still unknown but the researchers believe that they are the main sources of the unusual isotopic signature in the atmosphere. “There’s something about the chemistry of the atmosphere that is creating this really weird anomaly that we only see in air,” said Young. “We have this beautiful tracer that can fingerprint air.”

Scientists can now use 15N15N to track nitrogen flow through the environment for microbial and geological research. Yeung plans to use the tracer to examine oxygen minimum zones in the ocean, which is the depth where molecular oxygen runs out and microbes begin consuming oxygen-containing nitrogen compounds instead. When analyzing the content of volcanic gases, scientists can now easily distinguish between nitrogen coming up from deep in the mantle, from materials buried in shallow sediments, and from the air.

An instrument that can detect 15N15N may one day be deployed in space to look for possible signs of life on other planets. “If a planet has an atmosphere with nitrogen, we should see some sort of upper atmospheric chemistry signature, similar to what we see on Earth,” said Yeung. If the planet has nitrogen-cycling organisms, however, then its atmosphere may have lower levels of 15N15N than would be expected, signaling that the planet may be a good place to look for life.

 

Researchers from Rice University and UCLA simulated high-energy chemistry in the upper atmosphere to reproduce enriched levels of 15N15N, molecules that contain only heavy isotopes of nitrogen. Credit: Laurence Yeung/Rice University

 

Laurence Yeung. Credit: Jeff Fitlow/Rice University

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