Intellectual Merit:

            Nitrogen is an essential often-limiting element for all life and while it is the dominant gas comprising the atmosphere (in the form of N₂) this form is largely inaccessible to the biosphere. Nitrogen fixation is the only biological process that can access N₂and this gas is returned to the atmosphere via the biological processes of denitrification and anammox. Nitrogen primarily consists of the isotope ¹⁴N; only ~ 0.4% is comprised of the heavier isotope ¹⁵N. The probability of two ¹⁵N atoms joining to for ¹⁵N¹⁵N is exceeding low making this the rarest of atmospheric N₂molecules. As¹⁵N¹⁵N is rare it is difficult to quantify but in this project we were able to evaluate its abundance with great accuracy using a newly developed high-resolution mass spectrometer. We unexpectedly found approximately 20 times form ¹⁵N¹⁵N in the atmosphere than expected. In studies of N₂generated by bacteria carrying denitrification and anammox we found nearly no appreciable excess of ¹⁵N¹⁵N indicating that microbial processes could not explain the anomalous enrichment of ¹⁵N¹⁵N in the atmosphere. Electrical discharge experiments, that simulate upper atmospheric reactions, however, were found to result in an excess of ¹⁵N¹⁵N greater than that present in the atmosphere. Consequently, the observation of excess ¹⁵N¹⁵N in atmospheric N₂reflects a balance of that influenced by upper atmospheric reactions and that biologically produced. These findings pave the way for better resolution of the global nitrogen budget and identify ¹⁵N¹⁵N as a potential tracer of biological processes on other planetary bodies. 

Broader Impacts:

            This project has established a new tracer, ¹⁵N¹⁵N, of the environmental nitrogen cycle that will have broad applications in a range of research disciplines including evaluation of the global nitrogen cycle, better quantification of denitrification in the oceans and in soils, and better understanding of upper atmospheric reactions. Two postdoctoral fellows were involved in the project and have gone onto professional or academic careers. The fundamentals concepts in isotope theory and instrument design developed as part of this research project were a focus of our short course in Stable Isotope Biogeochemistry in May of 2016 and 2018. Students participating in this course were from academic positions, industry and graduate education programs and are both domestic and international. In 2016students joined us from Brazil and Korea.

Last Modified: 09/24/2018
Modified by: Nathaniel E Ostrom