Award: EAR-1424720

Intellectual Merit: The naturally occurring stable isotope ratios of nitrate (15N/14N and 18O/16O) have long provided a tool for tracking environmental sources and biological transformations. However, divergent interpretations of fundamental nitrate isotope systematics exist among disciplinary divisions. In an effort to transcend disciplinary boundaries of terrestrial and marine biogeochemistry, (a) we used a quantitative model for coupled nitrogen and oxygen isotopes of nitrate founded on benchmarks established from microbial cultures, to reconcile decades of nitrate isotopic measurements in freshwater and seawater and move toward a unified understanding of cycling processes and isotope systematics. Model findings indicate that denitrification operates within the pervasive context of nitrite reoxidation mechanisms, specifically highlighting the relative importance of nitrification in marine denitrifying systems and anammox in groundwater aquifers. (b) We tested the predictions borne out of the model in an impacted aquifer in Cape Cod. Results suggest that denitrification and anammox are tightly coupled, occurring simultaneously in anaerobic aquifers. (c) We demonstrated that the co-occurrence of denitrification and anammox is pervasive among different sediment types, thus explaining convergent patterns in nitrate isotope dynamics among aquatic environments. Finally, (d) our results led us to re-examine a commonly used method to quantitate the stable isotope ratios of nitrite, i.e, the azide method. We demonstrated important salinity matrix effects on the resulting isotope ratio measurements, as well as pH effects – the latter which are difficult to minimize, leading us to advise caution when reporting nitrite isotope measurements effectuated with the azide method. Broader Impacts: Funding for this study has helped reconcile fundamental tenets of nitrate isotope dynamics among disciplinary divisions, demonstrating that parallel nitrogen transformations can manifest as distinct isotopic patterns given differences in the 18O/16O of ambient water. The insights from this study thus contribute to a shared understanding of N isotope dynamics that transcends ecosystem distinctions. Funding from this study contributed to the training of a graduate student, Danielle Boshers, and a research assistant, Holly Westbrook. Both became versed in N isotope biogeochemistry and in analytical techniques related to mass spectrometric analyses of stable isotpe ratios. Boshers has since secured a technical position at the USGS, and Westbrook has recently begun graduate school at the University of South Carolina in a parallel discipline. Products: The work so far has resulted in two peer-reviewed publication in high-profile journals (Granger & Wankel, PNAS 2016; Boshers et al. ES&T 2019), as well as two publications in preparation that will be submitted for publication peer-reviewed journals (Granger et al. in prep; Granger and Boshers, in prep). The work was presented in 6+ talks and/or posters at national and international conferences. The field and experimental data have been submitted to the BCO-DMO data repository. Last Modified: 03/05/2019 Submitted by: Julie Granger