Given its central role in biogeochemistry, its multiple valences and the reactive and unreactive gaseous forms, the nitrogen cycle has long proven a difficult biogeochemical system to unravel. The oxidized form of nitrogen, nitrate, has been of particular interest due to its hydrologic mobility and role as a common groundwater contaminant. While the use of δ15N and δ18O of nitrate have long helped us over the last decade to distinguish sources of contaminant and biogeochemical processes, such as nitrate reduction, the addition of Δ17O (the triple isotope approach) has quantified the variability and importance of atmospheric contributions of nitrate, as well as the fate and transport of nitrate in natural environments. In the process of following this atmospherically sourced nitrate through hydrologic systems we have been further able to elucidate the loss and transformation processes that influence nitrate of all origins in the environment. Here, we will highlight the utility of the triple isotope system in a semi-arid system with reference to studies in other settings. The Tucson basin has four main sources of nitrogen- atmospheric deposition, terrestrial N fixation, anthropogenic fertilizer, and sewage effluent. The triple isotope system enabled the identification of denitrification as a major loss pathway after effluent discharge. We were also able to quantify the contribution of atmospheric nitrate directly to the regional groundwater system, and identify spatial patterns in surface water and groundwater. By bringing additional nitrogen budget and tracer data to bear we were able to constrain the biogeochemical cycling of N in the Tucson basin. This study offers lessons for those working on the N cycle in other settings and the triple isotope system offers a unique tool to help isolate the different processes that influence nitrate concentrations in natural waters.
Meixner T., Michalski G.M., Dejwahk N., Riha K.M., Lohse K.A., Gallo E.L., McIntosh J.C., Brooks P.D. (2013): Insights on Biogeochemistry from the Triple Isotope System of Nitrate (Invited). Abstract H53I-01 presented at 2013 Fall Meeting, AGU, San Francisco, CA, 9-13 Dec..