Δ17O-NO3-: Application of a stable isotopic tracer to forest nitrogen saturation

Background/Question/Methods

Atmospheric nitrate (NO₃^-) deposition is of particular concern in eastern US forests, where it is estimated that between 60 and 80 percent of chronic N deposition is in the form of NO₃^-. We sampled nitrate in streams draining four watersheds at the Fernow Experimental Forest in Parsons, West Virginia, each representing a different stage of forest N saturation. The designation of a specific N saturation stage in these watersheds is based on long-term records of nitrate dynamics in the streams sampled. We used these well-characterized watersheds to develop and calibrate a tool to quickly assess N saturation status and NO₃^- source apportionment in forested catchments. This tool uses a mass-independent isotopic anomaly (Δ¹⁷O-NO₃^-) present in atmospheric nitrate to definitively attribute streamwater and soil solution NO₃^- pools to atmospheric and biogenic sources.  As Δ¹⁷O-NO₃^- is not subject to fractionation by biological processes such as nitrification and denitrification, it serves as a reliable quantitative measurement of atmospheric and biogenic NO₃^- source contributions to overall stream and soil solution nitrate pools.

Results/Conclusions

This poster outlines the development of the Δ¹⁷O-NO₃^- analytical methodology. Nitrate concentrations and preliminary isotopic composition are used to illustrate the potential for using isotopic tools to characterize saturation status. In particular, preliminary data on the calibration of this isotopic tool for N saturation stage determination are presented and specifics of the Δ¹⁷O-NO₃^- analytical methodology and its application to forest N saturation are discussed.