Measurement of the stable isotope ratio of dissolved N2 in 15N tracer experiments

Stephen K. Hamilton,Nathaniel E. Ostrom

Limnol. Oceanogr. Methods 5:233-240 (2007) | DOI: 10.4319/lom.2007.5.233

ABSTRACT: Stable isotope addition experiments seeking to trace the denitrification of combined forms of nitrogen (N) to gaseous N2 in aquatic environments typically need to measure the stable isotope ratio of dissolved nitrogen gas. This measurement presents challenges because of the potential for contamination of samples by N in air, and because field experiments conducted in-situ often do not produce a marked enrichment in the isotope ratio of the N2 pool. Field experiments also require numerous samples, sometimes processed under arduous conditions, and thus methods have to be convenient and low in cost. This paper describes the methods for sampling and measurement of the N isotope ratio of dissolved nitrogen that were developed for the isotope addition experiments in the Lotic Intersite Nitrogen Experiment (LINX), a cross-site study examining N biogeochemistry in headwater streams. Headspace equilibration was performed in the field and gas samples were stored in re-evacuated glass vials (Exetainers®). Samples were processed and stored underwater to minimize the potential for contamination of samples by air. Isotope ratios were measured using a gas chromatograph interfaced to the isotope ratio mass spectrometer and equipped with a custom sample entry system.