Routine Measurement of Dissolved Inorganic 15N in Precipitation and Streamwater

The difficulty of measuring 15N in dilute solutions has limited the potential of ecosystem labeling experiments and model comparisons. By concentrating the N from large (up to 20 L) water samples on ion-exchange resin columns, we obtained enough N for accurate and reproducible 15N measurement using the Teflon tape diffusion method. Analysis of standards demonstrated > 95% recovery of inorganic N from samples, and δ15N values comparable to those obtained using standard distillation methods. The value of the blank at our laboratory was 0.16 μmoles N. Analytical precision was within 2 ‰ δ15N when samples of streamwater from the Bear Brook Watersheds (1800 eq ˙ ha−1 ˙ yr−1 N addition at 192 ‰ δ15N) were kept frozen until analysis. The analytical process is lengthy, but once set up can easily be performed for large numbers of samples.