Routine measurement of dissolved inorganic 15N in precipitation and streamwater

The difficulty of measuring N-15 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 N-15 measurement using the Teflon tape diffusion method Analysis of standards demonstrated >95% recovery of inorganic N from samples, and delta(15)N values comparable to those obtained using standard distillation methods. The value of the blank at our laboratory was 0.16 mu moles N. Analytical precision was within 2 parts per thousand delta(15)N when samples of streamwater from the Bear Brook watersheds (1800 eq.ha(-1).yr(-1) N addition at 192 parts per thousand delta(15)N) were kept frozen until analysis. The analytical process is lengthy, but once set up can easily be performed for large numbers of samples.