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Analysis of δ15N and δ18O to differentiate NO3− sources in runoff at two watersheds in the Catskill Mountains of New York

January 1, 2002

To quantify the movement of atmospheric nitrogen deposition through two forested watersheds in the Catskill Mountains of New York, dual‐isotope analysis (δ15N and δ18O) was used to differentiate NO3 derived from precipitation from NO3 derived by microbial nitrification and to quantify the contributions of these sources to NO3 in drainage waters. Samples of stream water, soil water, precipitation, snowmelt, and O‐horizon soil were collected during the March and April snowmelt period of 1994 and throughout an 18‐month period from August 1995 through February 1997. The mean δ18O‐NO3 value of precipitation was +50.5‰, whereas the mean values for stream water and soil water were +17.7‰ and +23.6‰, respectively. The mean δ15N‐NO3 of precipitation was −0.2‰, that of soil water was +1.4‰, and that of stream water was +2.3‰; these values showed greater overlap among the three different waters than did the δ18O‐NO3 values, indicating that δ15N‐NO3 was not as useful for source separation. Soil water δ18O‐NO3 values decreased, and δ15N‐NO3 values increased, from the O to the B and C horizons, but most of the differences among horizons were not statistically significant. Nitrate derived by nitrification in incubated soil samples had a wide range of δ15N‐NO3 values, from +1.5‰ to +16.1‰, whereas δ18O‐NO3 values ranged more narrowly, from +13.2‰ to +16.0‰. Values of δ18O‐NO3 indicated that NO3 in stream water is mainly derived from nitrification. Only during a high‐flow event that exceeded the annual flood was precipitation a major contributor to stream water NO3. Values of δ18O‐NO3 and δ15N‐NO3 changed at differing rates as NO3 cycled through these watersheds because δ18O‐NO3 values change sharply through the incorporation of oxygen from ambient water and gas during nitrification, whereas δ15N‐NO3 values change only incrementally through fractionation during biocycling processes. The results of this study show that most NO3 is first cycled through the biota and nitrified before entering the stream.

Publication Year 2002
Title Analysis of δ15N and δ18O to differentiate NO3− sources in runoff at two watersheds in the Catskill Mountains of New York
DOI 10.1029/2001WR000292
Authors Douglas A. Burns, Carol Kendall
Publication Type Article
Publication Subtype Journal Article
Series Title Water Resources Research
Index ID 70024740
Record Source USGS Publications Warehouse