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WATER RESOURCES RESEARCH,
VOL. 38, NO. 5,
1051,
doi:10.1029/2001WR000292,
2002
Analysis of δ15N and δ18O to differentiate NO3− sources in runoff at two watersheds in the Catskill Mountains of New York
Douglas A. Burns
U.S. Geological Survey,
Troy,
New York,
USA
Carol Kendall
U.S. Geological Survey,
Menlo Park,
California,
USA
Abstract
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.
Published 11
May
2002.
Index Terms: 1040 Geochemistry: Isotopic composition/chemistry; 1615 Global Change: Biogeochemical processes (4805); 1803 Hydrology: Anthropogenic effects; 1806 Hydrology: Chemistry of fresh water; 1871 Hydrology: Surface water quality.
Read Full Article (file size: 526216 bytes) Cited by
Citation: Burns, D. A., and C. Kendall
(2002),
Analysis of δ15N and δ18O to differentiate NO3− sources in runoff at two watersheds in the Catskill Mountains of New York,
Water Resour. Res.,
38(5),
1051,
doi:10.1029/2001WR000292.
Copyright 2002 by the American Geophysical Union.
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