Analysis of δ15N and δ18O to differentiate NO-3 sources in runoff at two watersheds in the Catskill Mountains of New York -: art. no. 1051

[1] To quantify the movement of atmospheric nitrogen deposition through two forested watersheds in the Catskill Mountains of New York, dual-isotope analysis (delta(15)N and delta(18)O) 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 delta(18)O-NO3- value of precipitation was +50.5parts per thousand, whereas the mean values for stream water and soil water were +17.7parts per thousand and +23.6parts per thousand, respectively. The mean delta(15)N-NO3- of precipitation was -0.2parts per thousand, that of soil water was +1.4parts per thousand, and that of stream water was +2.3parts per thousand; these values showed greater overlap among the three different waters than did the delta(18)O-NO3- values, indicating that delta(15)N-NO3- was not as useful for source separation. Soil water delta(18)O-NO3- values decreased, and delta(15)N-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 delta(15)N-NO3- values, from +1.5parts per thousand to +16.1parts per thousand, whereas delta(18)O-NO3- values ranged more narrowly, from +13.2parts per thousand to +16.0parts per thousand. Values of delta(18)O-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 delta(18)O-NO3- and delta(15)N-NO3- changed at differing rates as NO3- cycled through these watersheds because delta(18)O-NO3- values change sharply through the incorporation of oxygen from ambient water and gas during nitrification, whereas delta(15)N-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.