Sources and Transformations of Nitrate from Streams Draining Varying Land Uses: Evidence from Dual Isotope Analysis

Knowledge of key sources and biogeochemical processes that affect the transport of nitrate (NO3-) in streams can inform watershed management strategies for controlling downstream eutrophication. We applied dual isotope analysis of NO3- to determine the dominant sources and processes that affect NO3- concentrations in six stream/river watersheds of different land uses. Samples were collected monthly at a range of flow conditions for 15 mo during 2004-05 and analyzed for NO3- concentrations, delta N-15(NO3), and Samples from delta O-18(NO3). two forested watersheds indicated that NO3- derived from nitrification was dominant at baseflow. A watershed dominated by Suburban land use had three delta O-18(NO3) Values greater than +25 parts per thousand, indicating a large direct contribution of atmospheric NO; transported to the stream during some high flows. Two watersheds with large proportions of agricultural land use had many delta N-15(NO3) values greater than +9 parts per thousand, suggesting an animal waste source consistent with regional dairy farming practices. These data showed a linear seasonal pattern with a delta O-18(NO3):delta N-15(NO3) of 1:2, consistent with seasonally varying denitrification that peaked in late summer to early Fall with the warmest temperatures and lowest annual streamflow. The large range of delta N-15(NO3) values (10 parts per thousand) indicates that NO3- supply was likely not limiting the rate of denitrification, consistent with ground water and/or in-stream denitrification. Mixing of two or more distinct sources may have affected the seasonal isotope patterns observed in these two agricultural streams. In a mixed land use watershed of large drainage area, none of the source and process patterns observed in the small streams were evident. These results emphasize that observations at watersheds of a few to a few hundred km(2) may be necessary to adequately quantify the relative roles of various NO3- transport and process patterns that contribute to streamflow in large basins.