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, δ ¹⁵N_NO3, and δ¹⁸O_NO3. Samples from two forested watersheds indicated that NO₃^- derived from nitrification was dominant at baseflow. A watershed dominated by suburban land use had three δ¹⁸O_NO3 values greater than +25‰, indicating a large direct contribution of atmospheric NO ₃^- transported to the stream during some high flows. Two watersheds with large proportions of agricultural land use had many δ¹⁵N_NO3 values greater than +9‰, suggesting an animal waste source consistent with regional dairy farming practices. These data showed a linear seasonal pattern with a δ¹⁸O _NO3:δ¹⁵N_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 δ ¹⁵N_NO3 values (10‰) indicates that NO ₃^- 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 km² may be necessary to adequately quantify the relative roles of various NO ₃^- transport and process patterns that contribute to streamflow in large basins.