Sources and transformations of nitrate from streams draining varying land uses: Evidence from dual isotope analysis

Douglas A. Burns, Elizabeth W. Boyer, Emily M. Elliott, Carol Kendall

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126 Scopus citations


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 15NNO3, and δ18ONO3. Samples from two forested watersheds indicated that NO3- derived from nitrification was dominant at baseflow. A watershed dominated by suburban land use had three δ18ONO3 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 δ15NNO3 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 NO315NNO3 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 15NNO3 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.

Original languageEnglish (US)
Pages (from-to)1149-1159
Number of pages11
JournalJournal of Environmental Quality
Issue number3
StatePublished - May 2009

All Science Journal Classification (ASJC) codes

  • Environmental Engineering
  • Water Science and Technology
  • Waste Management and Disposal
  • Pollution
  • Management, Monitoring, Policy and Law


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