Ecosystems

, Volume 4, Issue 6, pp 536–544

Tracing the Sources of Exported Nitrate in the Turkey Lakes Watershed Using 15N/14N and 18O/16O isotopic ratios

  • John Spoelstra
  • Sherry L. Schiff
  • Richard J. Elgood
  • Ray G. Semkin
  • Dean S. Jeffries

DOI: 10.1007/s10021-001-0027-y

Cite this article as:
Spoelstra, J., Schiff, S., Elgood, R. et al. Ecosystems (2001) 4: 536. doi:10.1007/s10021-001-0027-y

Abstract

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. 15N/14N and 18O/16O isotopic ratios of nitrate can therefore be used to distinguish between these two sources of nitrate in surface waters and groundwaters. Two forested catchments in the Turkey Lakes Watershed (TLW) near Sault Ste. Marie, Ontario, Canada were studied to determine the relative contributions of atmospheric and microbial nitrate to nitrate export. The TLW is reasonably undisturbed and receives a moderate amount of inorganic nitrogen bulk deposition (8.7 kg N · ha−1· yr−1) yet it exhibits unusually low inorganic nitrogen retention (average = 65% of deposition). The measured isotopic ratios for nitrate in precipitation ranged from +35 to +59‰ (VSMOW) for δ18O and −4 to +0.8‰ (AIR) for δ15N. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately −1.0‰ for δ18O and a value of about 0 to +6‰ for δ15N, thus, the isotopic separation between atmospheric and soil sources of nitrate is substantial. Nitrate produced by nitrification of ammonium appears to be the dominant source of the nitrate exported in both catchments, even during the snowmelt period. These whole catchment results are consistent with the results of small but intensive plot scale studies that have shown that the majority of the nitrate leached from these catchments is microbial in origin. The isotopic composition of stream nitrate provides information about N-cycling in the forested upland and riparian zones on a whole catchment basis.

Key words: nitrate, stable isotopes, forested catchments, 15N, 18O, nitrogen saturation, nitrification

Copyright information

© Springer-Verlag New York Inc. 2001

Authors and Affiliations

  • John Spoelstra
    • 1
  • Sherry L. Schiff
    • 1
  • Richard J. Elgood
    • 1
  • Ray G. Semkin
    • 2
  • Dean S. Jeffries
    • 2
  1. 1.Department of Earth Sciences, University of Waterloo, 200 University Avenue, West Waterloo, Ontario, Canada N2L 3G1CA
  2. 2.Environment Canada, National Water Research Institute, Aquatic Ecosystems Conservation Branch, 867 Lakeshore Road,, P.O. Box 5050, Burlington, Ontario, Canada L7R 4A6CA