Nitrate sources and cycling at the Turkey Lakes Watershed: A stable isotope approach
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Date
2004
Authors
Spoelstra, John
Journal Title
Journal ISSN
Volume Title
Publisher
University of Waterloo
Abstract
<p class=MsoNormal><span style="mso-spacerun: yes"> </span>Stable isotopic analysis of nitrate (<sup>15</sup>N/<sup>14</sup>N and <sup>18</sup>O/<sup>16</sup>O) was used to trace nitrate sources and cycling under undisturbed conditions and following harvest at the Turkey Lakes Watershed (TLW), located near Sault Ste. Marie, Ontario, Canada. <span style="mso-spacerun: yes"> </span>
<p class=MsoNormal><span style="mso-spacerun: yes"> </span><span style="mso-spacerun: yes"> </span>Bulk precipitation collected biweekly at the TLW from 1995 to 2000 had nitrate isotope values that ranged from +42. 4 to +80. 4‰ for <span style='font-family:Symbol'>d</span><sup>18</sup>O and -6. 3 to +2. 8‰ for <span style='font-family:Symbol'>d</span><sup>15</sup>N. <span style="mso-spacerun: yes"> </span>An incubation experiment indicated that the isotopic composition of atmospheric nitrate was not compromised by collection methods whereby unfiltered bulk precipitation samples remain in the collector for up to two weeks. <span style="mso-spacerun: yes"> </span>
<p class=MsoNormal><span style="mso-spacerun: yes"> </span>The first direct measurement of the isotopic composition of microbial nitrate produced <i>in situ</i> was obtained by eliminating precipitation inputs to three forest floor lysimeters and subsequently watering the area with a nitrate-free solution. <span style="mso-spacerun: yes"> </span>Microbial nitrate had <span style='font-family:Symbol'>d</span><sup>18</sup>O values that ranged from +3. 1 to +10. 1‰ with a mean value of +5. 2‰, only slightly higher than values predicted based on the <span style='font-family:Symbol'>d</span><sup>18</sup>O-H<sub>2</sub>O of the watering solution used. <span style="mso-spacerun: yes"> </span><span style='font-family:Symbol'>d</span><sup>18</sup>O values of soil O<sub>2</sub> (+23. 2 to +24. 1‰) down to a depth of 55cm were not significantly different from atmospheric O<sub>2</sub> (+23. 5‰) and therefore respiratory enrichment of soil O<sub>2</sub> did not affect the <span style='font-family:Symbol'>d</span><sup>18</sup>O values of microbial nitrate produced at the TLW. <span style="mso-spacerun: yes"> </span>
<p class=MsoNormal><span style="mso-spacerun: yes"> </span>Nitrate export from two undisturbed first-order stream basins was dominated by microbial nitrate, with the contribution of atmospheric nitrate peaking at about 30% during snowmelt. <span style="mso-spacerun: yes"> </span>Clear-cutting of catchment 31 in 1997 resulted in elevated nitrate concentrations, reaching levels that exceeded the drinking water limit of 10 mg N/L. <span style="mso-spacerun: yes"> </span>Isotopic analysis indicated that the source of this nitrate was predominantly chemolithoautotrophic nitrification. <span style="mso-spacerun: yes"> </span>The <span style='font-family:Symbol'>d</span><sup>18</sup>O values of microbial nitrate in stream 31 progressively increased during the post-harvest period due to an increase in the proportion of nitrification that occurred in the summer months. <span style="mso-spacerun: yes"> </span>Despite drastic alteration of nitrogen cycling in the catchment by the harvest, <span style='font-family:Symbol'>d</span><sup>15</sup>N-nitrate values in shallow groundwater did not change from the pre-harvest. <span style="mso-spacerun: yes"> </span>Denitrification and plant uptake of nitrate in a small forested swamp in catchment 31 attenuated 65 to 100% of surface water nitrate inputs following harvest, reducing catchment-scale nitrate export by 35 to 80%.
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Keywords
Earth Sciences, nitrate, isotopes, forested catchments, nitrification, wetlands, forest harvest