Nitrate sources and cycling at the Turkey Lakes Watershed: A stable isotope approach

Stable isotopic analysis of nitrate (15N/14N and 18O/16O) 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. Bulk precipitation collected biweekly at the TLW from 1995 to 2000 had nitrate isotope values that ranged from +42. 4 to +80. 4‰ for d18O and -6. 3 to +2. 8‰ for d15N. 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. The first direct measurement of the isotopic composition of microbial nitrate produced in situ was obtained by eliminating precipitation inputs to three forest floor lysimeters and subsequently watering the area with a nitrate-free solution. Microbial nitrate had d18O 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 d18O-H2O of the watering solution used. d18O values of soil O2 (+23. 2 to +24. 1‰) down to a depth of 55cm were not significantly different from atmospheric O2 (+23. 5‰) and therefore respiratory enrichment of soil O2 did not affect the d18O values of microbial nitrate produced at the TLW. 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. 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. Isotopic analysis indicated that the source of this nitrate was predominantly chemolithoautotrophic nitrification. The d18O 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. Despite drastic alteration of nitrogen cycling in the catchment by the harvest, d15N-nitrate values in shallow groundwater did not change from the pre-harvest. 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%