Tracing the sources of exported nitrate in the turkey lakes watershed using 15N/14N and 18O/16O isotopic ratios

Nitrate produced by bacterially mediated nitrification in soils is isotopically distinct from atmospheric nitrate in precipitation. N-15/N-14 and O-18/O-16 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 parts per thousand (VSMOW) for delta O-18 and -4 to +0.8 parts per thousand (AIR) for delta N-15. Nitrate produced from nitrification at the TLW is expected to have an average isotope value of approximately -1.0 parts per thousand for delta O-18 and a value of about 0 to +6 parts per thousand for delta N-15, 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.