Fig. 1. Values of ¹⁸O_nitrate vs. ¹⁵N_nitrate for fertilizer (open diamonds), fertilized soil (black circles), and unfertilized soil (black triangles) from the irrigated cropping systems study designated Rotation U located at Agriculture & Agri-Food Canada's Research Centre in Lethbridge, AB, Canada, determined using the denitrifier technique. Also shown are typical isotopic ranges of NO₃^– derived from NH₄NO₃ fertilizers and soil organic N (after Mengis et al. [2001] and references therein). Note that the range given for soil organic-N-derived NO₃^– is based on the analysis of soil water samples. Also note that Mayer et al. (2004) determined ¹⁵N values of synthetic NO₃^– fertilizer as high as 3.2, and Kendall (1998) lists ¹⁸O values of NO₃^– from soil nitrification as low as –10.