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Fig. 1. Values of 
18Onitrate vs. 15Nnitrate 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.
