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Fig. 2. Ranges of 
18O and 15N values for potential NO3– sources and values measured in Mississippi River and tile drain samples. The domain of soil organic matter is shaded to show the range of greatest (darkest) to lowest (lightest) frequency. The domain of soil organic matter partially overlaps that of reduced N fertilizer, manure, and sewage with respect to 15N. The 18O of NO3– from these reduced N sources was calculated as described in the discussion. The large arrow represents a denitrification vector; as denitrification proceeds, the 15N and 18O values of the remaining NO3– progressively increase in the direction of the vector. The size of circles represents NO3–N concentration; shaded circles indicate tile drain samples. In all cases, analytical errors are smaller than the circle size. Synthetic NO3– Fertilizer refers to synthetic fertilizer applied as NO3– (e.g., the NO3– in NH4NO3) and Reduced N Fertilizer refers to NO3– produced by nitrification of applied NH3 (as NH3 or the NH4+ in NH4NO3) and urea.
