Rapeseed plant tissue and composite soil samples from field fertility plots were assayed for total N and inorganic N content, respectively, and corresponding 15N abundance. Lower δa 15N values were obtained for subsurface horizons containing residual fertilizer-N in both dryland (−2.0) and irrigated (ave. −3.5) plots compared to the surface horizons (3.4 and ave. 8.6). A systematic decline in the δa15N values of the plant tissue was observed with irrigation application (20 and 30 cm) and application of N fertilizer (220 kg/ha). This was attributed in both instances to the increased contribution of fertilizer N to the plant. Calculation of the mass balance of the two isotopes in the fertilized dryland treatment indicated that no significant fractionation had occurred during N uptake by plants but the differences in the δa15N values represented changes in soil N. While this study has clearly demonstrated the potential of utilizing variations in natural 15N abundance to trace the fate of applied fertilizer N, it has also shown that it is of utmost importance to recognize the N transformations that have taken place in the soil-plant system and the isotope effects that accompany them.
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Karamanos et al. (1980) studied this question.