Variations in Natural Nitrogen-15 Abundance as an Aid in Tracing Fertilizer Nitrogen Transformations
1980
Karamanos, R. E. | Rennie, D. A.
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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