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Variations in Natural Nitrogen-15 Abundance as an Aid in Tracing Fertilizer Nitrogen Transformations¹

ABSTRACT

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 _a 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 _a¹⁵N 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 _a¹⁵N 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.

¹ Publication no. R219 of the Saskatchewan Institute of Pedology, Saskatoon, Sask., Canada. This paper was presented in part before Comm. II, 11th Congress ISSS, Edmonton, Canada, 26 June 1978. This study is part of a F.A.O./I.A.E.A. (Vienna) coordinated research program and was supported by the National Research Council of Canada.

² Professional Research Associate and Professor, respectively, Dep. of Soil Science, Univ. of Saskatchewan, Saskatoon, Sask., Canada S7N 0W0.

Received for publication February 9, 1979. Accepted for publication September 10, 1979.