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Isotopic Fractionation Associated With Symbiotic N₂ Fixation and Uptake of NO₃^– by Plants ¹

Department of Biology and Center for the Biology of Natural Systems, Washington University, St. Louis, Missouri 63130

Isotopic fractionation associated with N₂ fixation and NO₃^– uptake by plants are relevant to the accuracy of estimates of N₂ fixation based on differences in the natural abundance of ¹⁵N between N₂ fixing and nonfixing plants. The isotope effect on N₂ fixation by soybeans (Glycine max [L] Merrill, variety Harosoy) and red clover (Trifolium pratense [L]) was determined from the difference in ¹⁵N abundance between atmospheric N₂ and the total N of plants grown hydroponically with N-free nutrient solution. In soybeans the isotope effect was found to be +0.98 ± 0.18 ( = 0.99902). In clover the isotope effect was +1.88 ± 0.14 ( = 0.99812). The magnitude of these inverse isotope effects is small. However, they would lead to an underestimation of the amount of N₂ fixed, since the N of atmospheric origin which finally appears in the plant is made richer in ¹⁵N by the inverse isotope effects than is atmospheric N₂, and, to that degree, is attributed to soil-derived N in the calculation.

Isotopic fractionation associated with NO₃^– uptake by plants does not have a critical effect on estimates of N₂ fixation which are based on natural abundance of ¹⁵N since the ¹⁵N abundance of soil-derived N in plants is measured directly (i.e. after the N has undergone fractionation). Nevertheless, such fractionation is of some interest from the point of view of deciding upon the most appropriate sampling time. The isotope effect on NO₃^– uptake by a nonnodulating isoline of soybeans (variety Harosoy), marigold (Tagetes erecta [L]) and ryegrass (Lolium perenne [L]) was estimated from the difference between the ¹⁵N abundance of the total N of plants grown hydroponically and that of NO₃^– supplied in the medium. It was found to be about –5 ( = 1.005).

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