Abstract
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‰ (β = 0.99902). In clover the isotope effect was +1.88 ± 0.14‰ (β = 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‰ (β = ∼1.005).
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Selected References
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