Isotopic Fractionation Associated With Symbiotic N2 Fixation and Uptake of NO3− by Plants

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.