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 "N between N2 fixing and nonfixing plants. The isotope effect on N2 fixation by soybeans (Glyine max ILI Merrill, variety Harosoy) and red clover (Trifolium pratense ILI) was determined from the difference in "N 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%o (f? = 0.99902). In clover the isotope effect was +1.88 ± 0.14%o (( = 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 "5N 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 "5N since the l"N abundance of soil-derived N in plants is measured directly (Le. 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 ILl) and ryegrass (Lolium perenne ILI) was estimated from the difference between the "N abundance of the total N of plants grown hydroponically and that of NO3-supplied in the medium. It was found to be about -5%o (8 = 1.005).A number of investiators have reported a small, but measureable, difference in the 5N abundance of symbiotic N2-fixing and nonfLxing plants growing at the same site (1,2,6,7,12,15,18). The difference is always in the same direction, with the N2-fixing plant being less abundant in "N than nonfixing plants. This difference has led to the suggestion (6) that the "N abundance of plants may be used as an index of the contribution of atmospheric N2 to the N economy of plants or plant communities.Such an index may be useful in estimating the contribution of symbiotic N2 fixation integrated over time despite the fact that the acetylene reduction assay (8) is considerably more sensitive in detecting the presence of symbiotic N2-fixing activity. Estimating the contribution of symbiotically fixed N2 over the lifetime of the plant or within an ecosystem poses very different problems from estimating the instantaneous rate of nitrogenase activity from the rate of acetylene reduction. Among the problems in using the 'The work reported in this paper was supported by Grant DEB 77-01869 from the National Science Foundation. 51 acetylene assay for the former purpose are (a) conversion of acetylene reduced to N2 reduced (4); (b) large environmental, diurnal and seasonal variations (8); (c) the determination...

Tansley Review No. 9515N natural abundance in soil‐plant systems

“…Kohl & Shearer (1980) observed a larger discrimination against ^"N in non-nodulating soybeans, ryegrass and marigold (A = -32, -4-4 and -4-0/^, respectively), but they used a higher concentration of NOg^ in the medium (7-5 compared with 5 rriM). In another study, of pearl millet {Pennisetum spp.…”

Estimates of N2 Fixation Based on Differences in the Natural Abundance of 15N in Nodulating and Nonnodulating Isolines of Soybeans

“…Amarger et al (2) measured the isotope effect on N2 fixation by four species of soybeans to be about -I%o. For the variety of soybeans used in these experiments, we have measured the isotope effect on N2 fixation to be +I%o (8). Had we applied this isotope effect to the estimates shown in Table IV, the estimates would have been higher by about 3% when 25% of the plant's N was fixed, about 6% when 50%o of the plant's N was fixed and by about 1 1% when 100%1o of the plant's N was fixed, given a 8-15N value of +6.6 for soil-derived N in the plant (the average of all unfertilized nonnodulating plants [ Tables I, II, III]).…”

“…This paper reports results of a test of the 15N abundance method of estimating the contribution of atmospheric N2 to the total N of N2 fixing plants in greenhouse and field experiments. Variation in 15N abundance in different plant parts and isotope effects associated with uptake of atmospheric N2 and NO3-are reported in companion papers (8,13).…”

Distribution of 15N Among Plant Parts of Nodulating and Nonnodulating Isolines of Soybeans

“…The unit used to express '5N abundance in this paper is _-15N which is defined elsewhere (8 in the few instances in which such variation occurred, is discussed later in this paper.) Table I shows A8-'5N values for above-ground portions of the plants; Table II shows values for below-ground portions (roots and nodules in the nodulating isoline and roots in the nonnodulating isoline).…”