Distribution of <sup>15</sup>N Among Plant Parts of Nodulating and Nonnodulating Isolines of Soybeans

Shearer, Georgia; Kohl, Daniel H.; Harper, James E.

doi:10.1104/pp.66.1.57

Cited by 74 publications

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“…However, because the natural abundance of "5N varies with depth from the soil surface (23) some limitations on the use of differences between the isotopic composition of fixing and nonfixing plants may occur unless the distribution of roots with depth is similar for both species. The magnitude of isotopic discrimination during N2 fixation is small and some preference is shown for the lighter "4N isotope (11), although the latter is not universally accepted (12 (18) and it has been suggested that "N enrichment is a result of the denitrifying capability of some rhizobia (20) although this has subsequently been rejected (10). A more recent hypothesis is that "N enrichment in nodules is indicative of the predominant type of N compound transported from the nodule (18).…”

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“…Many investigators have reported small but measurable differences in the natural abundance of "N in N2-fixing and nonfixing plants growing in association, and some have suggested that these differences may be used to provide a quantitative integrated measurement over time ofthe contribution ofN2 to the N economy of the fixing plant (2,5,13,16,20 (22). This raises the question as to whether the isotopic composition of the host plant is influenced by environmental parameters or by the Rhizobium strain with which it is infected.…”

Section: Introductionmentioning

confidence: 99%

“…This further complicates, but does not invalidate, the use of small variations in the natural abundance of 1"N to estimate the contribution of symbiotically fixed N2 to the N in legume herbage. Some other implications of the observed differences are also discussed.Legumes obtain their N requirements from two sources: symbiotic N2 fixation and assimilation of soil mineral N. Although a quantitative partitioning of both sources is important in understanding the N nutrition of legume crops in mono-culture or in mixed species stands, there are relatively few reports in the literature of field measurements of symbiotic N2 fixation, largely because of the difficulties of making such measurements (14).Many investigators have reported small but measurable differences in the natural abundance of "N in N2-fixing and nonfixing plants growing in association, and some have suggested that these differences may be used to provide a quantitative integrated measurement over time ofthe contribution ofN2 to the N economy of the fixing plant (2,5,13,16,20 (22). This raises the question as to whether the isotopic composition of the host plant is influenced by environmental parameters or by the Rhizobium strain with which it is infected.…”

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Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

Steele¹,

Daniel²,

O’Hara³

1983

Plant Physiol.

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Lotus pedculatus L., Medicago sativa L., Macroptilwum atropwrpureum, Glycine max, and Trifoliun repens L. were grown in a N-free medium and inoculated with one of ten Rhizobium strains. Dry matter, N content, and 8"N values were determined for various plant parts.Nodules, with the exception of those from lucerne, were enriched in 1"N relative to atmospheric N. Considerable variation was found in 81"N values of plant herbage (-4.5 to +0.8). The extent of isotopic discrimination was dependent on both the host plant and the infecting rhizobial strain. This further complicates, but does not invalidate, the use of small variations in the natural abundance of 1"N to estimate the contribution of symbiotically fixed N2 to the N in legume herbage. Some other implications of the observed differences are also discussed.Legumes obtain their N requirements from two sources: symbiotic N2 fixation and assimilation of soil mineral N. Although a quantitative partitioning of both sources is important in understanding the N nutrition of legume crops in mono-culture or in mixed species stands, there are relatively few reports in the literature of field measurements of symbiotic N2 fixation, largely because of the difficulties of making such measurements (14).Many investigators have reported small but measurable differences in the natural abundance of "N in N2-fixing and nonfixing plants growing in association, and some have suggested that these differences may be used to provide a quantitative integrated measurement over time ofthe contribution ofN2 to the N economy of the fixing plant (2,5,13,16,20 (22). This raises the question as to whether the isotopic composition of the host plant is influenced by environmental parameters or by the Rhizobium strain with which it is infected.Nodules of some N2-fixing plants are elevated in "N compared to other plant tissues (18) and it has been suggested that "N enrichment is a result of the denitrifying capability of some rhizobia (20) although this has subsequently been rejected (10). A more recent hypothesis is that "N enrichment in nodules is indicative of the predominant type of N compound transported from the nodule (18). Inasmuch as "N enrichment in nodules will affect the "N concentration in harvested plant material, it will influence estimates of N2 fixation based on variations in the natural abundance of "5N.

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Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

Steele¹,

Daniel²,

O’Hara³

1983

Plant Physiol.

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“…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).…”

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

1980

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Estimates of the contribution of biologically fixed N to the total N of nodulating soybeans (Glycine max (L) Merrill, variety Harosoy) grown under a variety of conditions were made from: (a) differences in N yield between nodulating and nonnodulating isolines; and (b) differences in 15N abundance between the two isolines. For plants grown in a greenhouse in nutrient-poor soil, both estimates showed a high level of N2 fixation; from 58 to 89% N fixed by differences in N yield and from 51 to 95% by differences in "5N abundance. Decreasing contributions of fixed N were estimated by both methods with increasing levels of added N03-. Results of field experiments carried out over two years on an unamended highly fertile midwestern soil showed a modest level of N2 fixation by both methods (7.3 to 51% by differences in N yield, and 5.4 to 46% by differences in 15N abundance). When the soil was amended with ground corn cobs, both methods showed higher contributions of fixed N. The two methods of estimating N2 fixation gave similar results. Both appear to be semiquantitative and the standard errors of the estimates were about the same (6% on the average).A number of investigators (1-4, 9, 10, 11, 14) have reported small differences in the 15N abundance between N2-fixing and non-N2-fixing plants, with the 15N abundance in N2-fixing plants being consistently lower than in nonfixing plants. These investigators have suggested that this difference could be used to indicate the contribution of atmospheric N2 to the total N of the N2-fixing plants. Such a method requires that: (a) there be a measurable difference in the "5N abundance between the two major sources of N to N2 fixing plants (atmospheric N2 and soil-derived N); and (b) this difference be maintained after alteration of 15N abundance resulting from isotopic discrimination on uptake of N. If portions of the plant, rather than the entire plant, are sampled, the method also requires that isotopic discrimination associated with metabolic processes, and subsequent transport of isotopically altered N compounds, does not result in large distortions of the original "5N abundance in the sources of the N.In applying the natural "5N abundance method, the 15N abundance in non-N2-fixing plants is used as a measure of the 15N content of soil-derived N, after it has undergone any fractionation that may occur during N transformations in the soil and on uptake into the plant. The 1 N abundance of the N2-fixing plant should

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“…Since the N2 fixed by the plants is drawn from an infinite reservoir, measured 8 (11,15,16), is sufficient to influence the 15N abundance of N in the whole plant, in spite of the small relative mass. The average A8-'6N value for the above ground portion of soybean plants on the last four sampling dates was +0.13 ± 0.14 compared to +0.98 ± 0.18 for the entire plant.…”

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

Kohl¹,

Shearer²

1980

Plant Physiol.

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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...

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

Cited by 74 publications

References 10 publications

Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

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

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

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

Cited by 74 publications

References 10 publications

Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

Effect of Rhizobial Strain and Host Plant on Nitrogen Isotopic Fractionation in Legumes

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

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

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

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

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