Nitrogen Isotope Distribution as a Presumptive Indicator of Nitrogen Fixation

Delwiche, C. C.; Zinke, Paul J.; Johnson, C. M.; Virginia, Ross A.

doi:10.1086/337037

Cited by 96 publications

(53 citation statements)

References 7 publications

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“…Kohl, Shearer & Commoner, 1971;Meints, Boone & Kurtz, I975fl), and on estimating the fractional contribution of N^-fixation to the N of Na-fixing plants (e.g. Amarger, Mariotti & Mariotti, 1977;Delwiche et al, 1979;Shearer & Kohl, 1986). Contemporary critics cautioned, in the former case, that internal isotope fractionations within ecosystems would preclude meaningful interpretation of source effects, i.e.…”

Section: Discussionmentioning

confidence: 99%

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

1997

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summary Equilibrium and kinetic isotope fractionations during incomplete reactions result in minute differences in the ratio between the two stable X isotopes, 15N and 14N, in various N pools. In ecosystems such variations (usually expressed in per mil [δ15N] deviations from the standard atmospheric N2) depend on isotopic signatures of inputs and outputs, the input‐output balance, N transformations and their specific isotope effects, and compartmentation of N within the system. Products along a sequence of reactions, e.g. the N mineralization‐N uptake pathway, should, if fractionation factors were equal for the different reactions, become progressively depleted. However, fractionation factors van. For example, because nitrification discriminates against 15N in the substrate more than does N mineralization, NH4+can become isotopically heavier than the organic N from which it is derived. Levels of isotopic enrichment depend dynamically on the stoichiometry of reactions, as well as on specific abiotic and biotic conditions. Thus, the δ15N of a specific N pool is not a constant, and 15N of a N compound added to the system is not a conservative, unchanging tracer. This fact, together with analytical problems of measuring 15N in small and dynamic pools of N in the soil‐plant system, and the complexity of the X cycle itself (for instance the abundance of reversible reactions), limit the possibilities of making inferences based on observations of 15N abundance in one or a few pools of N in a system. Nevertheless, measurements of δ15N might offer the advantage of giving insights into the N cycle without disturbing the system by adding 15N tracer. Such attempts require, however, that the complex factors affecting 15N in plants be taken into account, viz. (i) the source(s) of N (soil, precipitation, NOX, NH3, N2‐fixation), (ii) the depth(s) in soil from which N is taken up, (iii) the form(s) of soil‐N used (organic N, NH4+, NO3−), (iv) influences of mycorrhizal symbioses and fractionations during and after N uptake by plants, and (v) interactions between these factors and plant phenology. Because of this complexity, data on δ15N can only be used alone when certain requirements are met, e.g. when a clearly discrete N source in terms of amount and isotopic signature is studied. For example, it is recommended that N in non‐N2‐fixing species should differ more than 5% from N derived by N2‐fixation, and that several non‐N2‐fixing references are used, when data on δ15N are used to estimate Na‐fixation in poorly described ecosystems. As well as giving information on N source effects, δ15N can give insights into N cycle rates. For example, high levels of N deposition onto previously N‐limited systems leads to increased nitrification, which produces 15N‐enriched NH4 and N‐depleted NO3. As many forest plants prefer NH4−they become enriched in 15N in such circumstances. This change in plant 15N will subsequently also occur in the soil surface horizon after litter‐fall, and might be a useful indicator of N saturation, especially since ...

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Section: Discussionmentioning

confidence: 99%

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

1997

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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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confidence: 99%

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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“…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.…”

mentioning

confidence: 95%

“…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.…”

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confidence: 99%

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

Kohl¹,

Shearer²

1980

Plant Physiol.

150

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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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Nitrogen Isotope Distribution as a Presumptive Indicator of Nitrogen Fixation

Cited by 96 publications

References 7 publications

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

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

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

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

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Nitrogen Isotope Distribution as a Presumptive Indicator of Nitrogen Fixation

Cited by 96 publications

References 7 publications

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

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

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

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

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

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

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