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Selection of Reference Plants and Methods for Estimating N₂ Fixation in Mycorrhizal Leguminous Plants

USDA-Forest Serv., Southeastern Forest Exp. Stn., Research Triangle Park, NC 27709

R. J. Volk

Dep. of Soil Science

C. B. Davey

Dep. of Soil Science and Forestry, North Carolina State Univ., Raleigh, NC 27695-7619

*Corresponding author.

ABSTRACT

The effects of plant species and mycorrhizae on reference plant suitability were examined with regard to estimating N₂ fixation in two leguminous plants via the ¹⁵N isotope dilution technique (ID). Atom percent excess (A % E) ¹⁵N values of ryegrass (Lolium perenne L. ‘PennSinn’), nonnodulated big trefoil (Lotus pedunculatus Cav. ‘Marshfield’), and nonnodulated subterranean clover (Trifolium subterraneum L. ‘Mt. Barker’), each with or without mycorrhizae, were compared at fertilizer N rates of 10 and 100 kg N ha^–1. Precision of the ID method was compared to the total nitrogen difference (ND) method. Atom percent excess ¹⁵N values of the reference plants had lower coefficients of variation than total N yields (mg plant^–1), and were unaffected by mycorrhizal treatment. Overall shoot values of A % E ¹⁵N were similar in trefoil and ryegrass, but were lower in clover in the 100 kg N ha^–1 fertilizer treatment. In roots, the A % E ¹⁵N values for clover were significantly larger than those for ryegrass. There were no differences between the values of N derived from the atmosphere (Ndfa) (percent and amount for shoot, root, and total plant) in clover, as estimated by the ID method, using nonnodulated clover and ryegrass as reference plants. Values for Ndfa (percent and amount), as estimated by the ND method, differed significantly from the ID values. The ND values were either smaller or larger than ID values, depending on N rate and inoculation treatment. In the 100 kg N ha^–1 rate, the ND method resulted in negative values for root Ndfa. Given the similarity of A % E ¹⁵N shoot values in all three reference plants, and the insensitivity of Ndfa to the reference plant used for its calculation, ryegrass was found to be an appropriate reference plant for determining N₂ fixation via the ID method in both big trefoil and subterranean clover.

Contribution from the Dep. of Forestry, North Carolina State Univ. Paper no. 11644 of the Journal Series of the North Carolina Agric. Res. Serv.

Received for publication June 14, 1988.