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  1. Vol. 21 No. 2, p. 330-335
     
    Received: Aug 15, 1980
    Published: Mar, 1981


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doi:10.2135/cropsci1981.0011183X002100020032x

Nitrogen Fixation of Alfalfa in the Seeding Year1

  1. G. H. Heichel,
  2. D. K. Barnes and
  3. C. P. Vance2

Abstract

Abstract

Alfalfa (Medicago saliva L.) obtains N from the soil and from the atmosphere by symbiotic nitrogen (N2) fixation. It is necessary to identify the contribution from each N source to evaluate N2-fixation of different alfalfa germplasms. Field experiments were undertaken using 15N as a tracer of N metabolism to determine the N2-fixation of two alfalfa populations in the seeding year. Additional objectives were to investigate the effects of establishment method, stage of plant development, and non-uniform distribution of isotope in the plant on assessment of N,-fixation capability.

During the seeding year the two populations averaged about 43% of their N needs from symbiosis and fixed an average of 148 kg/ha of N during the growing season. The isotope-dilution and A-value methods gave similar assessments of performance over four harvest intervals. Nitrogen fixation measured by isotope dilution was least (8 to 20 kg/ha) in the first and fourth harvest intervals when 25 to 30% of the N needs were met by symbiosis; it was greatest (47 to 87 kg/ha) during the second and third harvest intervals, when the populations derived more than 60% of their N from symbiosis.

The proportion of N derived from symbiosis for whole plants of seeded and transplanted materials differed significantly (P<0.05) among harvests, but the average over all harvests was not different for plants established by the two methods. A significant (P<0.05) interaction of sample type (herbage vs. whole plant) with harvest in transplants showed that equilibration of isotope in initially unlabeled seedlings occurred slowly early in the growing season.

In contrast to glasshouse evaluations of nitrogenase activity, significant (p<0.05) differences in rates of N2-fixation per plant were observed between the two populations at several harvests and over the growing season. However, a 14% difference in average plant stand precluded differences between populations in N2 fixation on a land area basis. The rates of N2 fixation of field grown plants mirrored growth rates, which varied with the changing field environment and especially with onset of fall dormancy. The use of stable N isotopes can be a powerful technique in assessing progress in breeding alfalfa for increased N2-fixation capability.

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