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This article in AJ

  1. Vol. 76 No. 5, p. 759-764
    Received: Aug 11, 1983

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Nitrogen Fixation in Soybeans: An Evaluation of Measurement Techniques1

  1. B. L. Vasilas and
  2. G. E. Ham2



Accurate estimates of N2 fixation by soybeans [Glycine max (L.) Merr.] are needed to evaluate specific interactions between Rhizobium, legume host, and environment. This study compares Kjeldahl-N, isotope dilution, and ‘A’-value techniques in the estimation of N2 fixation by field-grown soybeans receiving different levels of soil N. Experiments were conducted in 1979 and 1980 at Becker (Alfic Udipsamment) and Rosemount (Typic Hapludoll), MN. After planting but before emergence, 15N-labeled (NH4)2SO4, was surface applied in solution at rates of 5, 20, and 100 kg N/ha in 1979; and 10 and 100 kg N/ha in 1980. A non-nodulating soybean line was used as the control plant. Plant samples for N and 15N analyses were collected at full bloom, full pod, and full seed at Becker; and at full bloom, beginning seed, and physiological maturity at Rosemount. Combined N had little or no effect on N accumulation, vegetative production, nor grain yields of nodulating plants. Percentagewise, N fertilizer utilization in general was similar by nodulating and non-nodulating plants and tended to increase as fertilizer rates increased.

Although rarely statistically different, ‘A’-values consistently produced the highest and the isotope dilution technique the lowest estimates of N2 ked. Even under conditions of very low soil N availability, there was excellent agreement between the difference method and the isotope dilution technique. These two methods produced essentially the same values under conditions of low soil N and high fertilizer N (Becker, 100 kg N/ha). At the lowest level of applied N, N2 fixation, measured by isotope dilution, contributed approximately 77 and 48% of total plant N at Becker and Rosemount, respectively. Maximum N2-fixation values of 198 and 167 kg N/ha were achieved at Becker and Rosemount, respectively.

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