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Published online 1 May 1986
Published in Agron J 78:549-551 (1986)
© 1986 American Society of Agronomy
677 S. Segoe Rd., Madison, WI 53711 USA
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The Use of Nitrogen-15-Depleted Ammonium Sulfate for Estimating Nitrogen Fixation by Leguminous Trees1

Christopher van Kessel and Patty Nakao2

The feasibility of using 15N-depleted (NH4)2SO4 for assessing biological N2 fixation of leguminous trees was investigated. A slowgrowing species, lebbeck [Albizia lebbeck (L.) Benth], and a fastgrowing species, leucaena [Leucaena leucocephala (Lam.) de Wit] were inoculated with appropriate Rhizobium strains and grown under four different levels of N. Each week, 0, S, 12.5, or 25 mg of 15N depleted (NH4)2SO4 was applied to the upper chamber of the Leonard assembly. After 10 and 18 weeks growth for leucaena and lebbeck respectively, plants were separated into shoots, roots, and nodules for analysis. The atom% 15N in shoots and roots was linearly correlated to the amount of applied 15N-depleted (NH4)2SO4. Significant differences in atom% 15N and percentage N derived from N2 fixation were measured in lebbeck and leucaena when, respectively, more than 8.3 and 2.9% of the total N was derived from (NH4)2SO4. Nodules derived a larger portion of their total N from N2 fixation than did the roots or shoots, indicating that most of the N present in the nodules was derived from N2 fixation. Nodules of both species at the no N treatment showed slightly higher atom% 15N values than the 15N-natural abundance level of atmospheric N2.

Key Words: Leucaena • Leucaena leucocephala (Lam.) de Wit • Lebbeck • Albizia lebbeck (L.) Benth • Nitrogen-15 dilution • Rhizobium • Nitrogen-allocation

1 Contribution from the NifTAL Project, Department of Agronomy and Soil Science, University of Hawaii, Paia, Hawaii 96779. Supported in part by grant DAN-0613-C-00-2064-00.

2 Research scientist and research associate.

Received for publication June 12, 1985.




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Copyright © 1986 by the American Society of Agronomy.