HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS		QUICK SEARCH:   [advanced] Author: Keyword(s): Year:  Vol:  Page:

This Article Full Text (PDF) Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Wagner, G. H. Articles by Zapata, F. Search for Related Content PubMed Articles by Wagner, G. H. Articles by Zapata, F. Agricola Articles by Wagner, G. H. Articles by Zapata, F.

Field Evaluation of Reference Crops in the Study of Nitrogen Fixation by Legumes Using Isotope Techniques¹

Nitrogen fixed by a legume crop can be measured in a field fertilized with labeled ¹⁵N when a suitable non-fixing reference crop is grown also. Legume and reference crop, respectively, are used with ¹⁵N fertilizer to determine available soil plus fixed N and available soil N. Nitrogen derived from fixation is computed from the difference between these two estimates of N availability. Suitability of a reference crop depends on its utilization of fertilizer and soil N in the same proportion as the legume crop. Because this cannot be evaluated directly, the acceptability of reference crop data was examined by determining whether the proportion of fertilizer and soil S taken up by legume and non-fixing reference crops is the same. Various reference crops for determining N fixation by broadbean (Vicia faba L.) and by soybean (Glycine max L. Merr.) were examined by this approach in field experiments on a Typic Eutrocrept using ¹⁵N and ³⁵S labeled ammonium sulfate to determine plant available amounts (A-values) of N and S in the soil.

Nitrogen A-values for broadbean were clearly higher than for any of the non-fixing crops at two harvest dates. At physiological maturity of the legume, N fixation estimated from A-value comparisons was approximately 140 kg/ha. Sulfur A-values were reasonably similar among all crops studied but somewhat lower for oil radish (Raphanus sativus L.) which proved to be an inappropriate reference because it took up much more S than the other crops.

Nitrogen A-values for inoculated soybeans at the R₃ stage of maturity were similar to those for the several reference crops including non-nodulating soybeans. At stage R₆, N fixation was clearly evident from A-value comparisons but no greater than 60 kg/ha. By creating a N deficient condition in the soil using sucrose to immobilize N in an experiment with soybeans, the percentage of N in the crop derived from fixation was increased from 30 to 80%.

Key Words: Soybeans • Broadbeans • ¹⁵N • A-values • Sulfur

² Professor of agronomy, Univ. of Missouri, Columbia, MO 65211 on sabbatical at IAEA and Technical Officer Seibersdorf Laboratory, IAEA Vienna, Austria, respectively.

Received for publication June 22, 1981.