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The (15)N abundance of soybean (Glycine max L. Merrill var Harosoy) nodules is usually greater than it is for other tissues or for atmospheric N(2). Results of experiments in which nodules were separated by size show that the magnitude of the (15)N enrichment is correlated with nodule mass. The results support the hypothesis that (15)N enrichment of nodules results from differential N isotopic fractionation for synthesis of nodule tissue versus synthesis of compounds for export from the nodule. The physiological significance of this hypothesis is that it requires that a substantial fraction of the N for nodule tissue synthesis in (15)N-enriched nodules be N recently fixed within the same nodule.

This paper expands upon previous reports of (15)N elevation in nodules (compared to other tissues) of N(2)-fixing plants. N(2)-Fixing nodules of Glycine max (soybeans), Vigna unguiculata (cowpea), Phaseolus vulgaris (common bean), Phaseolus coccineus (scarlet runner bean), Prosopis glandulosa (mesquite), and Olneya tesota (desert ironwood) were enriched in (15)N. Nodules of Vicia faba (fava beans), Arachis hypogaea (peanut), Trifolium pratense (red clover), Pisum sativum (pea), Lathyrus sativus (grass pea), Medicago sativa (alfalfa), and Lupinus mutabilis (South American lupine) were not; nor were the nodules of nine species of N(2)-fixing nonlegumes. The nitrogen of ineffective nodules of soybeans and cowpeas was not enriched in (15)N. Thus, (15)N elevation in nodules of these plants depends on active N(2)-fixation. Results obtained so far on the generality of (15)N enrichment in N(2)-fixing nodules suggest that only the nodules of plants which actively fix N(2) and which transport allantoin or allantoic acid exhibit (15)N enrichment.

Soybean (Glycine max L. Merrill) nodules are usually more enriched in (15)N than other tissues. We show that both bacteroids and nodule cortex are considerably more enriched in (15)N than nodule cytosol, with bacteroids being slightly more enriched than the cortex. Hence, (15)N enrichment occurs in cells of both plant and bacterial origin.

Non-nodular tissue of soybean (Glycine max L. Merrill) plants grown hydroponically in the absence of added N have a (15)N abundance close to that of atmospheric N(2). In contrast, nodules are usually enriched in (15)N. In this paper, we report measurements of the (15)N abundance of foliar tissue and nodules of soybeans inoculated with 11 variably efficient strains of Rhizobum japonicum and grown hydroponically with no added N. The efficiency of the 11 symbioses varied over a wide range as judged by a 16-fold difference in N content. The degree of (15)N enrichment of nodules was closely correlated with N(2)-fixing efficiency (milligrams N fixed per milligram N in the nodules).These results confirm prior preliminary data based on six variably efficient R. japonicum strains. The strong correlation between (N)N enrichment of soybean nodules and N(2)-fixing efficiency is consistent with the hypothesis that new nodule tissue is synthesized from a pool of recently fixed N within the same nodule.