Increase of natural N enrichment of soybean nodules with mean nodule mass.

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.