Natural abundance of 15N in root nodules of soybean,lupin, subterranean clover and lucerne

The total nitrogen of root nodules of yellow lupins (Lupinus luteus) and soybeans (Glycine max), when grown with N₂ of air as the sole source of nitrogen, became progressively enriched with ¹⁵N relative to other parts of the plants. Nodules of subterranean clover (Trifolium subterraneum) and lucerne (Medicago sativa) were not enriched with ¹⁵N. Analysis of the distributions of ¹⁵N amongst nodule fractions showed highest specific enrichment in coarse plant cell fragments and bacteroids in soybean and in lupins the soluble protein was also highly enriched. In terms of the total μg of ¹⁵N excess, the bacteroids contained most in soybean nodules and the soluble protein contained most in lupin nodules.