Variation in Natural Abundance of 15N among Plant Parts and in 15N/14N Fractionation during N2 Fixation in the Legume-Rhizobia Symbiotic System

Abstract

Sixteen legumes were grown in N-free media so that N was supplied entirely by symbiotic N₂ fixation. The plant tissues were analyzed for natural ¹⁵N abundance (expressed as δ¹⁵N per mil relative to air N₂) with a ratio mass spectrometer. The nodules of desmodium, centro, siratro, soybean and winged bean showed high enrichment in ¹⁵N (∼+9‰), while red clover showed slight enrichment (∼+2‰). The nodules of 9 other forage legumes (Townsville stylo, white clover, alsike clover, common vetch, Chinese milk vetch, senna, alfalfa, ladino clover, and hairy vetch) showed little enrichment in ¹⁵N. In all the legumes investigated, particularly in the ureide-transporting plants such as desmodium, centro, siratro, soybean, winged bean and field bean, the δ¹⁵N value of the shoots was negative (∼−3.2‰). The δ¹⁵N value of the shoots in winged bean and field bean varied by about 1‰ depending on the Rhizobium strains used. The isotopic mass balance of 13 legumes indicated that isotopic fractionation occurs during N₂ fixation by the legume-rhizobia symbiosis with a preference for ¹⁴N over ¹⁵N, resulting in a δ¹⁵N value of −0.2 to −2‰ in the whole plant. The results indicate that ¹⁵N/¹⁴N isotopic discrimination with a preference for the lighter atom may occur in both N₂ fixation and export of fixed N from nodules.