Strain of Rhizobium lupini determines natural abundance of 15N in root nodules of Lupinus spp

Abstract Discrepancies between French and Australian results in measurement of the natural enrichment of 15 N in nodules of Lupinus spp were due to the strains of Rhizobium lupini used. The Australian inoculant strain, WU425, produced nodules enriched in 15 N above values for the entire plants, on all lines of L. luteus and L. albus tested. The total nitrogen and the natural abundance of 15 N in nodules produced by the French strain, LL13, were much lower. In tests in Australia and in France with L. luteus inoculated with 15 other strains of R. lupini from diverse origins, 9 produced nodules with elevated 15 N abundance, but of these 8 were less than with strain WU425. Most strains producing 15 N-enriched nodules were slower-growing on yeast extract-mannitol agar; most faster-growing strains produced little or no nodular 15 N enrichment. For strains with 15 N-enriched nodules, enrichment increased with increasing nodule N·plant −1 , and shoot nitrogen tended to be depleted in 15 N. There was no relationship between symbiotic effectiveness of strains (mg N 2 fixed·plant −1 ) and 15 N enrichment of nodules.