Effect of rhizobial strain and host plant on nitrogen isotopic fractionation in legumes.

Lotus pedunculatus L., Medicago sativa L., Macroptilium atropurpureum, Glycine max, and Trifolium repens L. were grown in a N-free medium and inoculated with one of ten Rhizobium strains. Dry matter, N content, and delta(15)N values were determined for various plant parts.Nodules, with the exception of those from lucerne, were enriched in (15)N relative to atmospheric N. Considerable variation was found in delta(15)N values of plant herbage (-4.5 to +0.8). The extent of isotopic discrimination was dependent on both the host plant and the infecting rhizobial strain. This further complicates, but does not invalidate, the use of small variations in the natural abundance of (15)N to estimate the contribution of symbiotically fixed N(2) to the N in legume herbage. Some other implications of the observed differences are also discussed.