Discrimination against N-15 in three N-2-fixing Trifolium species as influenced by Rhizobium strain and plant age

When estimating N-2 fixation with the N-15 natural abundance method, the N-15 abundance in plants grown with N-2 in air as only N source ( B), must be known in order to account for discrimination against N-15 during N-2 fixation. Trifolium hybridum L., T. pratense L. and T. repens L. were grown without N in the nutrient solution in a greenhouse. The aim was to establish B values in these species in symbioses with Scandinavian Rhizobium genotypes, and to test the hypothesis that B is related to N-2 fixation efficiency and plant age. Choice of Rhizobium strain significantly influenced B in shoots of T. hybridum and T. repens, and N content in all Trifolium species. B in shoots was partly correlated to amounts of N-2 fixed, while plant age only had a marginal effect. Soil suspension inoculum or single strain inocula gave very similar B in shoots of T. pratense.