Abstract
When estimating N2 fixation with the 15N natural abundance method, the 15N abundance in plants grown with N2 in air as only N source (B), must be known in order to account for discrimination against 15N during N2 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 N2 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 N2 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.
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