Does carbon isotope discrimination correlate with biological nitrogen fixation?

It has recently been reported that N₂ fixation and carbon isotope discrimination (△) are negatively correlated. To further test this hypothesis, a greenhouse experiment was conducted to investigate if △ is correlated with the efficiency of lentil (Lens culinaris cv Laird) in fixing atmospheric nitrogen. Lentil seed was inoculated with one of 10 Rhizobium leguminosarum strains that varied in their effectiveness in symbiotic N₂ fixation. Carbon-13 discrimination was positively correlated with N₂ fixation (r² = 0.60*). Although the amount of N₂ fixed ranged from 1.5 mg N to 13.5 mg N shoot⁻¹, the range of △ values was only 25.8 to 26.6‰. It is unlikely that variability of such small magnitude could be of any practical use in selecting for N₂-fixing efficiency.