DOES CARBON-ISOTOPE DISCRIMINATION CORRELATE WITH BIOLOGICAL NITROGEN-FIXATION

It has recently been reported that N2 fixation and carbon isotope discrimination (DELTA) are negatively correlated. To further test this hypothesis, a greenhouse experiment was conducted to investigate if DELTA 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 DELTA values was only 25.8 to 26.6 parts per thousand. It is unlikely that variability of such small magnitude could be of any practical use in selecting for N2-fixing efficiency.