Dinitrogen fixation in Illinois bundleflower

Symbiotic N₂ fixation capacity may affect productivity of the perennial legume Illinois bundleflower [Desmanthus illinoensis (Michx.) MacMill. ex B.L. Rob. & Fernald]. Our objective was to estimate N₂ fixation of three Illinois bundleflower accessions managed for forage. Herbage yield ranged from 1.02 Mg ha^-1 to 3.69 Mg ha^-1 in Year 1, and 2.99 Mg ha^-1 to 8.27 Mg ha^-1 in Year 2. Accessions differed in herbage yield, aboveground N yield, and N₂ fixed at certain locations in Year 1, but not in Year 2. Percentage of N derived from the atmosphere (%Ndfa) varied with location but not with accession in either year. The ¹⁵N natural abundance method gave lower estimates of %Ndfa than the ¹⁵N enrichment method. In yr 1, N₂ fixation ranged from 0 to 30 kg ha^-1 N (¹⁵N natural abundance method), 11 to 43 kg ha^-1 (¹⁵N enrichment method), and 0 to 50 kg ha^-1 N (total N difference method), and in Year 2 these estimates at two locations were 60 to 67 kg ha^-1 N, 79 to 127 kg ha^-1 N, and 67 to 142 kg ha^-1 N, respectively. Differences in N₂ fixation among locations could have been due to rhizobial strains. At the high-yielding location with the greatest N ₂ fixation, over one-half of the nodules were occupied by indigenous rhizobial strains, whereas at other locations, strains from commercial inoculant accounted for most nodules. Symbiotic N₂ fixation by Illinois bundleflower could be enhanced by new rhizobial inoculums.