Dinitrogen fixation in Kura clover and birdsfoot trefoil

The quantification of symbiotic N₂ fixation by legumes is essential to determine their impact on N budgets. Kura clover (Trifolium ambiguum M.B.) and birdsfoot trefoil (Lotus corniculatus L.) are two promising pasture legumes for which such estimates are scarce. Dinitrogen fixation by 2- and 3-yr-old stands of these species was determined at two locations in Minnesota using the ¹⁵N isotope dilution method. Species were evaluated for forage and N yields, the percent of N derived from the atmosphere (%Ndfa), and the amount of fixed N₂ at each of four harvests. The results for each parameter varied with location, legume species, and harvest, as demonstrated by a three-way interaction among these terms. Across environments, both species produced the greatest amount of forage, total N, and fixed N yields at the first harvest. The yields decreased with successive forage harvests at one location but stabilized at the other. The %Ndfa for Kura clover was stable across harvests and environments. The %Ndfa for birdsfoot trefoil varied during the season, with a peak at the end of the season. The yearly total forage, total N, and fixed N yields of Kura clover were stable across environments, and the total N and fixed N yields were greater than those of birdsfoot trefoil at one location. Overall, Kura clover fixed about 155 kg N ha^-1 yr^-1, with a %Ndfa of 57; birdsfoot trefoil had amounts of fixed N₂ that varied with locations (avg. of 145 kg N ha^-1 yr^-1), and it obtained about 62% of its herbage N from fixation.