Dinitrogen Fixation, and N and Dry Matter Distribution During 4 Year Stands of Birdsfoot Trefoil and Red Clover1

Birdsfoot trefoil (Lotus corniculatus L.) and red clover (Trifolium pratense L.) are important perennial forage legumes for use as hay or pasture in crop rotations. Despite their traditional usage as a source of N for cropping systems, little information is available on the amounts of N2 that birdsfoot trefoil and red clover fix during long‐term stands. We conducted field experiments with the 15N isotope dilution method over a stand life of 4 years to determine the inter‐ and intra‐annual fixation of N2 and the distribution of dry matter among organs for birdsfoot trefoil and red clover managed for hay production. Each legume species showed significant variation of N2 fixation within a year and among years of the stand. Red clover annually produced 22% greater average herbage dry matter compared with birdsfoot trefoil during the first 3 years of the stand. Nevertheless, the two species had similar quantities of dry matter in roots plus crowns and fixed similar average annual quantities of N2 ranging from about 60 to 120 kg N/ha. Dinitrogen fixation of birdsfoot trefoil exceeded that of red clover by 42% in Year 4 because it obtained a significantly greater proportion of its total N from fixation with similar herbage and root dry matter production. Over the 4‐year stand life, both species averaged 92 kg N/ha annually from N2 fixation. Throughout the experiments, birdsfoot trefoil and red clover distributed 10% or less of their dry matter to roots plus crown, components which usually had a N concentration 0.5 to 0.6 that of herbage. Therefore, the greatest return of fixed N2 to a subsequent crop would result from adoption of management practices that achieve a plowdown of a substantial growth of herbage rather than only roots plus crowns soon after hay harvest.