DINITROGEN FIXATION AND NITROGEN TRANSFER AMONG RED-CLOVER CULTIVARS

Red clover (Trifolium pratense L.) is an important perennial forage legume used for hay or as pasture in crop rotations. Despite its traditional usage as a source of nitrogen (N) for cropping systems, little information is available on the amounts of atmospheric dinitrogen (N2) that red clover fixes or transfers to an associated grass during long-term stands. Field research was undertaken in 1989 and 1990 to compare N2 fixation and N transfer potentials of one experimental and three common red clover cultivars seeded in binary mixtures with orchardgrass (Dactylis glomerata L.). Dinitrogen fixation and N transfer were estimated by N-15 isotope dilution using orchardgrass pure stands as a reference. Over the 2-yr study, percentage legume N derived from N2 fixation ranged from 96.4 to 96.7 % among the red clover cultivars. Total-season fixed-N yields in red clover herbage ranged from 72.6 to 159.2 kg ha-1. Dinitrogen fixation and fixed-N yields usually did not differ among red clover cultivars in either year. Percentage N in orchardgrass herbage derived from N2 fixation by red clover ranged from 43.7 to 70.5%. Total-season transferred-N yields in orchardgrass herbage was 16.9 kg ha-1 in 1989 and 57.8 kg ha-1 in 1990. Neither N-transfer nor transferred-N yield differed among cultivars in either year. It is concluded that, under the conditions of this study, the red clover cultivars tested generally did not differ in their abilities to fix atmospheric N2 or to transfer fixed-N to associated orchardgrass.