DINITROGEN FIXATION AND NITROGEN TRANSFER AMONG RED-CLOVER CULTIVARS

Red clover (Trifolium pratense L.) is an important perennial forage le gume used for hay or as pasture in crop rotations. Despite its traditi onal usage as a source of nitrogen (N) for cropping systems, little in formation is available on the amounts of atmospheric dinitrogen (N2) t hat red clover fixes or transfers to an associated grass during long-t erm stands. Field research was undertaken in 1989 and 1990 to compare N2 fixation and N transfer potentials of one experimental and three co mmon red clover cultivars seeded in binary mixtures with orchardgrass (Dactylis glomerata L.). Dinitrogen fixation and N transfer were estim ated by N-15 isotope dilution using orchardgrass pure stands as a refe rence. Over the 2-yr study, percentage legume N derived from N2 fixati on ranged from 96.4 to 96.7 % among the red clover cultivars. Total-se ason 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 a mong 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 tra nsferred-N yield differed among cultivars in either year. It is conclu ded 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.