Harvest management and cultivar effects on dinitrogen fixation and nitrogen transfer in forage legume-grass mixtures

Field research was undertaken (1) to compare cultivars of birdsfoot trefoil (Lotus corniculatus L.) and red clover (Trifolium pratense L.) in terms of N[subscript]2 fixation and N transfer potentials and (2) to determine how harvest management (3-cut vs. 6-cut systems) affected amounts of N[subscript]2 fixation and N transfer by the same cultivars of birdsfoot trefoil and red clover. An experimental cultivar of birdsfoot trefoil and one of red clover, as well as three common, commercially available cultivars of each species,were seeded in binary mixtures with orchardgrass (Dactylis glomerata L.). Dinitrogen fixation and N transfer were estimated by [superscript]15N isotope dilution using orchardgrass pure stands as a reference. Dinitrogen fixation averaged (two-year means) 95.1 and 97.2% by birdsfoot trefoil and red clover, respectively. Dinitrogen fixation usually did not differ among cultivars of either birdsfoot trefoil or red clover. Nitrogen transfer to associated orchardgrass averaged 26.8% and 57.1% from birdsfoot trefoil and red clover, respectively. Nitrogen transfer did not differ among cultivars of either birdsfoot trefoil or red clover. Dinitrogen fixation by birdsfoot trefoil averaged 97.8 and 95.5% under the 3- and 6-cut systems, respectively. Dinitrogen fixation by red clover averaged 93.3 and 95.4% under the 3- and 6-cut systems, respectively. Harvest management had a significant effect on N[subscript]2 fixation by birdsfoot trefoil but not by red clover. Nitrogen transfer by birdsfoot trefoil averaged 43.4 and 47.0% under the 3- and 6-cut systems, respectively. Nitrogen transfer by red clover averaged 41.5 and 43.2% under the 3- and 6-cut systems, respectively. Harvest management generally did not affect N transfer from either birdsfoot trefoil or red clover to associated orchardgrass. It is concluded that, under the conditions of this study, (1) the birdsfoot trefoil and red clover cultivars tested generally did not differ in their abilities to fix atmospheric N[subscript]2 or to transfer fixed-N to associated orchardgrass, and (2) harvest management did not affect N[subscript]2 fixation by red clover, or N transfer by either birdsfoot trefoil or red clover; harvest management did affect N[subscript]2 fixation by birdsfoot trefoil.