HARVEST MANAGEMENT EFFECTS ON PRODUCTIVITY, DINITROGEN FIXATION, AND NITROGEN TRANSFER IN BIRDSFOOT-TREFOIL ORCHARDGRASS COMMUNITIES

Dinitrogen fixation and N transfer need to be quantified to help devel op optimum harvest recommendations for legume-grass mixtures. Field re search was undertaken to determine how harvest management (three-cut v s. six-cut systems) influenced the productivity, N-2 fixation, and N t ransfer of birdsfoot trefoil-orchardgrass communities. Four birdsfoot trefoil (Lotus corniculatus L.) cultivars were seeded in binary mixtur es with orchardgrass (Dactylis glomerata L.) Dinitrogen fixation and N transfer were estimated by N-15 isotope dilution using pure stands of orchardgrass as a reference. Herbage dry matter yields were greater u nder the three-cut system, averaging 8.8 Mg ha(-1) compared with 6.4 M g ha(-1) for the six-cut system (means of cultivars and years). Percen tage legume N derived from fixation (%Ndfa) was only slightly greater for the three-cut system. Mean %Ndfa for the three-cut system was 98% compared with 95% for the six-cut system. Total-season yields of fixed N in birdsfoot trefoil herbage, percentage N in orchardgrass herbage derived from fixation, and total-season yields of transferred N in orc hardgrass herbage generally were not affected by harvest management. W e conclude that six-cut management reduced dry matter yield compared w ith three-cut management. Relatively little difference was observed in biological N-2 fixation or N transfer, however.