HARVEST MANAGEMENT EFFECTS ON DINITROGEN FIXATION AND NITROGEN TRANSFER IN RED-CLOVER ORCHARDGRASS MIXTURES

The role of harvest management on the N nutrition of perennial forage legume-grass mixtures requires further investigation. Closer examination would provide a better understanding of the roles of defoliation. N2 fixation, and N-transfer in the management of red clover (Trifolium pratense L.)-orchardgrass (Dactylis glomerata L.) mixtures. Periodic harvesting of shoots of forage legumes removes a primary source of energy for maintaining N2 fixation. Nodule structure and function and the initiation of new nodules is affected, also. The ability of nodules to remain functional after removal of the vegetative shoots may depend upon a number of factors (e.g., plant species, microsymbiont, nodule morphology, rate of shoot regrowth, nutrient availability, and competition between plant parts for energy). Field research was undertaken to determine how harvest management (three-cut vs. six-cut systems) affected amounts of N2 fixation and N transfer in four red clover-orchardgrass mixtures. Dinitrogen fixation and N transfer were estimated by N-15 isotope dilution using pure stands of orchardgrass as a reference. Over the 2-yr study, percentage legume N derived from fixation (%Ndfa) ranged from 91 to 96% and from 91 to 97% under the three- and six-cut systems, respectively. Total-season yields of fixed-N in red clover herbage ranged from 70 to 242 and from 59 to 174 lb/acre under the three- and six-cut systems, respectively. Differences in %Ndfa and fixed-N yield between the two harvest managements generally were not significant (P less-than-or-equal-to 0.05). Percentage N in orchardgrass herbage derived from fixation (%Ngdfa) ranged from 18 to 44 and from 33 to 50% under the three- and six-cut systems, respectively Total-season yields of transferred-N in orchardgrass herbage ranged from 9 to 27 and from 17 to 29 lb/acre under the three-and six-cut systems, respectively. Differences in %Ngdfa and transferred-N yield between the two harvest managements generally were not significant (P less-than-or-equal-to 0.05). It is concluded that, under the conditions of this study, the intensive harvest management system did slightly increase N2 fixation by red clover but not N transfer from red closer to associated orchardgrass.