Graduate Thesis Or Dissertation
 

Nitrogen nutrition of seedling alfalfa (Medicago sativa L.)

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  • Nitrogen fertilization of seedling legumes is a controversial practice intended to provide N to the plant during periods when seed N, soil N, and N₂ fixation may not meet plant requirements; eg. prenodulation, postharvest, or in the event of an inoculation failure. Field experiments were conducted in 3 consecutive years (1980, 1981, and 1982) on a Woodburn silt loam soil (Aquultic Argixeroll) containing low soil N to evaluate the effect of 9 rates of NH₄ NO₃ -N (0-224 kg N ha⁻¹) on nodulation, acetylene reduction (AR), percentage herbage N, percentage nitrate N, and dry matter yield of seedling alfalfa (Medicago sativa L. cv. 'Apollo'). The R. meliloti at the field site were found to be ineffective at N₂ fixation, thus an inoculation treatment was added in 1982 by the use of a split-block design. The response of the variables to applied N was evaluated at 10 weeks, and over a postharvest time course. At ten weeks after planting, only the uninoculated, ineffectively nodulated plants exhibited increased herbage yield and %N in response to the applied N. All rates of N fertilization decreased nodulation and AR in a curvilinear response, regardless of rhizobial effectiveness. The applied NH₄NO₃ continued to decrease AR and nodulation of seedlings through the third week after the initial harvest. Acetylene reduction activity of control plants dropped to 68% of preharvest levels at 2 to 4 days postharvest, then recovered during the next 4 weeks. Herbage nitrate N was increased in one year out of three. Dry matter yield and total herbage N of uninoculated plants were increased during the initial growing season, and unaffected the following spring. Since the inoculated plants showed little response to the N rates, except for decreased N₂ fixation, it was concluded that during establishment, managerial emphasis should be placed on inoculation rather than N fertilization to obtain a consistent source of N for herbage protein. An ancillary investigation was conducted to characterize native Rhizobium taken from uninoculated, ineffectively nodulated, M. sativa nodules. Results suggested that Phaseolus vulgaris L. may be an alternate host for the native Rhizobium. Agricultural implications of this finding were discussed.
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