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Fertilizer-N Effects on N₂ Fixation by Cowpea and Soybean¹

While it is established that high levels of combined N inhibit the development and function of legume root nodules, the application of low-level "starter" doses of fertilizer-N is sometimes recommended to improve nodulation and N₂ fixation in the long term. The amounts of combined N which are required to optimize legume symbiosis are poorly defined. Before screening a number of rhizobia1 isolates for effectiveness with cowpea [Vigna unguiculata (L.) Walp.] we set out to determine what levels of fertilizer-N are optimal for symbiosis in pot conditions.

We examined the effects of fertilizer-N applied at or soon after sowing, on nodulation, N, fixation and growth of two cultivars of cowpea and, for comparison one of soybean [Glycine mar (L.) Merr.] in the greenhouse. Two experiments were done to examine (a) the effects of fertilizer N before flowering and (b) the effects of fertilizer-N throughout the growth cycle and on final yield. The levels of N found to stimulate symbiosis during the pre-flowering period were an order of magnitude higher than those previously reported. Applications of 36 and 72 mg N/plant increased nodule weights and acetylene reduction activities up to fourfold and sixfold, respectively, but trends varied with legume host and time of harvest. When 30, 90, 180, or 360 mg N/plant were applied as NH₄NO₃, KNO₃, or urea, symbiotic responses again varied and no single N treatment was optimal for all three legumes. Synergistic effects of applied N on total amounts of N fixed were observed with each host with various N treatments. The strongest synergism occurred in soybean with urea at 30 mg N/plant; for every milligram of urea-N absorbed, an additional 15 mg of N were fixed symbiotically.

Key Words: Vigna unguiculata (L.) Walp. • Glycine Max (L.) Merr. • ¹⁵N • Ammonium nitrate • Potassium nitrate • Urea • Nodulation • Acetylene reduction

² Associate scientist, visiting fellow, and research assistant, respectively.

Received for publication January 7, 1982.

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