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Symbiotic Nitrogen Fixation in Soybeans. III. Effect of Supplemental Nitrogen and Intervarietal Grafting¹

Two field experiments were conducted to determine the effect of 1) grafting onto root genotypes with different nitrogen fixation rates, and 2) supplemental nitrogen applied at the time of declining nodule activity (end of flowering), on nodulation, seed yield, and protein yield of ‘Chippewa 64’ and ‘Clay’ soybeans (Glycine max (L.) Merr.). The experiments were conducted in situations of low available soil nitrogen to ensure a strong reliance on symbiotic nitrogen fixation. In the first study, seedling shoots of both varieties were grafted onto four roofstocks: ‘Wayne’, ‘Altona’, Chippewa 64 and Clay.

Differences were evident among root genotypes, independent of shoot genotype, for acetylene reduction activity per plant, but not for nodule fresh weight. for Chippewa 64 shoots, seed yield and seed protein content were significantly altered by grafting onto other rootstocks, whereas for Clay shoots only seed protein content was significantly changed from that of the selfgrafted controls. The effects of the root genotypes on seed yield were related in part to the respective differences in ability to support nitrogen fixation.

In the second study, supplemental nitrogen (0, 224, and 448 kg per ha) was applied as ammonium nitrate at the end of flowering to Chippewa 64 and Clay soybeans. The application of nitrogen caused a decline in acetylene reduction activity for both varieties. Seed yield and seed protein content for Clay, and seed protein content and vegetative protein yield for Chippewa 64, were all increased by the application of nitrogen.

Key Words: Glycine max (L.) Merr. • Acetylene reduction • Protein yield

² Formerly Graduate Research Assistant, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 55101 (now Research Scientist, Division of Tropical Agronomy, CSIRO, Brisbane, Australia) and Associate Professor, Department of Agronomy and Plant Genetics, University of Minnesota, St. Paul, MN 5510

Received for publication July 14, 1973.