Cultivar and Rhizobium Strain Effects on Nitrogen Fixation and Remobilization by Soybeans1

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Abstract
Maximum utilization of the N2‐fixation capability of leguminous plants such as soybean (Glycine max L. Merr.) in crop production requires an understanding of factors that control seasonal expression of N2‐fixation activity. The purposes of this investigation were to determine the seasonal N2‐fixation pattern for a determinate soybean cultivar nodulated by an efficient Rhizobium strain and the influence of determinate soybean cultivar and Rhizobium strain on the relationship between N2‐fixation capability and reproductive tissue N demand. Greenhouse experiments were conducted in a nil‐N culture system that was sterile until inoculated with the appropriate Rhizobium strain.The seasonal N2‐fixation pattern for the ‘Ransom’ cultivar nodulated by R. japonicum strain USDA 3I1b110, revealed that 65% of the total seasonal N2 fixation occurred after N began to accumulate in reproductive tissue and that N2 fixation during reproductive development and remobilization of vegetative tissue N met 83% and 17%, respectively, of the reproductive tissue N requirement.In a second experiment, ‘Davis’ and Ransom soybeans fixed over 100% more N2 during the growing season when nodulated by strain USDA 3I1b110 than when nodulated by strain USDA 3I1b31. When Ransom and Davis soybeans were nodulated by strain USDA 3I1b110, they achieved essentially the same yields of seed dry matter and seed N by expression of N2‐fixation capability at different times in the growth cycle. High N2‐fixation capability before reproductive development coupled with remobilization of vegetative tissue N was associated with the yield potential of Davis soybeans while high N2‐fixation capability during reproductive growth was associated with the yield potential of Ransom soybeans. With either cultivar, nodulation by strain 3I1b31 restricted the amount of N stored in vegetative tissue for remobilization to developing seed.