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Dinitrogen Fixation Measured by ¹⁵N Isotope Dilution in Two Canadian Soybean Cultivars¹

Adapted cultivars must be developed if significant production of soybeans [Glycine max (L.) Merr.] is to be realized in western Canada. Selection and breeding for ability to support symbiotic N₂ fixation is important in this development.

¹⁵N isotope dilution techniques were used to determine the percent plant N derived from the atmosphere (%Ndfa), i.e., fixation, and actual amounts of N fixed by two promising cultivars, X005 and Maple Presto. Precent Ndfa for the two cultivars was almost identical under comparable conditions. Depending on soil type and plant growth stage, %Ndfa varied from 38 to 70% in lysimeter experiments and maximized at 67% in the field. X005 fixed significantly more N₂ (115 kg N/ha) than Maple Presto (82 kg N/ha) in the field because X005 had a higher N yield, possibly due to a slightly longer growing season. The N yield of both cultivars, when inoculated, did not respond to increasing rates of fertilizer N [Ca (NO₃)₂]. The highest percent fertilizer use efficiency (%FUE) was 51% when uninoculated and 44% when the seed was inoculated. Although N₂ fixation occurred in both cultivars when fertilizer up to 160 kg N/ha was provided, the cultivars differed in their tolerance to fertilizer N with respect to N₂ fixation. For X005, N₂ fixation was constant as N fertilizer increased from 0 to 80 kg/ha but decreased by 24% where 160 kg N/ha was applied. For Maple Presto, N₂ fixation was constant as N fertilizer increased from 0 to 40 kg/ha but decreased by 21% where 80 or 160 kg N/ha was applied. Both cultivars had similar %Ndfa and amount of N fixed/ha to soybeans in other countries.

Key Words: ¹⁵N • N₂ fixation • Fertilizer use efficiency • Rhizobium japonicum • Glycine max (L.) Merr.

² Research scientists, Research Station, Agriculture Canada, Lethbridge, Alberta, Canada T1J 4B1.

Received for publication November 23, 1981.