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Time Course of Nitrogen Fixation in Field-Grown Soybean Using Nitrogen-15 Methodology¹

The use of ¹⁵N techniques allows for the quantitative evaluation of N₂ fixation and distribution and their impact on the N balance in various soil-plant systems. The A-value approach was used in this investigation to assess N₂ fixed at various growth stages in fieldgrown soybean [Glycine max (L.) Merrill] cv. Chippewa in a Typic Eutrocrepts soil. At physiological maturity (R7), the amount of N derived from fixation (Ndfs) was 102 kg/ha, equivalent to 47% of total N assimilated, while the contributions from soil (Ndfs) and ¹⁵N-labelled fertilizer (Ndff) accounted for 50 and 3%, respectively. Up to growth stage V6, which occupied half of the total duration of growth, Ndfa was less than 5% of N₂ fixed by physiological maturity. A rapid increase in Ndfa occurred from R1 onwards, and during the reproductive stages (Rl-R4), which spanned less than one-third of the total duration of growth, this represented about 45% of total Ndfa. An almost equal portion of N (approximately 43%) was fixed from pod-filling (R5) to physiological maturity (R7), a period slightly more than one-fifth of the total duration of growth. Therefore, substantial N₂ fixation occurred during periods of active sink development and contributed more than 65% of the plant's N accrued during pod fill (R3-R7). Nitrogen assimilated between R3 and R7 (when N₂ fixation was high) seemed to be the predominant source of N for pod development. Thus there was a greater contribution from fixed N (55%) than soil N (43%) in pods and seeds at the end of R7. After grain removal, it was estimated that the growth of cv. Chippewa in this soil led to a net soil depletion of 54 kg N/ha.

Key Words: Nitrogen assimilation • Nitrogen partitioning • Soybean isolines • Glycine max (L.) Merrill

² Scientific officer, IAEA Agric. pb., Seibersdorf, Austria; scientific officer, Joint FAO/IAEA Division, A-1400 Vienna, Austria; scientific officer, IAEA Agric. Lab., Seibersdorf. Austria: consultant, Natl. Academy of Sci., Washington, DC.

Received for publication January 21, 1986.

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