The use of 15 N techniques allows for the quantitative evaluation of N 2 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 N 2 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 (Ndfa) was 102 kg/ha, equivalent to 47% of total N assimilated, while the contributions from soil (Ndfs) and 15 N‐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 N 2 fixed by physiological maturity. A rapid increase in Ndfa occurred from R1 onwards, and during the reproductive stages (R1‐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 N 2 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 N 2 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.
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Zapata et al. (1987) studied this question.