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This article in AJ

  1. Vol. 79 No. 1, p. 172-176
     
    Received: Jan 21, 1986
    Published: Jan, 1987


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doi:10.2134/agronj1987.00021962007900010035x

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

  1. F. Zapata,
  2. S. K. A. Danso,
  3. G. Hardarson and
  4. M. Fried2

Abstract

Abstract

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.

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