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This article in AJ

  1. Vol. 69 No. 3, p. 377-380
    Received: July 24, 1976

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Alfalfa Root Nodule Distribution and Inhibition of Nitrogen Fixation by Heat1

  1. D. N. Munns,
  2. V. W. Fogle and
  3. B. G. Hallock2



Greenhouse and field trials were done to test the likelihood that impairment of N fixation contributes to late summer decline in productivity of Medicago sativa L.

In greenhouse solution cultures, acetylene reduction and N fixation by nodules were impaired by daily heating of the culture solutions to 32 C, and eliminated by repeated exposures to 36 C or a single exposure to 40 C. However, in a field trial conducted on a Yolo silt loam (Typic Xerorthent, fine silty mixed non-acid thermic), alfalfa showed no signs of N deficiency and responded little to application of 500 kg NH4NO3/ha although soil temperatures above 32 C were recorded for 5 to 6 hours each day during a 6-day hot spell following a late summer cut, with peak soil temperatures above 40 C at 2 cm depth.

Lack of response in the field trial is explained by observations of temperature profiles and nodule distribution. Only the topmost 5 cm of soil heated above 30 C, and this contained less than 10% of the nodules. Most of the nodules were at depth 10 to 30 cm, and remained at nearly optimal temperatures of 22 C to 27 C.

Heat inhibition of nitrogen fixation should not significantly limit growth of alfalfa in the field unless nodule-formation is constrained to shallow depths.

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