Quantifying dinitrogen (N2) fixation in soybeans by 15N isotope dilution: the question of the nonfixing control plant

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Abstract
The stable isotope 15N was used to trace the contribution of soil, fertilizer, and atmospheric N to the plant N of soybeans (Glycine max L. Merrill). 15N isotope dilution is the best technique to quantify N2 fixation (percent nitrogen derived from the atmosphere, % Ndfa) but requires reference to an appropriate nonfixing control plant so that the contribution of soil and (or) fertilizer N to the N yield of the fixing plant may be determined. The nodulating soybean line in nonfixing mode, i.e., uninoculated or inoculated with an ineffective strain of Rhizobium japonicum, was shown to be the best nonfixing control plant. The nonnodulating isoline, even when near isogenic, overestimated the % Ndfa and was judged to be an improper control. Of three nonlegumes tested only Hordeum vulgare was useful as a nonfixing control plant. In two lysimeter studies in a growth room the % Ndfa of Glycine max cv. Harosoy was 59% while that of cv. X005 was 38%. The average % Ndfa of cv. Chippewa soybeans under field conditions was 17%. The precision of the isotopic dilution technique, as measured by the coefficient of variation, was as good as 1.3% in one lysimeter study but averaged 6.2% in growth room lysimeters or in the field.