Comparison of nitrogen balance and nitrogen 15 isotope dilution to quantify nitrogen fixation in field grown legumes

Rennie, R.J.

Agronomy Journal 76(5): 785-790


ISSN/ISBN: 0002-1962
Accession: 005010387

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Two techniques are commonly used to estimate N2 fixation integrated over the growing season in field-grown legumes [Phaseolus vulgaris and Glycine max]; total N balance (NB) (the classical difference method), and methods based on the principle of 15N isotope dilution. Many researchers do not have access to 15N or to isotope ratio mass spectrometers and thus rely on the NB method. A comparison of these 2 techniques of estimating N2 fixation in several experiments over 3 yr showed that the NB method generally resulted in a lower estimate of N2 fixation and was consistently less precise (higher experimental error). Good agreement existed between the 2 methods in 70% of the experiments for estimating the amount of N2 fixed but in only 60% of the coefficients for percentage N2 fixed. Even when high correlation coefficients were found, the unacceptable characteristics of the regression equations (i.e., deviation from a slope of unity and a y-intercept of zero) made predictions difficult. Nitrogen balance was most reliable in experiments when soil N was low, so that non-fixing plants showed signs of N deficiency by anthesis. Still, it was not possible to determine when NB would agree with 15N isotope dilution for estimates of N2 fixation. Negative values for fixing systems or positive values for non-fixing systems for N2 fixation were found when using NB. In only 1 circumstance will NB accurately quantify N2 fixation: when the fertilizer use efficency (or, in unfertilized experiments, soil N uptake) of the fixing system is identical to that of the non-fixing system. Since this occurs sporadically, NB cannot be used with confidence to estimate N2 fixation in field-grown legumes.