Comparison of N Balance and ¹⁵N Isotope Dilution to Quantify N₂Fixation in Field‐Grown Legumes¹

Abstract

Abstract Two techniques are commonly used to estimate N 2 fixation integrated over the growing season in field‐grown legumes: total N balance (NB) (the classical difference method), and methods based on the principle of 15 N isotope dilution. Many researchers do not have access to 15 N or to isotope ratio mass spectrometers and thus rely on the NB method. A comparison of these two techniques of estimating N 2 fixation in several experiments over 3 years showed that the NB method generally resulted in a lower estimate of N 2 fixation and was consistently less precise (higher experimental error). Good agreement existed between the two methods in 70% of the experiments for estimating the amount of N 2 fixed but in only 60% of the experiments for percentage N 2 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 15 N isotope dilution for estimates of N 2 fixation. Negative values for fixing systems or positive values for non‐fixing systems for N 2 fixation were found when using NB. In only one circumstance will NB accurately quantify N 2 fixation: when the fertilizer use efficiency (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 N 2 fixation in field‐grown legumes.