Evaluation of the N₂-fixation capacity of four soybean genotypes by several methods

Plant-N derived from N₂-fixation (N₂-fixed) and the percentage of plant-N derived from N₂-fixation (% Ndff) were determined for four field-grown soybean lines by several techniques. The objective was to determined which technique would distinguish between the lines on the basis of their N₂-fixing abilities. Variables in the techniques included method (total N or ¹⁵N isotope dilution), plant sample (seed, shoot at physiological maturity and whole plant) and placement of control (non-nodulating soybean) plots. Estimates of N₂-fixed generated by isotope dilution were. on average, higher than those generated by the difference method (total N). Greater distinction between the genotypes was possible using the difference method than with isotope dilution. Separation of genotypes on the basis of N₂-fixed as estimated by isotope dilution was not affected by the tissue sample used (seed, shoot or whole plant).