Effect of Method and Plant Sample on Nitrogen Fixation Estimates in Soybean

Abstract

AbstractThe labor involved in collecting total plant dry matter limits the number of soybean [Glycine max (L.) Merr.] genotypes that can be evaluated for N2 fixation. Therefore, a 2‐yr experiment was conducted on a Flanagan silt loam (fine, montmorillonitic, mesic Aquic Argiudoll) to determine if genotypic differences in plant N derived from fixation (N2 fixed) and percentage of plant N derived from fixation (% Ndfa) could be detected by isotope dilution or difference (total N) method analysis of whole plant samples (whole plant sample including abscised material), shoot samples (whole plant samples‐abscised material), or seed samples. Five experimental soybean lines derived from a ‘Williams’ ✕ ‘Kanrich’ cross and ‘Harosoy’ were tested. Estimates of N2 fixed and % Ndfa generated by isotope dilution analysis of whole plant samples ranged from 36 to 113 kg ha−1 and 17 to 48%, respectively. More genotypic separations were possible with isotope dilution than with the difference method. Separation on the basis of % Ndfa by isotope dilution was not affected by plant sample. More genotypic separations on the basis of N2 fixed were possible by isotope dilution analysis of whole plant or shoot samples than by analysis of seed samples.