ESTIMATE OF SYMBIOTICALLY FIXED NITROGEN IN FIELD-GROWN SOYBEANS USING VARIATIONS IN N-15 NATURAL ABUNDANCE

The use of variations in natural abundance of15N between nitrogen fixing and non nitrogen fixing soybeans was investigated for quantitative estimate of symbiotic nitrogen fixation. Isotopic analysis of 4 varieties of inoculated and non-inoculated soybeans growing under field conditions, with and without N-fertilizer was determined. It was found that inoculated soybeans had a significantly lower15N content than non-inoculated ones. Estimates of the participation of fixed N to the total nitrogen content of inoculated soybeans were calculated from these differences. They were compared to estimates calculated from differences in N yield between inoculated and noninoculated plants and to the nitrogenase activity, measured by the C2H2 reduction assay over the growing season. Estimates given by the15N measurements were correlated with the C2H2 reducing activity but not with the differences in the N yield. This shows that the isotopic composition was dependent on the amount of fixed nitrogen and consequently that the estimates of fixed nitrogen based on natural15N abundance should be reliable. The absence of correlation between estimates based on15N content and estimates based on N yield was explained by differences in the uptake of soil nitrogen between inoculated and non inoculated soybeans. © 1979 Martinus Nijhoff, The Hague/Kluwer Academic Publishers.