Estimate of symbiotically fixed nitrogen in field grown soybeans using variations in¹⁵N Natural abundance

Summary

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.