Nitrogen fixation by nodulated soybean under tropical field conditions estimated by the nitrogen 15 isotope dilution technique

Boddey, R.M.; Chalk, P.M.; Victoria, R.L.; Matsui, E.

Soil Biology and Biochemistry 16(6): 583-588


ISSN/ISBN: 0038-0717
Accession: 005986477

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The contribution of biological N2 fixation to the N nutrition of nodulated soybean was estimated using the 15N isotope dilution technique and a non-nodulating soybean isoline as a non-fixing control plant. The plants were grown in the field in concrete cylinders (60 cm dia ) and harvested at 7 stages of plant growth. Labeled N was added to the soil either as labeled organic matter before planting or in 7 small additions (2 kg N ha-1) of (NH4)2SO4 during the growing period. There was good agreement between isotope dilution estimates of N2 fixation for the 2 labeling methods. Acetylene reduction assays on intact root systems greatly underestimated N2 fixing activity. The difference in total N between nodulated and non-nodulated plants generally gave higher estimates compared with the isotope technique. The data indicate that this was because nodulated plants recovered more N from the soil than the non-nodulated plants. After 92 days of growth, the soybean derived approximately 250 kg N ha-1 from biological N2 fixation.