Biological N2-fixation and growth of soybeans as affected by inoculation and residual 15N

ABSTRACT

The effect of inoculation with Bradyrhizobium japonicum either solely or in combination with Azotobacter chroococcum was examined for the growth and N2 fixation of soybean plantsSoybeans were fertilized with 15N-urea or ammonium sulfate and the isotope dilution technique was used for the quantification of N2 fixationDry mass and N uptake by inoculated plants were higher than those of the uninoculated plants Inoculation with sole or mixed inoculants induced 80-90% of dry matter and N yields of the plants fertilized with the full dose of N [40 mg N kg-1 soil]Those two parameters were higher in case of urea fertilization as compared with ammonium sulfate Similar trend was observed between the two seasons, where the dry mass and N uptake were higher with the first soybean crop comparable with the succeed soybean crop Generally, the nodulated soybean produced higher dry matter yield and accumulated more nitrogen than the non-nodulated isoline Nitrogen derived from air [Ndfa%] was ranged from 42 to 65% depending on the inoculation treatments Fixed N2 is the most important source of N utilized by the undulating soybean, while the non-nodulating isoline uptakes more nitrogen from soil and fertilizer