Effects of nitrogen and phosphorus supply, and of<i>Rhizobium</i> and VAM fungus inoculants on dinitrogen fixation in soybean
The effect of inoculation of N 2 fixation by soybean plants, grown in sandy soil was studied in pot experiments. Bradyrhizobium japonicum (Rh) and/or mycorrhizæ, in the presence of basic application of a P fertilizer (super or rock P), and two levels of 15 N-labelled ammonium sulfate (20 and 100 mg N per kg soil), were used. Highest N 2 fixation was observed after a dual inoculation (Rh+VAM), followed by single inoculation (Rh) and by mycorrhizal infection. Higher doses of N fertilizer depressed the capacity of the plant nodules and the inoculants for N 2 fixation.