In the infertile sandy soils of northeast Thailand, field experiments were conducted to evaluate the effects on rates of biological nitrogen fixation (BNF) of inoculating soybeans with selected strains of Bradyrhizobium japonicum and adding herbicides. Inoculating soybean seeds with effective B. japonicum strains increased the acetylene reduction by nodulated root systems, and increased the nodule numbers and dry weights associated with soybean roots. The amounts of Nz fixed, as measured by N-15 isotope dilution methods, were also increased by adding the B. japonicum. Inoculation of soybeans with a nif -strain of B. japonicum resulted in increased nodule weights and numbers, but did not increase the acetylene reduction rates or the amounts of Nz fixed. There was a significant straincultivar interaction. Application of metolachlor, alachlor, fluazifop butyl or sethoxydim did not reduce the levels of Nz fixed by B. japonicum in association with soybeans, but paraquat did significantly reduce the Nz fixed by the inoculated plants.

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