Abstract: Broadhost range conjugative plasmid transfer between R.fredii QB1131 (pLV1016) and lux3 or between QB1131(pLV1016) and indigenous bacteria on filter,in broth,and in microcosms containing sterile or nonsterile soil,planted with or without soybean were studied. The donors,recipients and transconjugants were enumerated on selective media 1 day after inoculation,and the growth rates of donor and recipient as well as the transfer rate constants (γ) of plasmid were calculated for three initial parental densities in each system.At the same initial density, γ value was the highest in filter mating and the lowest in soil microcosm mating,which was not influenced by soybean spermosphere or by soil sterilization or nonsterilization.But in microcosm experiments,it was inversely related to initial parental density and directly related to donor growth rates.In nonsterile soil,plasmid pLV1016 could transfer from R.fredii to indigenous bacteria Rhizobium sp.or Pseudomonas sp.

Key words: Conjugative plasmid, Horizontal plasmid transfer, Rhizobium fredii, root morphology, fine root, root anatomy, N availability, mycorrhiza.