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Efficiency, Competitiveness, and Persistency of Chinese Rhizobium fiedii in Iowa Soils

Loynachan, Dep. of Agronomy, Iowa State Univ., Ames, IA 50011

A.G. Atherly

Dep. of Genetics, Iowa State Univ.,, Ames, IA 50011.

^* Corresponding author.

Native Bradyhizobium japonicum occupy the majority of Midwest soybean [Glycine max (L.) Merr.] nodules despite inoculation with more effective N₂-fixing bradyrhizobia. The objective of this study was to determine whether three fast-growing strains of Rhizobium fiedii obtained from soybean fields in China could displace the highly competitive native strains from the nodules of commonly grown Iowa-soybean cultivars. Before introducing these nonnative organisms into Iowa soils, we evaluated their effectiveness under greenhouse conditions compared with the effectiveness of B. juponicum strains USDA 110 and USDA 123. There was a dominant strain-by-cultivar interaction, and two of the three fast-growers were as effective in N₂ fixation as was USDA 123, but were less effective than was USDA 110. Furthermore, we evaluated the competitiveness of the fast-growers against native bradyrhizobia in six different soils in the greenhouse. Nodule occupancy by the fast-growers ranged from 1.5 to 38.7%. Results indicated significant cultivar-by-soil, strain-by-soil, and soil-by-strain-by-cultivar interactions. For field tests, two separate sites were selected each year in the summers of 1987 and 1988. Each of the three strains was introduced into soils at approximately 10⁶ colony forming units (CFU) cm^–1 row. Nodule occupancies for the four site years on Corsoy 79 and Williams 82 soybean cultivars ranged from 3.8 to 13.3%. The saprophytic survivability of fast-growers, based on nodule occupancy of field-collected soil in the greenhouse, was tested 2 and 3 yr after inoculation. Fast-growing rhizobia isolated from nodules were identified by their acid production on an artificial medium, and the reliability of this method was verified by comparing restriction-endonuclease-digestion patterns of plasmid DNA of test and reference strains. The nodule occupancies by strains of R. fredii on Williams 82 ranged from 8.6 to 13.3% and 4.4 to 11.370, 2 and 3 yr after inoculation, respectively. Thus, the strains of R. fiedii were able to persist in soil reasonably well as saprophytes.

Received for publication March 6, 1991.