Factors affecting competition of three strains of rhizobia nodulating groundnut,Arachis hypogaea*

The nitrogen (N₂) fixing ability of three strains of rhizobia (NC 92, NC 43.3, and TAL 176) was compared in groundnut cv. Robut 33–1. The competitiveness of these strains in pot culture in a sand-vermiculite medium and with native rhizobia in the field was also investigated. In pot culture, NC 43.3 formed more nodules than TAL 176 and NC 92. Nodules formed by NC 43.3 and NC 92 fixed more N₂ (as measured by total N content in the plants at 42 days after sowing) than nodules formed by TAL 176. TAL 176 was a poor competitor compared with NC 92, NC 43.3, or with native rhizobia in the field. NC 92 when mixed with NC 43.3 (10⁶ cells seed^-1 of each strain) formed only 21% of the nodules, but when independently inoculated in the soil containing native rhizobia, the two-strains formed similar percentages of nodules. Thirty percent of the nodules in two strain combinations of NC 43.3 and NC 92 showed double occupancy. Strain NC 43.3 formed nodules earlier than NC 92 and TAL 176 and this may be one of the factors responsible for its better N₂-fixation and competitiveness. Nodules formed earlier by one strain (NC 92 or TAL 176) were found to have no effect on the subsequent nodulation by the other (TAL 176 or NC 92) strain. Although NC 92 and NC 43.3 were equally competitive with native rhizobia in the field and NC 43.3 fixed more N₂ than NC 92 in pot culture, earlier experiments indicated that only inoculation with NC 92 increased pod yield in field trials.