Competitiveness of Rhizobium trifolii Strains Associated with Red Clover ( Trifolium pratense L.) in Mississippi Soils

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Abstract
Five strains of Rhizobium trifolii were evaluated in competition with indigenous populations in nodulating red clover ( Trifolium pratense L.) cv. Kenland in two different soils in Mississippi. Double antibiotic resistance acquisition was used to measure the proportion of nodules occupied by the introduced mutant strains. In vertisol soil, strains RP113-7, 162BB1, LM1, and 162P17 were recovered in at least 94% of the assayed nodules, whereas TA1 was found in 83.8% of the nodules. At an ultisol location, significant differences were detected within the introduced rhizobia. Strain RP113-7 was recovered at very high rates (99.2% of the assayed nodules), whereas strains 162BB1, LM1, 162P17, and TA1 were all found in 84.9 to 96.0% of the nodules sampled. Forage yield and percent crude protein levels were lower with the less effective but competitive strain (TA1) at both locations. Results indicated that more effective strains of R. trifolii can increase red clover production and symbiotic nitrogen fixation under different environmental conditions in Mississippi.