Symbiotic Efficiency of Slow and Fast-Growing Soybean Rhizobia of Central India

Soybean rhizobia are pre-dominantly slow growing but fast growing strains that evolved from natural populations have been reported globally including from several locations in India but have not been evaluated. The symbiotic effectivity of three slow and three fast-growing rhizobial strains isolated from Vertisols of Central India was evaluated on soybean var. 9752 in sterilized sand microcosms in a green-house. At 40 days growth, there was variation in nodulation parameters and growth of soybean with both types of rhizobia but overall, the slow growers were superior to fast growers with respect to nodule number (56%), nodule dry matter (25%), shoot dry matter (11%) and total nitrogen uptake (22%). The slow-growers (genetic homology to Bradyrhizobium japonicum) fixed ~ 25% more nitrogen than fast-growers with homology to Rhizobium (Agrobacterium) radiobacter. The results have very important implications for strain selection during biofertilizer production to maximize biological nitrogen fixation.