Rhizosphere populations of Rhizobium strains and nodulation of Leucaena leucocephala

Abstract

Populations of two Rhizobium strains (NGR8 and CB81) in the rhizosphere of Leucaena leucocephala were estimated in field experiments with varying levels of antibiotically marked strains as seed inoculation treatments. The population level varied with soil type and strain of Rhizobium. Multiplication in the rhizosphere was very slow in a prairie soil but was more rapid in a sandy podzolic soil and nodulation was three weeks earlier in the sandy soil than in the prairie soil. Survival of these two strains in soil stored in the laboratory also suggested that they (especially NGR8) were not well suited to the prairie soil. Nodule representation of strain CB81 on the prairie soil decreased from 100% three months after sowing to between 12% and 16% two years after sowing. The results suggest that on this soil indigenous rhizobia form effective nitrogen fixing associations with Leucaena leucocephala and that any improvement in nitrogen fixation will require strains of Rhizobium that are more effective than the indigenous strains and better competitors for nodule formation.