Ecology of stem-nodulating Rhizobium and Azorhizobium in four vegetation zones of Senegal.

The occurrence and distribution of Azorhizobium and Rhizobium strains that induce stem nodulation of Sesbania rostrata were determined in four vegetation zones in Senegal. Based on tests with 16 Rhizobium and 10 Azorhizobium strains nodulating S. rostrata, a method was devised to distinguish among the strains. In all vegetation zones, members of both genera were more abundant in rhizosphere than nonrhizosphere soil under S. rostrata, Cassia obtusifolia, Acacia senegal, and Hystic suaveolens, and Rhizobium was present at higher densities than Azorhizobium. Azorhizobium was more abundant on the leaves and stems than Rhizobium in three of the vegetation zones, and the density of Azorhizobium but not Rhizobium was far greater on the leaves of S. rostrata than the three nonhost species in all four zones. Approximately 90% of the stem nodules and 39–48% of the root nodules on S. rostrata in all four zones were formed by Azorhizobium.