Seasonal changes of bacterial communities in the rhizosphere of Acacia senegal mature trees inoculated with Ensifer strains in Burkina Faso and Niger

Acacia senegal is of paramount importance in sub Saharan Africa because it produces gum arabic. Inoculation with specific rhizobial strains improves the vigour of the trees but also positively enhances the gum arabic production of mature trees. A host-specific cocktail of Ensifer strains was used to inoculate mature A. senegal trees in Burkina Faso and Niger. Rhizosphere soils were sampled during dry and rainy seasons from 2006 to 2008. Total microbial biomass and soil inorganic nitrogen were assessed, and DNA extracted and used to analyse diversity of A. senegal rhizosphere bacterial communities on Denaturing Gradient Gel Electrophoresis after PCR amplification of the 16S rDNA. In Niger, results showed a significant increase of total bacterial diversity due to the season but no effect of the rhizobial inoculation was observed. However, a significantly higher microbial biomass was found in soil from uninoculated trees than from inoculated trees. In Burkina Faso, no effect of both season and rhizobial inoculation was observed on total bacterial diversity, but higher microbial biomass values were obtained compared to Niger, especially for 2006. However, there were no significant differences between inoculated and uninoculated trees. Rhizobial inoculation significantly enhanced gum arabic production in Niger but not in Burkina Faso.