Many fast growing tree species have been introduced to promote
biodiversity rehabilitation on degraded tropical lands. Although it
has been shown that plant productivity and stability are dependent on
the composition and functionalities of soil microbial communities,
more particularly on the abundance and diversity of soil symbiotic
micro-organisms (mycorrhizal fungi and rhizobia), the impact of tree
introduction on soil microbiota has been scarcely studied. This
research has been carried in a field plantation of Acacia holosericea
(Australian Acacia species) inoculated or not with an ectomycorrhizal
fungus isolate, Pisolithus albus IR100. After 7 year’s plantation, the
diversity and the symbiotic properties of Bradyrhizobia isolated from
the plantation soil or from the surrounding area (Faidherbia albida
(Del.) a. Chev. parkland) and able to nodulate F. albida, a native
Sahelian Acacia species, have been studied. Results clearly showed
that A. holosericea modified the structure of Bradyrhizobia
populations and their effectiveness on F. albida growth. This negative
effect was counterbalanced by the introduction of an ectomycorrhizal
fungus, P. albus, on A. holosericea root systems. In conclusion, this
study shows that exotic plant species can drastically affect genotypic
and symbiotic effectiveness of native Bradyrhizobia populations that
could limit the natural regeneration of endemic plant species such as
F. albida. This effect could be counterbalanced by controlled
ectomycorrhization with P. albus. These results have to be considered
when exotic tree species are used in afforestation programs that
target preservation of native plants and soil ecosystem
rehabilitation.