Soil bacterial diversity responses to root colonization by an ectomycorrhizal fungus are not root-growth-dependent.

The hypothesis tested in this present study was that the ectomycorrhizosphere effect on the bacterial community was not root-growth-dependent. The impacts of ectomycorrhizal infection (Pisolithus albus COI007) and a chemical fertilization to reproduce the fungal effect on root growth were examined on (1) the structure of bacterial community and (2) fluorescent pseudomonad and actinomycete populations in the mycorrhizosphere of Acacia auriculiformis using both culture-independent and culture-dependent methods. A. auriculiformis plants were grown in disinfested soil in pots with or without addition of the ectomycorrhizal fungus or N/P/K fertilization (to reproduce the fungal effect on root growth) for 4 months and then transferred to 20-L pots filled with nondisinfested sandy soil. The fungal and fertilizer applications significantly improved the plant growth after 4-month culture in the disinfested soil. In the nondisinfested cultural substrate, these positive effects on plant growth were maintained. The total soil microbiota was significantly different within the treatments as revealed from DNA analysis [denaturing gradient gel electrophoresis (DGGE)]. The structure of fluorescent pseudomonad populations was also affected by fungal and fertilizer applications. In contrast, no qualitative effect was observed for the actinomycete communities within each treatment, but fungal inoculation significantly decreased the number of actinomycetes compared to the fertilizer application treatment. These results show that the mycorrhizosphere effect is not root-growth-dependent but is mainly due to the presence of the ectomycorrhizal fungus and more particularly to the extramatrical mycelium.