Characterization of culturable bacterial populations associating with Pinus sylvestris--Suillus bovinus mycorrhizospheres.

Bacterial isolations were carried out on Pinus sylvestris--Suillus bovinus mycorrhizospheres obtained directly from boreal pine forest. When samples were taken during dry weather, the numbers of bacterial colony-forming units were significantly higher in uncolonized short roots and external mycelia than in mycorrhizal roots and soil outside the mycorrhizosphere. In contrast, the colony-forming unit counts were similar in all hypogeous samples after rainy weather. Culturable bacteria were absent from most Suillus bovinus sporocarps. The bacteria isolated from all types of mycorr hizo sphere samples, i.e. short roots, mycorrhizal roots, and external mycelia, consisted primarily of Burkholderia spp., whereas most isolates from soil outside the mycorrhizosphere were identified as Paenibacillus spp. This study shows that mycorrhizal external mycelia can expand the habitat favourable for common rhizosphere bacteria into the soil far from the immediate rhizosphere. Some of these bacteria may help the trees with nitrogen acquisition, since potentially diazotrophic bacteria harbouring nitrogenase reductase (nifH) genes were isolated from mycorrhizal root tips.