Impacts of conversion from natural forest to cedar plantation on the structure and diversity of root-associated and soil microbial communities

Natural broadleaved forests have been widely converted to plantations with economically valuable trees mainly belonging to the family Pinaceae in temperate regions. Furthermore, mixed natural forests dominated by Pinaceae have been converted into pure cedar plantations involving members of the Cupressaceae in many areas. The objective of this study was to determine the impacts of such conversion with respect to the changes in dominant tree species and tree stand types (i.e. pure vs mixed) on soil properties, including the diversity and community structures of bacteria and fungi colonizing roots, rhizospheres, surface and subsurface bulk soils. Our results showed that vegetation apparently affected the bacterial and fungal community structures, but differently among these compartments. Bacterial alpha diversity was significantly lower in a mixed natural forest than in a pure cedar plantation. In addition, roots in the natural forest supported greater proportions of specific bacteria such as Bradyrhizobiaceae and Burkholderia bryophila, which have been often reported as plant-growth-promoting rhizobacteria. Fungal alpha diversity was not different between vegetation, but ectomycorrhizal (ECM) fungi such as Piloderma olivaceum, Cenococcum geophilum, Tuber pseudosphaerosporum, Sebacina sp. Tomentella sp. and Russula sp. were detected in the natural forest but less than 0.2% in the cedar plantation. The rhizosphere effects on soil pH and bacterial and fungal abundance were observed only in the natural forest, but not in the cedar plantation. Therefore, our results suggested that conversion from mixed natural forest to pure cedar plantation reduces specific bacteria with plant-growth-promoting properties as well as ECM fungi, although it appears to increase bacterial diversity. This has practical implications in that it suggests that specific planting of ECM-associated trees might be an effective strategy when attempting to generate mixed forests from starting points of pure cedar plantations.