Composition and assembly processes of rhizosphere bacterial communities of arbuscular mycorrhizal and ectomycorrhizal tree species.

How the mycorrhizal type of tree species regulates the composition and assembly processes of rhizosphere bacterial communities remains unclear. We selected five arbuscular mycorrhizal (AM) and five ectomycorrhizal (EcM) tree species from two typical forests across different climatic zones, namely a subtropical evergreen broadleaved forest at Tiantong Station in Zhejiang and a temperate broadleaved Korean pine forest at Maoershan Station in Heilongjiang. We analyzed the rhizosphere soil physicochemical properties and bacterial community composition, and quantified the assembly processes of bacterial communities by using niche breadth and null model analyses. The results showed that forest type was the dominant factor driving variation in rhizosphere soil bacterial communities (explaining rate 62.6%), with a small contribution from mycorrhizal type (2.6%). However, mycorrhizal effects differed significantly among forest types and bacterial abundance levels, including all species, abundant species, and rare species. Soil nitrogen availability, especially ammonium, was a key factor regulating the structure of rhizosphere bacterial communities. The standardized effect size (SES) values of rhizosphere bacterial communities in both forests were all greater than 2, indicating that deterministic processes dominated the assembly of these communities. The influence of mycorrhizal type on the assembly processes of rhizosphere bacterial communities exhibited significant forest-type dependence. At Tiantong Station, deterministic processes had a stronger influence on AM tree species, whereas at Maoershan Station, this influence was stronger for EcM tree species. This study revealed the dominant role of deterministic processes in the assembly of rhizosphere bacterial communities of mycorrhizal tree species, and this role was dually regulated by forest type and mycorrhizal type.