Ectomycorrhizal (Dipterocarp) and arbuscular mycorrhizal (non-dipterocarp) tree

The type of mycorrhizal associations (i.e. ecto- or arbuscular
mycorrhizal) formed by trees is of fundamental importance for a range of
soil properties and processes in forest ecosystems, yet their importance
for the distribution of other important soil biota such as bacteria is
still largely unknown. This is especially so in diverse tropical forests
where trees of different mycorrhizal types are intermingled in a highly
heterogeneous biotic and abiotic environment. Here we used an experimental
common garden that helped us to assess how abiotic and biotic variation
differentially influenced bacterial communities associated with trees
planted in a secondary tropical forest of Borneo. We used high-throughput
amplicon sequencing to characterize rhizosphere bacterial communities of
13 climax tree species (8 ectomycorrhizal and 5 arbuscular mycorrhizal) in
relation to plant traits, plant neighborhood, and abiotic environment.
Rhizosphere bacterial (RB) communities differed significantly between EM
and AM trees but not among EM species and only marginally among AM
species. Foliar nutrients, especially potassium, showed relationships with
RB community composition. Rhizosphere bacterial communities were related
to the density and size of neighboring ectomycorrhizal but not arbuscular
mycorrhizal trees. Diversity of RB on AM trees responded positively to AM
neighbors and negatively to EM neighbors but RB diversity associated with
EM trees was unaffected by neighborhood. Rhizosphere bacterial communities
of AM trees were more responsive to environmental variation such as light
availability and position on a slope. Plant-growth-promoting taxa of RB
assorted similarly to total RB but more strongly. Synthesis: Our results
suggest that the distribution of rhizosphere bacterial communities is
linked to plant functional group and plant neighborhood. Because
rhizosphere bacteria play important roles in nutrient cycling that
influence plant species composition, it is likely that their
distributional patterns are important for understanding ecosystem
processes and plant demographics.