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.