Host identity and neighborhood trees affect belowground microbial communities in a tropical rainforest

The roots and rhizospheres of trees harbor diverse microbial communities that can modulate plant competition and facilitation, thereby influencing plant community dynamics. Understanding the factors structuring microbial communities is valuable for predicting how plant communities assemble. In temperate forests, host identity, biotic neighborhood, abiotic environment and geographic distance shape microbial communities, but the importance of these factors is less well studied in highly diverse tropical forests. In this study, we used high-throughput amplicon sequencing to characterize the arbuscular mycorrhizal fungal (AMF) and rhizosphere bacterial (RB) communities of five tree species in an 8-year-old common garden planted into the understory of a selectively logged old-growth forest in Malaysian Borneo. We assessed the influence of host tree species, host traits and neighboring tree identity on the composition and diversity of both communities. The AMF and RB communities differed amongst host tree species; the tree species with the most distinct AMF communities associated with the lowest diversity of AMF. Alpha diversity of AMF correlated negatively with leaf phosphorus and potassium content. Density and abundance of AMF neighbor trees growing near focal trees influenced AMF community composition and was positively correlated with RB alpha diversity. Our results highlight the importance of considering both host tree identity and biotic neighborhood of trees in studies of microbial communities in tropical forests. Important next steps will be to elucidate the functional significance of shifts in AMF and RB community compositions and their implications for community and ecosystem dynamics in tropical forests.