Microbial rhizosphere and topsoil communities in young reforestation plantations

Relationships between microbial communities of the rhizosphere and plants can affect the successful establishment of plantations and their modification for forest restoration efforts. Recognising these fungal and bacterial assemblages is important for Acacia mangium plantations, as they are widely established in reforestation programs across the tropics. However, the microbial community associated with the rhizosphere of A. mangium and of plants often thriving within or in the proximity of A. mangium plantations is understudied. We hypothesise that microbial assemblages of the topsoil, hosted in the rhizosphere of the planted species and of the co-dominant plant species in the landscape, may differ. In the examined 2-year-old plantation, the plant community is dominated by the eudicotyledonous A. mangium and by monocotyledonous species (Cocos nucifera, Imperata cylindrica, and Saccharum spp.), whereas in the 10-year-old plantation, eudicotyledonous trees (Swietenia macrophylla, and Pterocarpus indicus) dominate the area. Given the diverse abundance of plants of different phylogenetic traits, we hypothesised a difference in the topsoil microbial community based between these two evolutionarily distinct Angiosperm lineages. Through high-throughput sequencing, we identified the taxonomic and putative functional traits of fungal and bacterial taxa in the rhizospheres of the cultivated and co-dominant plant species. Disproving our hypothesis, we found comparable taxonomic and functional community composition of fungi and bacteria among the rhizospheres tested and between these and the topsoil of the same plantation, without significant differences among mono- and eudicotyledonous species. Still, C. nucifera seemed to display a markedly unique rhizosphere community, and ectomycorrhizal fungi were abundant in the 2- and 10-year-old A. mangium plantings. Results support the role of topsoil as a reservoir for the microbial community, on which plants act by filtering the taxa hosted in their rhizosphere. In our context, a non-significant difference in the microbial composition may have reflected comparable ecological adaptations and needs, but further research is suggested, given the small sample size of this study. Our research provides insights into the microbial assemblages of the rhizospheres and topsoil of A. mangium plantations in a region of the Philippines, helping to disentangle the relationship between the soil microorganisms and plant rhizospheres during forest restoration.