Early afforestation influences on soil fungal communities but bacterial and oomycetes are slower to respond

Microorganisms underpin forest soil multifunctionality and delivery of ecosystem services. As climate and land-use change drive shifts in land cover, afforestation of pastoral land is becoming increasingly important. The New Forest Trial Series (NFTS) offers a novel system to assess how local soil microbiomes respond to interactions between site conditions, tree species, and functional traits. We investigated how tree species, their functional traits, and site conditions shape early soil fungal, bacterial, and oomycete communities during afforestation. In the first experiment, replicated plots of Pinus radiata , Podocarpus totara , and Cupressocyparis ovensii were sampled across three NFTS sites (Mangatoa, Te Apiti, and Rewanui). Fungal communities showed compositional shifts related to both tree species and site, whereas bacterial communities varied by site only. Oomycete communities remained similar across all samples. In the second experiment, plots planted with seven tree species (the three above plus Leptospermum scoparium , Eucalyptus fastigata , Fraxinus excelsior , and Sequoia sempervirens ) were sampled at Rewanui. Tree species again influenced soil fungal microbiomes, but not those of bacteria or oomycetes. Trait-centred analysis revealed that mycorrhizal type (ectomycorrhizal vs. arbuscular) explained more variance in fungal composition than tree species. Whether species were native or introduced to New Zealand had no effect on soil microbial composition. The influence of tree species and mycorrhizal association on fungal communities appeared to be direct, with no evidence of mediation via modification of soil chemistry. These sites were sampled ∼4.5 years after afforestation. As tree-specific effects on site conditions intensify over time, secondary impacts on soil microbiomes are expected, providing opportunities to observe how both primary and secondary effects of tree selection interact to influence site development, soil microbiology, and ecosystem service delivery. These results establish an early-stage baseline showing that fungi respond rapidly to tree selection, while other microbial groups remain stable. This provides a foundation for forecasting how afforestation choices influence soil development, ecosystem functions, and potential disease dynamics as forests mature. • Fungal communities responded to both tree species and site differences • Mycorrhizal type (EM vs AM) explained more fungal variation than tree identity • Soil chemistry did not mediate microbial shifts ∼4.5 years after planting • Results highlight primary and secondary tree selection effects on soil microbes