Host identity and fungal source pools shape mycorrhizal assembly under carbon removal interventions in large-scale afforestation
1. Large-scale afforestation is central to global carbon dioxide removal (CDR) commitments. Soil inoculation and enhanced rock weathering (ERW) may increase tree biomass carbon stocks during woodland establishment. Both can alter soil fungal communities, but whether they change the root-colonising mycorrhizal fungi that support tree nutrition and growth is largely unknown. 2. In a fully-factorial field trial on former upland pasture in Wales (11.5 ha; ~25,000 saplings), we tested the effects of soil inoculation (addition of soil from nearby mature, donor forests) and ERW (basalt amendment) on root-associated ectomycorrhizal (EcM) and arbuscular mycorrhizal (AM) fungi, and tree growth in mixed-broadleaf and conifer plantations, two and four years after planting. To identify key colonisation sources, we also characterised mycorrhizal fungi in roots and soils of surrounding hedgerows, source tree nursery, non-forested control plots, donor woodlands and soil inoculum. 3. Soil inoculation caused a subtle shift in conifer EcM community composition (3% of variation) but had no detectable effect in broadleaves. Basalt amendment raised soil pH but did not change mycorrhizal composition, diversity, or colonisation. 4. Root-associated EcM communities were distinct from those detected in plantation soils and shared far more taxa with the tree nursery than with the donor inoculum (41-48% vs. 2-9% of plantation EcM taxa) whereas AM communities largely reflected fungi present in pre-existing pasture. 5. Host identity was the dominant factor structuring mycorrhizal communities, overshadowing both treatments, and was the strongest predictor of aboveground biomass. 6. Synthesis and applications: This field-scale evaluation shows early mycorrhizal assembly during woodland establishment is strongly structured by tree species identity and pre-existing fungal source pools, limiting the effect of inoculation at planting. Mycorrhizal management should therefore target the nursery stage and be tailored to individual tree species. Basalt amendment for ERW is unlikely to harm mycorrhizal communities during establishment. Because soil fungal communities did not reflect root symbiosis, our findings also stress that monitoring of belowground outcomes in CDR-focused planting should include root sampling rather than relying on soil DNA alone.