Mycocentric fertilisation of ectomycorrhizae-inoculated Pinus radiata during containerised production alters root microbiome and growth outcomes

During containerised production in forest nurseries, seedlings are often intensively fertilised to maximise seedling survival and growth. However, this practice can inadvertently harm the development of a robust root microbiome needed for plant resilience post-planting. In this controlled study using Pinus radiata, we combined reduced fertilisation and seedling inoculation with a diverse ectomycorrhizal (ECM) community as an alternative to intensive, industry-standard fertilisation. After 9 months, we investigated growth responses and changes to ECM and non-ECM root fungal communities. Reduced fertilisation improved the belowground growth and mycorrhizal root colonisation of inoculated seedlings. Isotopic nutrient tracing determined that, under moderate fertilisation, more photosynthetically fixed carbon was allocated belowground, and root-tip nitrogen (N) accumulation, a proxy for N uptake from the soil, also increased. Fertilisation level resulted in shifts in both ECM and non-ECM fungal community composition and substantial changes in the abundance of certain fungi. This study demonstrates that employing more-mycocentric fertilisation regimes may improve plantation nursery outcomes and that interactions between ECM and non-ECM fungi within inocula should be considered when studying the role of the soil microbiome in supporting P. radiata growth during containerised production.