Do neighbours matter? Soil mycobiome and mycorrhizal colonization of Ulmus laevis seedlings grown in the vicinity of arbuscular or ectomycorrhizal trees

The influence of arbuscular (AM) or ectomycorrhizal (ECM) trees and a mixture of tree mycorrhizal types on the soil mycobiome is not yet well understood. To address this knowledge gap, a pot experiment was designed in which AM Ulmus laevis seedlings were grown in the presence of AM ( Acer campestre ) and ECM ( Quercus robur , Carpinus betulus ) trees. A possible ECM symbiosis of U. laevis , a topic that is often misinterpreted in Ulmus research, has also been analysed. Soil mycobiomes were identified by Illumina sequencing of ITS2 rDNA, ectomycorrhizas by Sanger sequencing ITS rDNA, and AM colonization through microscopic observations of AM structures in Ulmus roots. The soil mycobiome differed between variants with AM trees only and those with admixture of ECM trees. These differences were observed at the taxonomic level, fungal trophic guilds, and in the contribution of ECM fungi. The structures resembling ECM on Ulmus roots were formed by non-ectomycorrhizal fungi. Microscopic analysis of Ulmus roots revealed, that Ulmus seedlings form only AM symbioses, regardless of the ECM trees in their vicinity. Further research on mature Ulmus trees in their natural habitats is needed to clarify their potential to enter into ECM symbiosis. The pot experiment results emphasise the pressing necessity for additional research into AM and ECM tree interactions in natural environments, as this is pivotal to comprehending changes in soil mycobiomes and their repercussions on temperate forest ecosystems. • The soil mycobiome differed between AM-only and AM and ECM-mixed treatments. • Ectomycorrhizal fungi do not colonize Ulmus roots. • Arbuscular colonization of Ulmus is similar, regardless of neighbouring trees.