Boreal forest podzol soils have vertically stratified horizons with different physico-chemical characteristics and high microbial diversity. Ectomycorrhizal fungi play key roles in accessing nutrients from both organic and mineral substrates. The role of associated bacteria in these processes is still poorly understood. The aim of the studies described in this thesis was to improve understanding of the distribution, diversity and community structure of fungi and bacteria on roots and in soil and their responses to environmental perturbations such as N-fertilisation.
In two microcosm studies a single-root tip microbiome method was used to sample bacteria associated with different ectomycorrhizal roots at defined time-points, using high throughput sequencing of both fungi and bacteria The first study revealed highly dynamic patterns of assembly of bacterial communities associated with ectomycorrhizal roots at different time points in organic soil. Bacterial community structure differed between roots colonised by different species of ectomycorrhizal fungi from different genera. The second study extended these results to include both organic and mineral horizons, demonstrating significant differences between fungal and bacterial communities colonising soil from different horizons. Responses of both fungi and bacteria to short-term N additions were context dependent, influenced by both soil horizon and the dominant ectomycorrhizal fungi colonising the roots. Bacterial communities associated with roots colonised by pairs of closely related fungal species within the same fungal genera were also shown to be statistically distinct.
Field studies of fungi and bacteria in a forest fertilised with 150 kg N ha⁻¹ 15 months previously, revealed 1017 unique fungal OTUs, (877 in the soil, 652 in the roots). N increased fungal diversity slightly in the O horizon soil but decreased it in the roots, particularly in the B horizon. Fungal community structure varied significantly between horizons and within each soil horizon the community structure of fungi colonising ectomycorrhizal roots was significantly different from that associated with the soil, suggesting that analyses of both soil and roots are necessary for accurate monitoring of environmental perturbations. 10925 unique bacterial OTUs were distinguished in total (8560 in the soil, 5512 in the roots). Bacteria displayed similar trends to the fungi but were less strongly influenced by N. These studies pave the way for more detailed functional studies of specific combinations of fungi and bacteria
