Nitrogen dynamics and mycorrhizal interactions between ectomycorrhizal and ericoid mycorrhizal plants

Mycorrhizal fungi are critical players in nutrient dynamics within forest ecosystems. Although typically associated with specific plant groups, evidence suggests that some species of ericoid mycorrhizal fungi can colonise ectomycorrhizal plants, and vice versa, with potential for nutrient exchange across these associations. Carbon (C) transfer from plants to mycorrhizal fungi and reciprocal nitrogen (N) transfer from fungi to plants are well-established processes. However, single studies report N loss from pine seedlings associated with mycorrhizal fungi and forest ground vegetation underscoring the complexity of these interactions.

Our study investigates whether such cross-functional colonisations may occur between Pinus sylvestris seedlings and Calluna vulgaris plants, and if they result in measurable N transfer, and evaluates the direction and magnitude of N movement between these plants and their mycorrhizal symbionts.

Therefore, we planted C. vulgaris plants in pots alongside ¹⁵N-labelled pine seedlings with varying degrees of interspecies connectivity: full root and hyphal contact, hyphal contact only, disrupted hyphal contact, and no contact. Some pots were enriched with additional nitrogen to assess the influence of nutrient availability on fungal-mediated nutrient transfer. Nitrogen transfer was quantified by measuring ¹⁵N content in roots and shoots of both species, as well as in fungal hyphae grown in ingrowth bags. To identify shared fungal taxa, we performed ITS sequencing on fungal communities associated with both C. vulgaris and pine roots.

To assess C exchange and hyphal connectivity, pine seedlings were ¹³C-labeled, allowing us to trace ¹³C allocation to fungal hyphae and C. vulgaris. Additionally, fungal biomass and enzyme activity were analysed to provide a detailed understanding of fungal contributions to nutrient dynamics.

In boreal forests, the field vegetation is frequently dominated by ericaceous dwarf shrubs, and their interactions with tree seedlings can therefore have far-reaching implications. This is particularly true if forest management practices change, for instance if the use of mechanical site preparation were to be reduced.  Our study aims to elucidate the mechanisms underlying nitrogen and carbon fluxes in mixed ectomycorrhizal-ericaceous systems, providing insights into nutrient sharing and potential ecological implications in forest ecosystems.