Effects of nitrogen deposition on the growth, metabolism and activity of ectomycorrhizal fungi

Sammanfattning: Elevated nitrogen (N) deposition is an environmental problem that can affect plants and microorganisms, which are vital for the terrestrial N and carbon (C) cycles. This thesis has investigated the response of ectomycorrhizal fungi (EMF) to increased N deposition. Ectomycorrhizal fungi form mutualistic interactions with the roots of woody plants, including almost all boreal and temperate forests tree species. Trees transfer photosyntetically-derived carbohydrates to EMF, which in exchange provide trees with water and nutrients, particularly N. In soil, a major part of N is present in organic forms, embedded in recalcitrant organic matter (OM), and it is believed that trees are dependent on degrading activities of EMF to gain access to this pool of N. This thesis provides evidences that EMF disrupt OM complexes when assimilating N using an oxidation mechanism. In order to release organic N, EMF can deploy a mechanism including Fenton chemistry involved in the production of hydroxyl radicals. It was also demonstrated that EMF can be equally efficient in degrading OM as saprotrophic brown-rot fungi. Little is known about how the ability of EMF to modify OM is affected by increased N deposition. Many studies have shown that degradation of OM decreases with increasing N deposition. However, in this thesis it was demonstrated that N addition did not decrease the oxidation of OM by EMF when grown in axenic cultures with sufficient amounts of readily available C. Furthermore, work in this thesis showed that elevated N deposition decrease the biomass of EMF, which has been interpreted as a result of reduced C allocation from the tree host. It was further demonstrated that during C-starvation, EMF can metabolize C and other nutrients from dying parts of its mycelia. This can influence the nutrient status of dead mycelia and affect the recalcitrance of EMF necromass, which is important to consider when studying C and N dynamics in forest soils, especially under increased N deposition when EMF are exposed to decreasing C supply from tree hosts. Taken together, it is suggested that the most important factor regulating growth, activity and metabolism in EMF, is the C allocation from the tree host. However, most experiments were conducted in non-symbiotic systems and symbiotic experiments are needed to understand how plant C allocation affects EMF processes in natural systems. One gram of boreal forest soil typically contains hundreds of meters of ectomycorrhizal hyphae, thus, a shift in their capacity to assimilate organically bound N through the modification of OM can have major impacts on the cycling of carbon and other nutrients in forest ecosystems.