Effects of Long‐Term Free‐Air Ozone Fumigation on δ15N and Total N in Fagus sylvatica and Associated Mycorrhizal Fungi

Patterns of nitrogen (N) isotope composition (δ 15 N) and total N contents were determined in leaves, fine roots, root-associated ectomycorrhizal fungi (ECM) of adult beech trees (FAGUS SYLVATICA), and soil material under ambient (1 × O 3 ) and double ambient (2 × O 3 ) atmospheric ozone concentrations over a period of two years. From fine root to leaf material δ 15 N decreased consecutively. Under enhanced ozone concentrations total N was reduced in fine roots and δ 15 N showed a decrease in roots and leaves. In the soil and in most types of mycorrhizae, δ 15 N and total N were not altered due to ozone fumigation. The number of vital ectomycorrhizal root tips increased and the mycorrhizal community structure changed in 2 × O 3 . Simultaneously, the specific rate of inorganic N-uptake by the roots was reduced under the double ozone regime. From these results it is assumed that 2 × O 3 changes N-nutrition of the trees at the level of N-acquisition, as indicated by enhanced mycorrhizal root tip density, altered mycorrhizal species composition, and reduced specific N-uptake rates.