Ectomycorrhizal fungi associated with Pinus sylvestris seedlings respond differently to increased carbon and nitrogen availability: implications for ecosystem responses to global change

The ectomycorrhizal (ECM) symbiosis can cause both positive and negative feedback with trees under elevated CO₂. Positive feedback arises if the additional carbon (C) increases both nutrient uptake by the fungus and nutrient transfer to the plant, whereas negative feedback results from increased nutrient uptake and immobilization by the fungus and reduced transfer to the plant. Because species of ECM fungi differ in their C and nitrogen (N) demand, understanding fungal species‐specific responses to variation in C and N supply is essential to predict impacts of global change. We investigated fungal species‐specific responses of ECM Scots pine (Pinus sylvestris) seedlings under ambient and elevated CO₂ (350 or 700 μL L^?1 CO₂) and under low and high mineral N availability. Each seedling was associated with one of the following ECM species: Hebeloma cylindrosporum, Laccaria bicolor and Suillus bovinus. The experiment lasted 103 days. During the final 27 days, seedlings were labeled with ¹⁴CO₂ and ¹⁵N. Most plant and fungal parameters were significantly affected by fungal species, CO₂ level and N supply. Interactions between fungal species and CO₂ were also regularly significant. At low N availability, elevated CO₂ had the smallest impact on the photosynthetic performance of seedlings inoculated with H. cylindrosporum and the largest impact on seedlings with S. bovinus. At ambient CO₂, increasing N supply had the smallest impact on seedlings inoculated with S. bovinus and the largest on seedlings inoculated with H. cylindrosporum. At low N availability, extraradical hyphal length increased after doubling CO₂ level, but this was significant only for L. bicolor. At ambient CO₂, increasing N levels reduced hyphal length for both H. cylindrosporum and S. bovinus, but not for L. bicolor. We discuss the potential interplay of two major elements of global change, elevated CO₂ and increased N availability, and their effects on plant growth. We conclude that increased N supply potentially relieves mycorrhiza‐induced progressive N limitation under elevated CO₂.