Relationships between fungal uptake of ammonium, fungal growth and nitrogen availability in ectomycorrhizal Pinus sylvestris seedlings

Nitrogen deposition and intentional forest fertilisation with nitrogen are known to affect the species composition of ectomycorrhizal fungal communiti es. To learn more about the mechanisms responsible for these effects, the r elations between fungal growth, nitrogen uptake and nitrogen availability w ere studied in ectomycorrhizal fungi in axenic cultures and in symbiosis wi th pine seedlings. Effects of different levels of inorganic nitrogen (NH4) on the mycelial growth of four isolates of Paxillus involutus and two isola tes of Suillus bovinus were assessed. With pine seedlings, fungal uptake of N-15-labelled NH4 was studied in shortterm incubation experiments (72 h) i n microcosms and in long-term incubation experiments (3 months) in pot cult ures. For P. involutus growing in symbiosis with pine seedlings, isolates w ith higher NH4 uptake were affected more negatively at high levels of nitro gen availability than isolates with lower uptake. More NH4 was allocated to shoots of seedlings colonised by a high-uptake isolate, indicating transfe r of a larger fraction of assimilated NH4 to the host than with isolates sh owing lower NH4 uptake rates. Thus low rates of N uptake and N transfer to the host may enable EM fungi avoid stress induced by elevated levels of nit rogen. Seedlings colonised by S. bovinus transferred a larger fraction of t he N-15 label to the shoots than seedlings colonised by P. involutus. Seedl ing shoot growth probably constituted a greater carbon sink in pot cultures than in microcosms, since the mycelial growth of P. involutus was more sen sitive to high NH4 in pots. There was no homology in mycelial growth rate b etween pare culture and growth in symbiosis, but N uptake in pure culture c orresponded to that during growth in symbiosis. No relationship was found b etween deposition of antropogenic nitrogen at the sites of origin of the P. involutus isolates and their mycelial growth or uptake of inorganic nitrog en.