XVI International Botanical Congess

Norway spruce forests on acidic soils belong to the most N-sensitive forest ecosystems in Europe. Since N loads have increased over the last decades, 'critical loads' are mostly exceeded. In this context, interactions of N with mycorrhizae as organs of the nutrient uptake are of importance. In greenhouse experiments, the mycorrhization of Norway spruce seedlings associated with Hebeloma or Laccaria was not affected by N loads of up to 100 kg/ha.y, in contrast to the mycorrhization with Cenococcum which was reduced even at N loads of 50 kg/ha.y. With increasing N loads, N contents in the plants are increased, but P, Ca, and Zn contents are decreased. Mycorrhization, however, compensated the P decrease to a certain extent. Additionally, mycorrhizal roots had a reduced nitrate reductase activity (NRA) suggesting that either extramatrical hyphae absorbed nitrate efficiently or mycorrhizae suppressed root NRA.