Effects of nitrogen fertilization on beech and Norway spruce and on the preformed defences of their fine roots against fungal pathogens

The main objective of this thesis was to determine the effect of nitrogen fertilization on beech and Norway spruce and on the pre-formed defences of their fine roots to fungal pathogens. I investigated the influence of nitrogen fertilization on root parameters of beech and Norway spruce in field experiments. The plots were situated at three sites in Switzerland on acidic and calcareous soils. For nine years, the trees had been treated with dry ammonium nitrate resulting in 0, 10, 20, 40, 80 and 160 kg N ha-1 year-1, respectively. Fine roots of beech and Norway spruce showed a significant decrease of fungistatic phenolics with increasing nitrogen fertilization. The in vitro growth of Heterobasidion annosum and Cylindrocarpon destructans was inhibited by the presence of most of the fungistatic phenolic compounds in the concentrations found in the roots. In this thesis I also addressed questions about the effect of increased nitrogen fertilization on fine roots of beech growing on different soil conditions. Relative length of fine roots and the root tip density of beech were reduced by nitrogen fertilization on acidic soil but not on calcareous soil. The nutrient concentrations in beech leaves decreased due to increased nitrogen fertilization. I suggest that increased nitrogen deposition may increase the susceptibility of trees to pathogens and may affect the water and nutrient uptake of trees.