Long-term effects of a high nitrogen supply to soil on the growth and nutritional status of young Norway spruce trees

Central European forests are characterised by increasing N inputs. In a pot experiment with 3-year-old trees, the effects of varied N supply in soil (in mineral or organic form) on the growth and nutrient uptake of Norway spruce were examined over a period of 20 months. A nutrient-poor mineral forest soil was used in the experiment. N was added to the soil once, at the onset of the experiment. High mineral N supply (NH4NO3) decreased root growth and the root/shoot ratio of newly grown plant biomass, which resulted in wide N/P and N/K ratios in needles, and significantly decreased needle K concentrations. On the contrary, needle Ca concentrations were increased in plants with additional mineral N supply. The organic N form (keratin) slowly became available to plants, and also decreased the root/shoot ratio. In contrast to many observations in short-term experiments, the specific root length and the number of root tips per cm of root length were not affected by the soil N supply. High supply of mineral N did not cause visual symptoms of toxicity or damage in the plant. Induced cation or P deficiency may be a harmful consequence of high soil N levels, but will occur only when the soil supply of these nutrients is critically low.