Nitrogen Load
 

Apart from the high ozone concentrations, the nitrogen load is one of the most important man-made impact on the forest. Since the fifties, deposition of plant available nitrogen (N) from the air has risen by 200% to about 30 kg N per hectare and year on an average for the Swiss forest. Forests, however, can only support 10-20 kg without negative consequences in the long term. In Switzerland, two third of this N originates in agricultural activities.  

The IAP takes part in the evaluation procedure of UN/ECE. In autumn 2002, a workshop on Critical Loads took place in Berne. The IAP was involved in the preparation of the background document.

 
Since the fifties, the deposition of nitrogen into the forest has increased considerably. Green: NH3-nitrogen (emitted by agriculture), red: NOx-nitrogen (traffic).
 
In 78% of the forest observation plots the nitrogen deposition is higher than the Critical Load of 10-20 kg nitrogen per hectare and year (see also the deposition map on the web site of the Swiss Federal Office of Environment and deposition measurements in the LWF observation plots).
Leaching of nitrate to the ground water serves as an indicator for nitrogen saturation of an ecosystem. Forest ecosystems are regarded as nitrogen saturated when more than 4-5 kg nitrogen in the lowlands and more than 1-2 kg nitrogen per hectare and year in the mountains is leached from the soil. In plots of the Swiss Plateau, leaching rates between 11 and 33 kg, in the Prealps between 7 and 25 kg and in southern Ticino even 54 kg nitrogen per hectare and year are measured.
  
 
Loads of nitrogen in the leaching water of 16 forest observation plots during 1998-2003. 
 
The high N deposition is causing increasing nutrient imbalances. In fertilization experiments, phosphorus and - depending on the soil conditions - also potassium and magnesium decrease in the leaves with increasing nitrogen fertilization. This may be the reason for the nutrient imbalances observed in mature forest trees.
 
Nitrogen fertilization experiments with young beech in afforestation plots cause distinct nutrient changes in the leaves. Left: phosphorus, right: magnesium.
 
The nitrogen deposition promotes growth. However, the aboveground tree parts are stimulated more than the roots. This may lead to decreased stability and increased drought sensitivity. 
 
Beech and Norway spruce are more frequently infested with pathogens and sucking insects when fertilized with nitrogen. It is suggested that the nutrient imbalances are playing a crucial role. 
 
In a nitrogen fertilization experiment, the proportion of dead shoots was growing with increasing nitrogen fertilization. The symptom is caused by a fungus, Apiognomonia errabunda. The changes are statistically significant already at a N fertilization with 10 kg per hectare and year during four years. 
 
Apart from ozone, nitrogen load is also impairing the symbiosis which is formed by the forest trees and root fungi (mycorrhiza). This may have negative consequences for the supply of the trees with water and nutrients as well as for the sensitivity against root pathogens.