The European NITREX project is investigating the effects of nitrogen deposition on coniferous forests. The WSL in Birmensdorf is a participant in this project. The study site is in the valley of Alptal in the canton of Schwyz. The present rate of nitrogen deposition is there roughly equivalent to the critical level, as currently defined. Nitrogen is leaching from the soil, both as nitrate and as dissolved organic compounds. The herbaceous vegetation also reflects a sufficient nitrogen supply. Chemical analysis of spruce needles, however, demonstrates a nitrogen deficiency.
In one plot nitrogen deposition is tripled through the application of an ammoniumnitrate solution. The effects of this increased loading are ascertained by way of comparison with a control plot. Although the nitrogen addition increases nitrate leaching, no effects on the vegetation have been found after two years, neither in the trees nor in the herbaceous layer.
These observations can be explained by the dynamic and heterogeneity of the ecosystem (i.e. temporal and spatial variability of the soil and vegetation).