1 Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), CH-8903 Birmensdorf, Switzerland
Nitrogen (N) deposition exceeds the critical loads for this element in most parts of Switzerland apart from the Alps. At 17 sites (8 broadleaved stands, 8 coniferous stands, and 1 mixed stand) of the Swiss Long-term Forest Ecosystem Research network, we are investigating whether N deposition is associated with the N status of the forest ecosystems. N deposition, assessed from throughfall measurements, was related to the following indicators: 1) nitrate leaching below the rooting zone (measured on a subset of 9 sites); 2) the N nutrition of the forest stand based on foliar analyses (16 sites); and 3) crown defoliation, a non-specific indicator of tree vitality (all 17 sites).
Nitrate leaching ranging from about 2 to 16 kg N ha-1 a-1 was observed at sites subjected to moderate to high total N deposition (> 10 kg ha-1 a-1). The C/N ratio of the soil organic layer, or, when it was not present, of the upper 5 cm of the mineral soil, together with the pool of organic carbon in the soil, played a critical role, as previous studies have also found. In addition, the humus type may need to be considered as well. For instance, little nitrate leaching (<2 kg N ha-1 a-1) was recorded at the Novaggio site, which is subjected to high total N deposition (>30 kg ha-1 a-1) but characterized by a C/N ratio of 24, large organic C stocks, and a moder humus type.
Foliar N concentrations correlated with N deposition in both broadleaved and coniferous stands. In half of the coniferous stands, foliar N concentrations were in the deficiency range. Crown defoliation tended to be negatively correlated with N concentrations in the needles. In the majority of the broadleaved stands, foliar N concentrations were in the optimum nutritional range or, on one beech plot with high total N deposition (>25 kg ha-1 a-1), above the optimum values. There was no correlation between the crown defoliation of broadleaved trees and foliar concentrations.
Keywords: critical loads for nitrogen, forest, atmospheric deposition, nitrate leaching, foliar concentrations, crown defoliation