Increase of nitrogen depositions, as oxidized (NO_x) and reduced (NH_x) compounds, has important implications on ecosystem nitrogen cycle. NO_x comes predominantly from fossil fuel combustion in car engines and industrial processes, while agricultural activities (cattle farming, fertilizations) are the main sources for NH_x emissions. Such fertilisations could stimulate growth and, therefore, productivity of forests, especially in temperate forests, where nitrogen is a limiting factor. On the other hand an excess of nitrogen can lead to an acidification of the soil and have a negative impact on the microbial fauna and structure of plants. NO_x and NH_x depositions can be separated with the help of stable isotopes with different ¹⁵N-values reported for NO_x originating from combustion sources. Consequently it was observed that the nitrogen isotopic composition of the vegetation reflects the isotopic signature of nitrogen sources. Our preliminary results on needles of Norway spruce trees exposed to NO_x emissions along a transect perpendicular to a highway close to Faido and Brittnau show a clear isotopic enrichment in ¹⁵N in trees growing closer to traffic pollution, indicating an uptake of atmospheric nitrogen by stomatal pathway. Carbon and oxygen isotopic composition measured in tree rings reveals the physiological response in plants. Trees more exposed to air pollution from traffic show an increase in δ¹³C and δ¹⁸O in tree rings, since mid sixties next to Erstfeld. This could mean a higher photosynthetic activity, enhanced by NO_x traffic emissions, under low or not changed stomatal conductance. Our results confirm that stable isotopes of carbon, oxygen and nitrogen measured in needles and tree rings represent an important tool to monitor the impact of NO_x load on tree physiology.