First detection of nitrogen from NOx in tree rings: a 15N/14N study near a motorway

Nitrogen isotope analysis (δ15N) of tree rings is potentially useful for evaluating the temporal development of the nitrogen (N) deposition to forests and for studying the long-term effects of N accumulation in ecosystems. To test this hypothesis, we investigated three sites across a pollution gradient in differing distances (20,150,1000 m) from a motorway in Switzerland, which was built in 1965. We sampled four Picea abies trees per site, whereby we extracted the tree ring cores with hot water and solvents before the isotope analysis to remove mobile N storage compounds, and determined the isotope variations in the stem wood for the period 1928–2000. While tree ring growth was not affected by the construction of the motorway, the δ15N values were increasing by up to 7.9‰ after 1965 at the most polluted site, indicating the uptake of NOx from car exhausts, although the signal was highly variable. Isotopically heavy NOx emissions were observed in an earlier study at the same location resulting in a δ15N-gradient of recent needles from +1.3‰ to −4.4‰ with increasing distance from the motorway. This gradient was also reflected in the tree rings, but dampened by a factor of about 2 compared to the needles. For the trees near the motorway, the total nitrogen concentration in the tree rings varied in parallel with the δ15N values (r2=0.52). This enabled us to apply a mass balance equation for reconstructing the isotope signal of N originating from the car exhausts for the period 1965–2000, with the δ15N of NO2 in the range +1.3‰–+6.4‰. The more distant sites were much less affected by the traffic and their isotope ratio reflected the influence of varying proportions of isotopically heavy (NO2) and light (NHx) deposition. We conclude that the analysis of tree ring 15N variations is a promising tool for the detection of the role played by nitrogen deposition to the forests.