Estimating the uptake of traffic-derived NO2 from 15N abundance in Norway spruce needles

The N-15 ratio of nitrogen oxides (NOx) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [delta(15)N(NO2) + 5.7 parts per thousand]. This high N-15 abundance was used to estimate lone-term NO2 dry deposition into a forest ecosystem by measuring delta(15)N in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO2 in a transect orthogonal to the highway, delta(15)N in the current-year needles of potted trees was 2.0 parts per thousand, higher than that of the control after 4 months of exposure close to the highway, suggesting a 25% contribution to the N-nutrition of these needles. Needle fall into the pots was prevented by grids placed above the soil, while the continuous decomposition of needle litter below the autochthonous trees over previous years has increased delta(15)N values in the soil, resulting in parallel gradients of delta(15)N in soil and needles with distance from the highway. Estimates of NO2 uptake into needles obtained from the delta(15)N data were significantly correlated with the inputs calculated with a shoot gas exchange model based on a parameterisation widely used in deposition modelling. Therefore, we provide an indication of estimated N inputs to forest ecosystems via dry deposition of NO2 at the receptor level under field conditions.