Estimating the uptake of traffic-derived NO from N abundance in Norway spruce needles.

The N ratio of nitrogen oxides (NO) emitted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [δN(NO) = +5.7‰]. This high N abundance was used to estimate long-term NO dry deposition into a forest ecosystem by measuring δN in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO in a transect orthogonal to the highway. δN in the current-year needles of potted trees was 2.0‰ 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 δN values in the soil, resulting in parallel gradients of δN in soil and needles with distance from the highway. Estimates of NO uptake into needles obtained from the δN 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 NO at the receptor level under field conditions.