M. Ammann á R. Siegwolf á F. Pichlmayer á M. Suter M. Saurer á C. Brunold Estimating the uptake of traf®c-derived NO 2 from 15 N abundance in Norway spruce needles Received: 7 November 1997 / Accepted: 16 September 1998 Abstract The 15 N ratio of nitrogen oxides (NO x ) emit- ted from vehicles, measured in the air adjacent to a highway in the Swiss Middle Land, was very high [d 15 N(NO 2 )  +5.7&]. This high 15 N abundance was used to estimate long-term NO 2 dry deposition into a forest ecosystem by measuring d 15 N in the needles and the soil of potted and autochthonous spruce trees [Picea abies (L.) Karst] exposed to NO 2 in a transect orthogonal to the highway. d 15 N 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 de- composition of needle litter below the autochthonous trees over previous years has increased d 15 N values in the soil, resulting in parallel gradients of d 15 N in soil and needles with distance from the highway. Estimates of NO 2 uptake into needles obtained from the d 15 N data were signi®cantly correlated with the inputs calculated with a shoot gas exchange model based on a parame- terisation widely used in deposition modelling. There- fore, we provide an indication of estimated N inputs to forest ecosystems via dry deposition of NO 2 at the receptor level under ®eld conditions. Key words 15 N á Forests á Spruce á Picea abies á NO 2 deposition Introduction Dry deposition of reduced or oxidised nitrogen com- pounds to nitrogen-limited ecosystems such as moors, heathlands, forests and lakes may represent a signi®cant contribution to the nitrogen nutrition of plants in these systems. The main consequences of the increased nitro- gen input are acidi®cation of soils (Wilson and Skef- ®ngton 1994), eutrophication of terrestrial and wetland ecosystems (Bobbink 1991), nutrient imbalances in the plants (Wellburn 1990), and, on the level of plant com- munities, the loss of biodiversity in sensitive ecosystems (Bobbink et al. 1992). Nitrogen input may also be a cause of forest decline in Europe (NihlgaÊrd 1985; Schulze 1989; Seith et al. 1994; Manderscheid and JaÈger 1993). In parts of Europe, ecosystem nitrogen loads are dominated by ammonium deposition, whereas the oxidised nitrogen compounds (NO 2 , HNO 3 ) contribute substantially in others (LoÈvblad and Erisman 1992; Duyzer and Fowler 1994). The precursor of the latter compounds is NO which is mainly emitted by combus- tion processes in, for instance, vehicles, power plants, garbage and oil burners, although recently much eort has been undertaken to reduce these emissions by cata- lytic reduction to N 2 . NO itself contributes a minor proportion of the deposition, but may be regarded as source of NO 2 in the photochemical equilibrium with ozone. Gas phase removal of NO 2 by the OH radical, producing nitric acid (HNO 3 ), is rather slow and deter- mines the NO 2 life-time, typically a few days in the troposphere (van Aaalst and Diederen 1985). HNO 3 and particulate nitrogen (Lovett and Lindberg 1993) are deposited readily (limited only by turbulent transport) on any surface, e.g. cloud droplets or plant cuticles (Cadle et al. 1991; Hanson and Garten 1992), and therefore contribute to nitrate input into the soils mainly by stem¯ow and throughfall (Eilers et al. 1992). In contrast, the pathway of NO 2 deposition in plants is mainly stomatal, i.e. by gas exchange (Hanson and Lindberg 1991; Duyzer and Fowler 1994). Most plants Oecologia (1999) 118:124 ± 131 Ó Springer-Verlag 1999 M. Ammann á M. Suter á C. Brunold Institute of Plant Physiology, University of Berne, CH-3013 Berne, Switzerland M. Ammann (&) á R. Siegwolf á M. Saurer Paul Scherrer Institute, CH-5232 Villigen, Switzerland e-mail: markus.ammann@psi.ch, Tel.: +41-56-3104049, Fax: +41-56-3104435 F. Pichlmayer Austrian Research Centre Seibersdorf, A-2444 Seibersdorf, Austria