Stomatal uptake and cuticular adsorption contribute to dry deposition of NH3 and NO2 to needles of adult spruce (Picea abies) trees.

•  In the present study NH 3 and NO 2 exchange between the atmosphere and needles of adult spruce ( Picea abies ) trees at a field site ('Höglwald') exposed to high loads of atmospheric nitrogen was assessed. •  Twigs were fumigated with different NH 3 (C NH3 ) or NO 2 (C NO2 ) concentrations using the dynamic chamber technique. Beside fluxes of NH 3 (J NH3 ) and NO 2 (J NO2 ), transpiration (J H2O ), leaf conductance for water vapour (g H2O ), photosynthetic activity (J co2 ), photosynthetic photon fluence rate (PPFR), air temperature (T) and relative air humidity (RH) were determined. •  Both fluxes, J NH3 and J NO2 , depended linearly on C NH3 and C NO2 in concentration ranges representative for the field site and g H2O as a measure of stomatal aperture. For both trace gases compensation points could be determined amounting to 2.5 nmol mol -1 for NH 3 and to 1.7 nmol mol -1 for NO 2 . •  The fluxes of NH 3 and NO 2 could not be explained exclusively by exchange through the stomata. In both NH 3 and NO 2 fumigation experiments additional deposition onto the needle surface was observed and increased with increasing C NH3 and C NO2 . 15 N[NH 3 ] fumigation experiments with adult spruce trees confirmed the results of gas exchange measurements and revealed that NH 3 -N deposited to spruce needles is subjected to long distance transport within the plant, supplying the plant with additional nitrogen from the atmosphere.