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

Abstract

In the present study NH3 and NO2 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 NH3 (CNH3) or NO2)(CNO2) concentrations using the dynamic chamber technique. Beside fluxes of NH3 (JNH3) and NO2 (JNO2), transpiration (JH2O), leaf conductance for water vapour (gH2O), photosynthetic activity (JCO2), photosynthetic photon fluence rate (PPFR), air temperature (T) and relative air humidity (RH) were determined. Both fluxes, JNH3 and JNO2, depended linearly on CNH3 and CNO2 in concentration ranges representative for the field site and gH2O as a measure of stomatal aperture. For both trace gases compensation points could be determined amounting to 2.5 nmol mol-1 for NH3 and to 1.7 nmol mol-1 for NO2. The fluxes of NH3 and NO2 could not be explained exclusively by exchange through the stomata. In both NH3 and NO2 fumigation experiments additional deposition onto the needle surface was observed and increased with increasing CNH3 and CNO2. 15N[NH3] fumigation experiments with adult spruce trees confirmed the results of gas exchange measurements and revealed that NH3-N deposited to spruce needles is subjected to long distance transport within the plant, supplying the plant with additional nitrogen from the atmosphere. © New Phytologist (2002).