NH₃ and NO₂ fluxes between beech trees and the atmosphere - correlation with climatic and physiological parameters

The dynamic-chamber technique was used to investigate the correlation between NH₃ and NO₂ fluxes and different climatic and physiological parameters: air temperature; relative air humidity; photosynthetic photon fluence rate; NH₃ and NO₂ concentrations; transpiration rate; leaf conductance for water vapour; and photosynthetic activity. The experiments were performed with twigs from the sun crown of mature beech trees (Fagus sylvatica) at a field site (Höglwald, Germany), and with 12-wk-old beech seedlings under controlled conditions. Both sets of experiments showed that NO₂ and NH₃ fluxes depended linearly on NO₂ and NH₃ concentration, respectively, in the concentration ranges representative for the field site studied, and on water-vapour conductance as a measure for stomatal aperture. The NO₂ compensation point determined in the field studies (the atmospheric NO₂ concentration with no net NO₂ flux) was 1.8-1.9 nmol mol^-1 . The NH₃ compensation point varied between 3.3 and 3.5 nmol mol^-1 in the field experiments, and was 3.0 nmol mol^-1 in the experiments under controlled conditions. The climatic factors T and PPFR were found to influence both NO₂ and NH₃ fluxes indirectly, by changing stomatal conductance. Whilst NO₂ flux showed a response to changing relative humidity that could be explained by altered stomatal conductance, increased NH₃ flux with increasing relative humidity (>50%) depended on other factors. The exchange of NO₂ between above-ground parts of beech trees and the atmosphere could be explained exclusively by uptake or emission of NO₂ through the stomata, as indicated by the quotient between measured and predicted NO₂ conductance of approx. 1 under all environmental conditions examined. Neither internal mesophyll resistances nor additional sinks could be observed for adult trees or for beech seedlings. By contrast, the patterns of NH₃ flux could not be explained by an exclusive exchange of NH₃ through the stomata. Deposition into additional sinks on the leaf surface, as indicated by an increase in the quotient between measured and predicted NH₃ conductance, gained importance in high air humidity, when the stomata were closed or nearly closed and/or when atmospheric NH₃ concentrations were high. Although patterns of NH₃ gas exchange did not differ between different months or years at high NH₃ concentrations (c. 140 nmol mol^-1 ), it must be assumed that emission or deposition fluxes at low ambient NH₃ concentration (0.8 and 4.5 nmol mol^-1 ) might vary significantly with time because of variation in the NH₃ compensation point.