NH and NO fluxes between beech trees and the atmosphere - correlation with climatic and physiological parameters.

The dynamic-chamber technique was used to investigate the correlation between NH and NO fluxes and different climatic and physiological parameters: air temperature; relative air humidity; photosynthetic photon fluence rate; NH and NO 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 NO and NH fluxes depended linearly on NO and NH concentration, respectively, in the concentration ranges representative for the field site studied, and on water-vapour conductance as a measure for stomatal aperture. The NO compensation point determined in the field studies (the atmospheric NO concentration with no net NO flux) was 1.8-1.9 nmol mol . The NH compensation point varied between 3.3 and 3.5 nmol mol in the field experiments, and was 3.0 nmol mol in the experiments under controlled conditions. The climatic factors T and PPFR were found to influence both NO and NH fluxes indirectly, by changing stomatal conductance. Whilst NO flux showed a response to changing relative humidity that could be explained by altered stomatal conductance, increased NH flux with increasing relative humidity (>50%) depended on other factors. The exchange of NO between above-ground parts of beech trees and the atmosphere could be explained exclusively by uptake or emission of NO through the stomata, as indicated by the quotient between measured and predicted NO 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 NH flux could not be explained by an exclusive exchange of NH through the stomata. Deposition into additional sinks on the leaf surface, as indicated by an increase in the quotient between measured and predicted NH conductance, gained importance in high air humidity, when the stomata were closed or nearly closed and/or when atmospheric NH concentrations were high. Although patterns of NH gas exchange did not differ between different months or years at high NH concentrations (c. 140 nmol mol ), it must be assumed that emission or deposition fluxes at low ambient NH concentration (0.8 and 4.5 nmol mol ) might vary significantly with time because of variation in the NH compensation point.