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

Abstract

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 (Ho$ glwald, 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 watervapour 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 1n8-1n9 nmol mol −" . The NH $ compensation point varied between 3n3 and 3n5 nmol mol −" in the field experiments, and was 3n0 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 (0n8 and 4n5 nmol mol −" ) might vary significantly with time because of variation in the NH $ compensation point.

Field investigations of nitrogen dioxide (NO<sub>2</sub>) exchange between plants and the atmosphere

“…They turn out to be a NO 2 sink when ambient concentrations exceed the NO 2 compensation point. Previous studies reported NO 2 compensation point concentrations ranging from 0.3 to 3 ppb depending on tree species (Rondón et al, 1993;Thoene et al, 1996;Weber and Rennenberg, 1996;Sparks et al, 2001;Geßler et al, 2000Geßler et al, , 2002Hereid and Monson, 2001). But Lerdau et al (2000) questioned the existence of such compensation points.…”

“…For that, a suitable guide is the stomatal conductance for H 2 O (g H 2 O ) because this parameter gives information about the condition of the plant affected by air temperature, radiation and water vapor deficit. Furthermore, NO 2 exchange is known to be strongly regulated by stomatal conductance (Thoene et al, 1991;Geßler et al, 2000;Teklemariam and Sparks, 2006;Chaparro-Suarez et al, 2011). Hence, our data were classified into seven g H 2 O classes.…”

“…Furthermore, discrepancies may also be due to the fact that some authors have neglected gas-phase reactions in the dynamic chambers all together, some excluded them by using corresponding set-ups, and some tried to compensate them by application of an empty chamber ("reference chamber") (Rondón et al, 1993;Geßler et al, 2000Geßler et al, , 2002Hereid and Monson, 2001;Sparks et al, 2001;Raivonen et al, 2009). Furthermore, different deposition velocities at leaf level may be related to different ages of the enclosed plant material.…”

“…In many reports it is not clear whether photochemical reactions of the NO-NO 2 -O 3 triad within the plant chambers were taken into account or an experimental setup was used which excluded reactions of NO 2 photochemistry (Geßler et al, 2000Hereid and Monson, 2001;Sparks et al, 2001;Raivonen et al, 2009). During field measurements chemical reactions of the NO-NO 2 -O 3 triad must be regarded as being part of the natural conditions; therefore the measured values must be corrected.…”

Dry nitrogen deposition estimates over a forest experiencing free air CO2 enrichment

“…Estimates of dry deposition of reactive N to individual plants or entire ecosystems have been made using dynamic and open-topped chambers (Gessler et al, 2000;Sparks et al, 2001) and by measuring precipitation throughfall after it passes through the plant canopy (Lovett et al, 2000). However, these methods preclude deposition to nonplant surfaces, chemical transformations within or above the canopy, and attempts to scale chamber measurements to whole-system fluxes are associated with a high degree of uncertainty (Hanson et al, 1989).…”