title: Bidirectional ammonia exchange above a mixed coniferous forest.
abstract: Two canopy compensation point models were used to study the bidirectional exchange of ammonia over a mixed coniferous forest subjected to high nitrogen deposition. The models were tested for 16 time series, average fluxes of which ranged between -270 and +1 ng m(-2)s(-1). The static model consisted of a bidirectional stomatal flux and a unidirectional cuticular flux component. The dynamic model also allowed for desorption of ammonia from the leaf surface and took into account ammonia fluxes from precedent periods. The apoplastic ammonium/hydrogen ion ratio (Gamma), which was derived to estimate the stomatal compensation point (chi(s)), amounted to 3300 in spring and 1375 during the summer/autumn. Empirical descriptions for cuticular resistances (R(w)) in the static model, developed as a function of micrometeorological conditions and codeposition effects, failed to reproduce the measured fluxes. A better match with measurements was obtained using the dynamic model, which succeeded in simulating net-emission during the daytime.
authors.affiliation: Research Institute for Nature and Forest, Gaverstraat 4, B-9500 Geraardsbergen, Belgium. johan.neirynck@inbo.be
authors.forename: J
authors.initials: J
authors.lastname: Neirynck
authors.forename: R
authors.initials: R
authors.lastname: Ceulemans
mesh_terms: D000641:Ammonia / Q000032:analysis / Q000378:metabolism* / Q000633:toxicity; D001530:Belgium; D002980:Climate; D004784:Environmental Monitoring / Q000379:methods; D004785:Environmental Pollutants / Q000633:toxicity*; D016468:Forestry; D006863:Hydrogen-Ion Concentration; D008954:Models, Biological; D009584:Nitrogen / Q000633:toxicity*; D018515:Plant Leaves / Q000378:metabolism*; D018526:Plant Transpiration; D012621:Seasons; D064028:Tracheophyta / Q000378:metabolism; D014197:Trees / Q000378:metabolism*
chemical_list: D004785:Environmental Pollutants; D000641:Ammonia; D009584:Nitrogen