THE IMPACT OF CANOPY EXCHANGE ON DIFFERENCES OBSERVED BETWEEN ATMOSPHERIC DEPOSITION AND THROUGHFALL FLUXES

To study the impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes, several-held experimen ts were performed at the Speulder forest in The Netherlands. Relevant information was obtained by (i) measuring open-field precipitation and throughfall fluxes with different time resolutions, using two canopy exchange models, (ii) by comparing results from surface wash experimen ts using real and artificial twigs, respectively, and (iii) by compari ng throughfall flux estimates with atmospheric deposition estimates fr om micrometeorological measurements and inferential modelling. Canopy uptake of gases through stomata was estimated using measured air conce ntrations and a stomatal conductance model. Specific information on ca nopy leaching of soil-derived sulphate was provided by a S-35 tracer e xperiment. Sulphur was found to behave conservatively within the canop y, with SO2 uptake more or less balancing leaching of soil-derived SO4 2-. Significant stomatal uptake of NO2, HNO2 and NH3 was calculated as well as uptake of H+ and NH4+ from water layers covering the tree sur face. Experiments did not indicate significant uptake of NO3- in solut ion. Canopy uptake of H+ and NH4+ was countered by leaching of K+, Ca2 + and Mg2+. Part of the leaching of K+, Ca2+ and Mg2+ (15%) took place along with weak organic acids. No significant canopy exchange was fou nd for Na+ and Cl-. Differences observed between atmospheric depositio n and throughfall fluxes could almost completely be explained by canop y exchange, the difference between NOy deposition and NO3- throughfall flux being the only exception. Copyright (C) 1996 Elsevier Science Lt d