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 field experiments 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 experiments using real and artificial twigs, respectively, and (iii) by comparing throughfall flux estimates with atmospheric deposition estimates from micrometeorological measurements and inferential modelling. Canopy uptake of gases through stomata was estimated using measured air concentrations and a stomatal conductance model. Specific information on canopy leaching of soil-derived sulphate was provided by a 35S tracer experiment. Sulphur was found to behave conservatively within the canopy, with SO2 uptake more or less balancing leaching of soil-derived SO2−4. Significant stomatal uptake of NO2, HNO2 and NH3 was calculated as well as uptake of H+ and NH4+ from water layers covering the tree surface. Experiments did not indicate significant uptake of N03− in solution. 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 found for Na+ and Cl−. Differences observed between atmospheric deposition and throughfall fluxes could almost completely be explained by canopy exchange, the difference between NOy deposition and N03 throughfall flux being the only exception.