The impact of canopy exchange on differences observed between atmospheric deposition and throughfall fluxes

Abstract

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 SO 2 uptake more or less balancing leaching of soil-derived SO 2− 4. Significant stomatal uptake of NO 2, HNO 2 and NH 3 was calculated as well as uptake of H + and NH 4 + from water layers covering the tree surface. Experiments did not indicate significant uptake of N0 3 − in solution. Canopy uptake of H + and NH4+ was countered by leaching of K +, Ca 2+ and Mg 2+. Part of the leaching of K +, Ca 2+ and Mg 2+ (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 NO y deposition and N0 3 throughfall flux being the only exception.