THE DRY DEPOSITION OF AMMONIA ONTO A DOUGLAS-FIR FOREST IN THE NETHERLANDS

The dry deposition flux of ammonia (NH3) to forest was measured using the micrometeorological gradient method. The measurements were carried out on a 36 m high tower in the forest, an approximately 18 m high Douglas fir stand. The deposition flux of NH3 was calculated from the turbulent exchange coefficient and the concentration gradient of NH3 measured between 20 and 36 m. The results indicate a strong uptake of NH3 by the forest, especially under moist or wet conditions. The uptake rate of NH3 by the canopy was significantly higher than could be explained by stomatal uptake alone. The canopy resistance for NH3 correlates with water vapour pressure deficit and is a factor of five to ten lower than the stomatal resistance. Only at vapour pressure deficits higher than 10 g m-3 is absorption of NH3 less efficient. The observations can be explained by assuming a strong uptake by the leaf surface under wet or moist conditions and a less strong, possibly stomatal, uptake under dry conditions. To estimate the annual flux to the canopy, the experimental results were parameterized using a resistance layer model. This model was run using the results of detailed meteorological measurements carried out continuously on the site in the year 1989. The annually averaged deposition velocity to the forest was between 2 and 3 cm s-1. Based on these calculations, and taking all uncertainties into account, the dry deposition flux of NH3 was estimated to lie between 21 and 44 kg N ha-1 yr-1;. These estimates are a little higher than previous estimates made on the basis of large-scale dispersion model calculations and significantly higher than the critical N load to coniferous forest (10-20 kg N ha-1 yr-1).