IntroductionSince the 1950s, in the Netherlands and several other western European countries an intensified livestock production system has developed. As a consequence, the quantities of NH x (NH 3 and NH4+) emitted into the atmosphere have increased. However, atmospheric NH x is eventually returned to the soil surface, which occurs through dry and wet deposition. In forests, dry deposition on leaf surfaces is enhanced by several processes, resulting in average deposited amounts of 30 kg NH 4 -N ha -1year -1. In addition, annually 10 kg NO 3 -N ha -1is deposited. Possible negative effects of increased NH 4 depositions are, first, soil acidification and, second, an imbalanced plant nutrition in the form of (relative) mineral nutrient deficiencies.The former effect may be caused by nitrification of atmospheric NH 4 . or by an increased excess of cation-over-anion uptake by tree roots. When more (or less) cations than anions are absorbed, plants maintain their internal charge balance by net excretion of H +(or OH -) ions. Therefore, an increased uptake of NH4+may increase the roots' H +production.Relative or absolute nutrient deficiencies may occur when N availability is excessively high relative to that of the mineral nutrients, or through increased mobilization and leaching of base cations, respectively.This thesis presents two lines of research, pertaining to these possible negative effects of increased NH x depositions in the Netherlands. The first one deals with the question whether or not nutrient uptake of trees can be considered an important source of soil acidification, relative to other H +sinks and sources. The second line of research was aimed at obtaining more understanding of forest fertilization and liming as possible tools to ameliorate acid-rain-induced nutritional disorders and soil acidification.Scots pine (Pinus sylvestris L.) was chosen as experimental tree species, because of its predominant contribution to the forested area in the Netherlands and other western European countries. A pot ...