We evaluated the effect of acidic deposition and nitrogen on Austrian forests soils. Until thirty years ago air pollution had led to soil acidification, and concerns on the future productivity of forests were raised. Elevated rates of nitrogen deposition were believed to cause nitrate leaching and imbalanced forest nutrition. We used data from a soil monitoring network to evaluate the trends and current status of the pH and the C？:？N ratio of Austrian forest soils. Deposition measurements and nitrogen contents of Norway spruce needles and mosses were used to assess the nitrogen supply. The pH values of soils have increased because of decreasing proton depositions caused by reduction of emissions. The C？:？N ratio of Austrian forest soils is widening. Despite high nitrogen deposition rates the increase in forest stand density and productivity has increased the nitrogen demand. The Austrian Bioindicator Grid shows that forest ecosystems are still deficient in nitrogen. Soils retain nitrogen efficiently, and nitrate leaching into the groundwater is presently not a large-scale problem. The decline of soil acidity and the deposition of nitrogen together with climate change effects will further increase the productivity of the forests until a limiting factor such as water scarcity becomes effective. 1. Introduction Forests in central Europe provide manifold ecosystem services. Besides their function for wood production and protection, their soils represent an efficient filter and purification layer for water passing through, where they retain carbon and nitrogen. The maintenance of carbon stocks in soils contributes to the mitigation of climate change. In addition, forest soils buffer acidic deposition. Over the last three decades several factors have been identified which compromise the provision of these ecosystem services. In the late 1970s, increasing evidence for forest decline arose [1]. The topic attracted a lot of public attention, followed by initiatives to reduce emissions and improve forest soils [2, 3]. In the mid 1980s, the nitrogen saturation hypothesis was brought up. Due to industrial processes the nitrogen input to forests has reached unprecedented levels [4–6]. It was expected that the release of ammonium into soils is harmful due to the proton generation during nitrification [7–9]. The formed nitrate is leached into the groundwater, and as a consequence base cations are lost. The nitrogen saturation hypothesis was expanded to nutrient imbalances; these occur as a consequence of high growth rates, caused by excess nitrogen, when other
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