Long-term additions of ammonium nitrate to montane forest ecosystems may cause limited soil acidification, even in presence of soil carbonate

Abstract. In Europe, nitrogen (N) deposition to forest ecosystems decreased in the last decades, but remains in many cases higher than the critical loads, i.e. higher than what could be considered as safe. The main concerns about N deposition are eutrophication and acidification. In a long-term experiment in a montane (1200 m a.s.l.) coniferous forest in Alptal, central Switzerland, we simulated higher N deposition by adding NH₄NO₃ to rain water. This treatment represented 22 kg ha^-1 y^-1 of N, compared to 12 kg ha^-1 y^-1 ambient bulk deposition. It was applied at the same time to a small catchment and to plots in a replicated block experiment (n=5). The site has a carbonate-rich parent material and is thus not particularly endangered by acidification. Nevertheless, while previous reports on this experiment concentrated on N as a nutrient, we examine here soil acidification processes as affected both by ambient and experimentally increased N deposition. In the last two and a half decades since study start in 1994, nitrate (NO₃^-) and especially sulfate (SO₄^2-) concentrations decreased in precipitation, while pH increased by slightly more than 1 unit. In spite of these reduced acid inputs, the pH of the soil decreased. First, the exchangeable acidity in the soil increased, especially in the N-addition treatment. This was mainly observed on small mounds of the irregular topography of the site because they are less well buffered than the wet depressions. This trend, however, was also limited in time as exchangeable acidity later declined again, to reach values not much higher than 26 years before. This was also the case in the N-addition treatment and can be considered as a progressive recovery mainly due to the reduced acid inputs and, at this site with a carbonate-rich sub-soil, to the biological cycling of base cations. The pH of the runoff from the experimental catchments decreased by 0.3 units, both in the control and under N addition. Decreasing Ca²⁺ and increasing Al³⁺ and Fe²⁺ concentrations in runoff are also showing that the recovery observed in the exchangeable soil acidity is not yet able to stop the slow acidification of water leaving the catchments.