Experimental work related to nitrogen deposition, nitrification and soil acidification--a case study

The leaching of major ions has been studied since August 1986 in two plots with different nitrogen fertilization levels and in a control plot in a 29-year-old stand of Norway spruce (Picea abies Karst.) in south-central Sweden. The fertilization started in 1967. The two fertilizer levels, both of which have caused a significant stimulation of the tree growth, correspond to an annual input of approximately 35 kg N ha(-1) and 75 kg N ha(-1) respectively, as NH4NO3. Phosphorus fertilizer is also applied. Field and laboratory incubations performed during 1986 showed that nitrification mainly occurs in the plot with the highest fertilization level, in accordance with the fact that nitrate could not be detected in the soil water in the other two plots. Fertilization has increased the ionic strength of the soil solution due mainly to sulphate in the phosphate fertilizer, but also nitrate at the highest fertilization level. This has caused an increase in total aluminium and a decline in pH. The preliminary data reported here are compared with results obtained at Swedish field sites with moderate to high levels of nitrogen deposition.