To determine the importance of Al-availability for soil micro-organisms 95 forest soils from Tyrol/Austria with comparable topography, vegetation, climatic conditions, soil type and with low soil pH (median = 3.9) were investigated for their physical (percentage of stable aggregates, water holding capacity), chemical (pH, electrical conductivity, contents of organic matter, concentrations of easily extractable aluminium, calcium, potassium, magnesium and phosphorus) and microbiological characteristics (microbial biomass and respiration, metabolic quotient, content of ATP, activities of protease and CM-cellulase, cfu-values of total and Al-tolerant bacteria and fungi). A highly significant negative correlation was detected between concentrations of KCl-extractable aluminium and soil pH. By the application of multivariate statistical methods, the effect of the concentration of KCl-extractable aluminium on abundance and activities of soil micro-organisms could be revealed. Al turned out to be of great importance for micro-organisms and often outmatched the significance of other well known soil properties like organic matter, pH or water holding capacity. However, due to very healthy trees at the sites under investigation no effect of Al or soil acidification on forest decline could be detected.

The contributions of different acidifying processes to the total protonload (TPL) of the soil in control plots (C) and ammonium sulphate treatedplots (NS) were studied in a Norway spruce stand in Southwest Sweden during 1988–1998. The annual deposition of inorganic nitrogen and sulphate was on average 18 kg N and 20 kg S ha⁻¹. In addition the NS treated plots received 100 kg N and 114 kg S ha⁻¹annually. The amounts of nutrients added to the ecosystem by wet and dry deposition and the leaching at 50 cm depth were calculated. The net atmosphericproton load, the proton load by nitrogen transformations in the soil, the sulphate sorption/desorption in the soil and the excess base cation accumulation in biomass were calculated. There was no leaching of inorganic nitrogen from control plots during the study period. The net atmospheric proton deposition, originating from sulphuric and nitric acid deposition, was the main contributor to TPL in control plots. The addition of ammonium sulphate increased the leaching of ammonium, nitrate, sulphate, magnesium and calcium but not of potassium. The TPL in NS plots was about ten times that in control plots. The nitrogen transformation processes were the main contributors to TPL to NS soil, in the beginning by ammonium uptake and later also by nitrification. The pH decreased by 0.4 units in the mineral soil. The between-year variation in TPL during the eleven year period in C plots (200–1500 molcha⁻¹yr⁻¹) and in NS plots (1000–13000 molcha⁻¹yr⁻¹) was mainly dependent on the sorption or release of sulphate. Both in C and NS, the TPL was buffered mainly by dissolving solid aluminium compounds, most probably some Al(OH)₃phase.