Importance of natural soil processes relative to atmospheric deposition in the mobility of aluminium in forested watersheds of the Black Forest

The dynamics of aqueous aluminium in the ARINUS experimental watersheds at Schluchsee (granite) and Villingen (quartz sandstone), Black Forest (South-west Germany), were studied in order to detect the processes and factors controlling its mobility. Aluminium speciation was performed in the seepage of typical soils (podsol, acidic brown earth, stagnogley) at 3 depths (organic layer, 30 cm and 80 cm of the mineral soil) as well as in streamwater. The studies concentrated on the variability in time and space of inorganic monomeric Al (Ali), and organic monomeric Al (Alo). Furthermore, the equilibrium speciation model WATEQF was used to calculate the distribution of inorganic Al species. Natural soil properties and processes, such as DOC mobilization and excess mineralization of NO3(2-) and SO4(2-), appeared to have great influence and outweigh the deposition effects upon Al mobilization in these systems which receive only low to moderate loads of acidic deposition.