Aluminium chemistry in two contrasted acid forest soils and headwater streams impacted by acid deposition, Vosges mountains, NE France

Al chemistry was studied in two acidic watersheds, one with a podzol, the o ther with an acid brown soil, in the Vosges mountains (N.E. France), by ana lysing both leaching and centrifugation soil solutions and spring waters ov er 3 yr. In the podzol, Al was mobilized in the eluvial horizons under the predominant influence of organic acidity, then leached down the profile as organic and F-bound Al. Strong undersaturation with respect to proto-imogol ite and imogolite showed that the proto-imogolite theory of podzolization c ould not apply. Al was transferred from the soil to spring water mostly as Al3+ and Al-F. Al3+, as well as additional minor species (AlOH2+, AlSO4+), originated from the redissolution of the top of the spodic horizons under t he influence of both soil solution acidity and the occurrence of mobile ani ons derived from atmospheric deposition. Conversely, in the acid brown soil , Al mobilization was regulated by nitrate and occurred mainly as Al3+. Mos t of Al was retained in the deep soil and only traces of monomeric Al reach ed spring water. In the podzol eluvial horizons, soil solutions were unders aturated with respect to all relevant mineral phases and their chemical com position agree with the concept of a mobilization of Al from the solid soil organic Al and a control of Al3+ activity by complexation reaction with th e solid and soluble soil organic matter and F. In the acid brown soil, soil solutions were found to be in equilibrium with natural alunite, and the fo rmation of this mineral, if confirmed, would account for the occurrence of 'open' vermiculites instead of the expected hydroxy-Al interlayered vermicu lites. Al solubility control in surface water of both watersheds remains un clear. The Al-F species in both watersheds and the likely control of Al sol ubility by alunite in the acid brown soil emphasize the influence of acid d eposition on Al chemistry in acid watersheds.