A comparative analysis of aluminum biogeochemistry in a northeastern and a southeastern forested watershed

This comparative biogeochemical analysis focused on the patterns and processes of aluminum cycling in two small watersheds, one in the west‐central Adirondacks of New York and the other on Cumberland Plateau of eastern Tennessee. Despite shared similarities in soil acidity, soil exchangeable aluminum concentrations, and elevated inputs of acidic deposition, the northern and southern sites exhibited strong differences in aqueous aluminum chemistry and transport. Soil and stream drainage waters in the northern watershed were more acidic, and contained higher concentrations of base cations, sulfate, nitrate, and organic carbon than waters in the southern ecosystem. Mean concentrations of biologically active labile inorganic aluminum, Al_i, ranged from 17 to 46 μmol L⁻¹ in soil solutions and stream water in the northern drainage basin, and from 0 to 2 μmol L⁻¹ in the southern system. The major differences in aluminum chemistry and transport between the two watersheds were related to different patterns of alkalinity generation and mobile anion transport in these contrasting systems. In the northern watershed, atmospheric inputs of acidity were partially neutralized through the release of mixed cations (Ca, Al, Mg, K, and Na) from soils and detritus. Because of the high mobility of sulfate and nitrate in the northern watershed, there was significant transport of Al through the soil profile and into stream water. At the southern watershed, soil sulfate adsorption, biological retention of nitrate, and base cation release were the major sources of acid neutralizing capacity for soil drainage waters and surface waters. Because these processes resulted in both the release of alkalinity and the removal of mobile strong acid anions, concentrations of labile Al remained low throughout most of the drainage profile in the southern watershed. At the ecosystem level, results indicated that aluminum cycling at the southern site was controlled more by biotic transfers (e.g., plant uptake and litterfall), while solution transfers (e.g., soil leaching) dominated aluminum cycling at the northern site.