Changes in the concentrations and speciation of aluminum in response to an experimental addition of ammonium sulfate to the bear Brook Watershed, Maine, USA

An understanding of the biogeochemistry of aluminum (Al) in acid-sensitive terrestrial and aquatic ecosystems is critical to assessments of the effects of acidic deposition. Bear Brook Watershed, Maine, USA includes paired watersheds, East Bear and West Bear. Starting in November 1989, experimental additions of ammonium sulfate ((NH₄)₂SO₄; 900 mol/ha-yr) have been made to West Bear Brook Watershed. Chemical analysis of soil and stream waters were conducted to evaluate the speciation of Al prior to (1987-89) and following (1989-92) the experimental treatments. Before the treatments, soilwater Al occurred largely as inorganic monomeric Al (Ali). Concentrations of organic monomeric Al (Alo), Ali and dissolved organic C (DOC) were high in soil solutions draining the E horizon, and decreased in the lower mineral soilwater (Bs horizon) and streamwater. Streamwater concentrations of monomeric Al (Alm) were largely in the form of Alo. After the (NH₄)₂SO₄ treatments were initiated in the West Bear Brook Watershed, concentrations of Alm increased in soilwater and streamwater, largely as Ali. These increases in Al accompanied decreases in pH and increases in concentrations of SO₄^2- and NO₃^- in drainage waters. Increases in stream concentrations of Al were particularly evident during high flow events. This pattern, coupled with the increases in concentrations of Ali in upper soilwaters in response to the (NH₄)₂SO₄ addition, suggests that episodic increases in Ali were due to inputs of water entering the stream from shallow hydrologic flowpaths.