RESPONSE OF ALUMINUM SOLUBILITY TO ELEVATED NITRIFICATION IN SOIL OF A RED SPRUCE STAND IN EASTERN MAINE

Elevated concentrations of soluble Al can impair tree growth and be to xic to aquatic biota, but effects of acidic deposition on Al solubilit y in forest soils are only partially understood because of complex int eractions with Hf and organic matter. We therefore evaluated Al solubi lity in two red spruce stands in eastern Maine, one of which received dry (NH4)(2)SO4 at a rate of 1800 equiv ha(-1) yr(-1) during 1989-1995 . Samples of soil (Spodosol Ca and Bh horizons) and soil solution were collected on five dates from 1992 to 1995. The treatment elevated nit rification, causing an increase in acid input that led to inorganic Al concentrations of greater than 60 mu mol L(-1) in both the Oa and Bh horizons. Solubility of Al was also lower in the Bh horizon of the tre ated stand than in the reference stand, a response related to higher D OC concentrations in the treated stand. Concentrations of CuCl2 and py rophosphate-extractable Al were higher in the Oa horizon of the treate d watershed than the reference stand, a result of accelerated weatheri ng of mineral particles caused by lower solution pH in the treated sta nd (3.47) than in the reference stand (3.69). Dissolved Al concentrati ons in these soils are the result of complex mechanisms through which mineral matter, organic matter, and pH interact to control Al solubili ty; mechanisms that are not incorporated in current Al solubility mode ls.