The Chemistry of Acidic Soils in Humid, Temperate Forested Watersheds with Emphasis on Phosphorus, Aluminum and Iron

Acidification of soils can result from natural processes (i.e. pedogenesis) and from atmospherically derived sulfur (S) and nitrogen (N) which originate from anthropogenic emissions. Research on the effects of acidification has typically focused on base cations, N and aluminum (Al). The Bear Brook Watershed in Maine (BBWM) (a long-term, whole-watershed acidification experiment) demonstrated increased episodic stream export of Al, iron (Fe), and phosphorus (P) in the treated watershed, suggesting that acidification can also affect P. This research evaluates (a) the physical and chemical distribution of P, (b) mechanisms controlling soil P accumulation, mobilization, and availability in humid, temperate forested ecosystems, and (c) the temporal progression of soil acidification at BBWM. We measured soil P in important, operationally defined chemical phases using P fractionation techniques. Studies at a forested watershed in Acadia National Park, ME demonstrated that mechanisms affecting the distribution of Al and Fe hydroxide in soils, stream sediments, and lake sediments also controlled P distribution in each of the landscape compartments. Phosphorus fractionation studies in acid forest soils across six watershed sites on two continents determined that the majority of soil P (i.e. 71 %) was associated with Al. In experimentally acidified watersheds it was the Al forms of P that were depleted in the upper soil horizons. This suggests that changes in soil acidity due to management, air pollutants or pedogenesis could shift P availability by altering acidity and the Al:P balance. Measurements of soil chemistry at BBWM in 1998 and 2006, (a period of declining SO42- deposition and continued experimental acidification) revealed little evidence of continued base cation depletion or recovery. There were, however, significant declines in forest floor mass, % carbon (C), and % N attributable to the 1998 ice storm. Forest type exerted a strong influence on soil response to natural disturbance, experimental acidification, and recovery from acidification. This study underscores the importance of long-term, quantitative soil monitoring in determining the trajectories of change in forest soils