Soilaluminum, iron, and phosphorus dynamics in response to long-term experimental nitrogen and sulfur additions at the Bear Brook Watershed in Maine, USA.

Atmospheric deposition of nitrogen (N) and sulfur (S) containing compounds affects soil chemistry in forested ecosystems through (1) acidification and the depletion of base cations, (2) metal mobilization, particularly aluminum (Al), and iron (Fe), (3) phosphorus (P) mobilization, and (4) N accumulation. The Bear Brook Watershed in Maine (BBWM) is a long-term paired whole-watershed experimental acidification study demonstrating evidence of each of these acidification characteristics in a northeastern U.S. forested ecosystem. In 2003, BBWM soils were studied using the Hedley fractionation procedure to better understand mechanisms of response in soil Al, Fe, and P chemistry. Soil P fractionation showed that recalcitrant P was the dominant fraction in these watersheds (49%), followed by Al and Fe associated P (24%), indicating that a majority of the soil P was biologically unavailable. Acidification induced mobilization of Al and Fe in these soils holds the potential for significant P mobilization. Forest type appears to exert important influences on metal and P dynamics. Soils supporting softwoods showed evidence of lower Al and Fe in the treated watershed, accompanied by lower soil P. Hardwood soils had higher P concentrations in surface soils as a result of increased biocycling in response to N additions in treatments. Accelerated P uptake and return in litterfall overshadowed acidification induced P mobilization and depletion mechanisms in hardwoods.