Soil Iron, Aluminum, and Phosphorus Dynamics in Response to Long-Term Nitrogen and Sulfur Additions at the Bear Brook Watershed in Maine

Atmospheric deposition of nitrogen (N-) and sulfur (S-) containing compounds affects soil chemistry in forested ecosystems through (1) acidification and the depletion of exchangeable base cations, (2) metal mobilization, particularly aluminum (Al), and iron (Fe), (3) mobilization of phosphorus (P), and (4) N accumulation and potential Nsaturation. The Bear Brook Watershed in Maine (BBWM) is a long-term paired watershed study that includes bimonthly aerial applications of dry (NH4)2S04 at the rates of 28.8 kg S ha-1yr-1 and 25.2 kg N ha-1 yr-1 to the treated West Bear watershed beginning in 1989. The adjacent untreated East Bear watershed serves as a reference. In 1998, studies of soil chemistry at BBWM revealed lower concentrations of exchangeable calcium (Ca) and magnesium (Mg) in all soil horizons of the treated West Bear watershed compared to East Bear, with the greatest differences evident in 0 horizons from softwood forest sites. Acid neutralization through cation exchange of base cations for H\u27 and Al associated with increased soil acidity results in these exchangeable base cation differences. In 2003, soils were again studied at BBWM to see if there was evidence of a shift in acid neutralization mechanisms from base cation exchange dominated processes to A1 andor Fe mobilization dominated mechanisms after an additional five years of treatment since the 1998 study. Soils were collected from sites adjacent to the 1998 sites including 0 to C horizons in both watersheds. The 2003 results were consistent with 1998 in revealing lower concentrations of exchangeable Ca and Mg, and higher concentrations of A1 and Fe, in the treated watershed soils compared to the reference, and were best expressed in the 0 horizons of softwood stands. However, there were few differences evident in soil chemistry between 1998 and 2003 to indicate a further progression of base cation depletion. The importance of forest type in these responses to acidification is related to differences in litter quality. Traditional pedological and ecological soil fractionation techniques (dithionite-citrate-bicarbonate, oxalate, sodium pyrophosphate, and the Hedley fractionation extraction procedures) were used to determine the relative importance of different speciation of Al, Fe, and the biologically limiting nutrient P in the treated and reference watershed. Fractionation results indicated significantly lower concentrations of organically bound A1 and amorphous Fe in the treated mineral soils compared to reference soils. These results also indicate a linkage between A1 and Fe mobilization and P availability at this site. Lower labile A1 and Fe concentrations in soil were accompanied by lower concentrations of labile P. In hardwood stands, treatments appear to increase P concentrations in the upper soil horizons through increased rates of biocycling due to treatment N. Altered soil processes in response to these acidifying treatments involve base cations, N dynamics, and various secondary phases of A1 and Fe, thereby altering P biogeochemistry over time