Soil Solution Dynamics in Response to Elevated Nitrogen and Sulfur Treatments at the Bear Brook Watershed in Maine

Base cation depletion in forest soils of northeastern North America is a continuing concern because of the ongoing elevated deposition of sulfur (S) and nitrogen (N), intensive-management practices, and the often poorly understood implications of land-use history on modem ecosystem function. At the Bear Brook Watershed in Maine (BBWM), one of two paired, forested watersheds has been subjected to experimental additions of S and N for over a decade. Treatments consist of 28.8 kg S ha-\u27 yr-\u27 and 25.2 kg N ha-\u27 yr-\u27 in the form of granular (NH&SO4 applied bimonthly by helicopter. Ceramic cup tension lysimeters were used to sample soil solutions in both the treated and reference watersheds in 2001 and 2002. Soil solutions in the treated watershed had higher concentrations of most analytes compared with the reference watershed, with the largest relative increases in NO3-N (1650%), A1 (480%) and SO4-S (300%). Acid neutralization mechanisms in the treated watershed have progressed through an initial stage of neutralization by base cation exchange, to an increasing role for A1 hydrolysis. Treatments have also increased nitrification of both treatment and native N sources resulting in much higher concentrations of NO3-N in the treated watershed compared with the reference. Higher concentrations of most solutes under softwood stands compared with hardwoods were primarily the result of higher aerosol interception capacity and differences in litter quality. Long-term soil solution data showed steady increases in soil solution AI, H+, and ~04-s concentrations in response to treatments when soil solution data from this study were compared with previous soil solutions collected at BBWM. In contrast, Ca concentrations increased to a peak of 140 peq L-\u27 after the first four years of treatment, and subsequently declined to near pre-treatment concentrations of 77 peq L in the treated watershed after 11 years of treatment. Soil solution SO4-S concentrations in the treated watershed were in excess of pre-treatment concentrations reflecting an equilibrium with SO4-S adsorption capacity at the lower ambient levels of deposition. Results from this study suggest net base cation depletion from the soil exchange complex may be occurring, which results in higher A1 in soil solution and export to streams. Softwood forest types may be more vulnerable to acidification-induced A1 mobilization than hardwoods due to initially lower base saturation and higher aerosol interaction capacity