Soil Solution Chemical Response to Two Decades of Experimental Acidification at the Bear Brook Watershed in Maine

摘要

We examined long-term changes in soil solution chemistry associated with experimental, whole watershed-acidification at the Bear Brook Watershed in Maine (BBWM). At BBWM, the West Bear (WB) watershed has been treated with bimonthly additions of ((NH_4)_2 SO_4) since 1989. The adjacent East Bear (EB) watershed serves as a biogeochemical reference. Soil solution chemistry in the EB watershed was relatively stable from 1989-2007, with the exception of declining SO_4-S concentrations associated with a progressive decline in SO_4-S deposition during this period. Soil solution chemistry in WB reflected a progressive change in acid-neutralization mechanisms from base cation buffering to Al buffering associated with treatment during this period. Total dissolved Al concentrations progressively increased over time and were ～4× higher in 2007 than in 1989. Treatment of WB was also associated with long-term increases in soil solution H~+, SO_4-S, and NO_3-N, whereas soil solution dissolved organic carbon (DOC) was unresponsive to treatment. For solutes such as Ca, H~+, and SO_4-S, changes in stream chemistry were generally parallel to changes in soil solution chemistry, indicating a close coupling of terrestrial and aquatic processes that regulate the chemistry of solutions in this first-order stream watershed. For other solutes such as Al and DOC, solute concentrations were higher in soil solutions compared with streams, suggesting that sorption and transformation processes along hydrolog-ic flow-paths were important in regulating the chemistry of solutions and the transport of these solutes.