Experimental Acidification Causes Soil Base-Cation Depletion at the Bear Brook Watershed in Maine

There h concern that changes in atmospheric deposition, climate, or land use have altered the biogeochemistry o f forests causing soil base-cation depletion, particularly C e The Bear Brook Watershed in Maine (BBWM) is a paired watershed experiment with one watershed subjected to elevated N and S deposition through bimonthly additions of (NH,)aO,. Quantitative soil excavations in 1998 measured soil pools of exchangeable base cations9 yr after treatments began. Stream sampling at the weirs on a weekly and event basin, and weekly precipi-tation sampling, were used for input-output estimates. The treated watershed had lower concentrations of exchangeable Ca and M g in ail horizons, with evidence for the greater depletion i n the 0 horizon compared to underlying m i n e d soh, and i n softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment-induced baseation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha- ' of exchangeable Ca and Mg, respectively, after accounting for soil m w differences between watersheds.?his was comparable with tbe total armulative excess stream Ca and Mg export i n West Bear after 9 yr of treatment o f 55 and 11 kg ha-', respectively. Model simulations o f watershed response to treatments predicted excess soil exchangeable Ca and M g losses in the treated watershed o f 47 and 9 kg ha-', respectively. These results indicate that the response to a step-incmase in N and S deposition during Ule first d d e of treat-ments in this experimental forested watershed was to invoke cation-exchange buffering, resulting in a net decline in soil exdungeable base cations. CCELERATED LEACHING of base cations from forest A soils has been recognized as a possible conse-quence of acidic deposition for over 30 yr (Hutchinso