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There is concern that changes in atmospheric deposition, climate, or land use have altered the biogeochemistry of forests causing soil base‐cation depletion, particularly Ca. 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 (NH₄)₂SO₄ Quantitative soil excavations in 1998 measured soil pools of exchangeable base cations 9 yr after treatments began. Stream sampling at the weirs on a weekly and event basis, and weekly precipitation sampling, were used for input‐output estimates. The treated watershed had lower concentrations of exchangeable Ca and Mg in all horizons, with evidence for the greater depletion in the O horizon compared to underlying mineral soils, and in softwoods compared to hardwoods. This difference between watersheds is interpreted to be treatment‐induced base …