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

Abstract

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‐cation depletion, which was reinforced by model simulations. The difference between watersheds was 66 and 27 kg ha−1 of exchangeable Ca and Mg, respectively, after accounting for soil mass differences between watersheds. This was comparable with the total cumulative excess stream Ca and Mg export in West Bear after 9 yr of treatment of 55 and 11 kg ha−1, respectively. Model simulations of watershed response to treatments predicted excess soil exchangeable Ca and Mg losses in the treated watershed of 47 and 9 kg ha−1, respectively. These results indicate that the response to a step‐increase in N and S deposition during the first decade of treatments in this experimental forested watershed was to invoke cation‐exchange buffering, resulting in a net decline in soil exchangeable base cations.