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a Dep. of Plant, Soil, and Environmental Sciences, Univ. of Maine, Orono, ME 04469
b USDA Forest Service, Northeastern Experiment Station, Durham, NH 03824
c Dep. of Geological Sciences, Univ. of Maine, Orono, ME 04469
d Senator George J. Mitchell Center for Env. & Watershed Research, Univ. of Maine, Orono, ME 04469
e Dep. of Environmental Sciences, University of Virginia, Charlottesville, VA 22904
* Corresponding author (ivanjf{at}maine.edu).
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.
Abbreviations: ANC, acid neutralizing capacity • ANOVA, analysis of variance • BBWM, Bear Brook Watershed in Maine • BSe, effective base saturation • CECe, effective cation-exchange capacity • HBEF, Hubbard Brook Experimental Forest • LOI, loss-on-ignition
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