Trends in Water Chemistry in a Maple Forest on a Steep Slope

Year-to-year variation in SO₄^2-,NO₃^-, Ca²⁺, K⁺, and Mg²⁺concentrations in forest floor and mineral soil percolatefrom a forested, podzolic soil at the Turkey Lakes Watershedon the Precambrian Shield was assessed for monotonic trendsbetween 1986 and 1995. Our objective was to examine howrapidly ion concentrations in soil percolate equilibratedafter stabilization of SO₄^2- concentrations inprecipitation. Significant negative trends were detected inmonthly Ca²⁺, and Mg²⁺ concentrations in forestfloor and SO₄^2-, Ca²⁺, and Mg²⁺ inmineral soil percolate during the 10-year-period. Thedecline in Ca²⁺ and Mg²⁺ was greater than annualdecreases in SO₄^2- and NO₃^- in forestfloor percolate and proportional to the reduction inSO₄^2- in mineral soil percolate. Response ofmineral soil percolate to a 15 mol_c L^-1SO₄^2- decrease in wet-only precipitation between1985 and 1986 was a gradual decline in SO₄^2-concentration through 1995. The five-year meanSO₄^2- concentration in bulk precipitation, forestfloor percolate, and mineral soil percolate decreased 8, 9and 18 mol_c L^-1 from 1986–90 to 1991–95.Microbial (mineralization of organic S) and sorption(release from and/or retention in the pool of insolubleSO₄^2-) processes in the soil were logicalexplanations for the observed changes in SO₄^2- inmineral soil percolate.