Ion Leaching in Forest Ecosystems along a Great Lakes Air Pollution Gradient

Abstract

Abstract A gradient of H + , SO 2− 4 , and NO − 3 deposition across the Great Lakes region raised concerns over impacts on soil solution chemistry and ion leaching in regional forest ecosystems. Ten study sites representing northern hardwood and oak ecosystems were established across the gradient of increasing deposition from Minnesota to Ohio. Lysimeters were installed at lower E and lower B horizon boundaries at each site and sampled over a 2‐yr period. In soil solutions collected at lower E horizon boundaries, H + and SO 2− 4 wet deposition were correlated with solution concentrations of SO 2− 4 r = 0.82** and 0.92**) and Mg 2+ ( r = 0.75** and 0.85**), and with the SO 2− 4 /inorganic anion ratio ( r = 0.71** and 0.70**). In soil solutions collected at lower B horizon boundaries, SO 2− 4 deposition was correlated with solution SO 2− 4 concentrations ( r = 0.92**). Greater ionic outputs from B horizons than atmospheric inputs indicated that net losses of Ca 2+ and Mg 2+ were occurring from all sites. Sulfate outputs above background levels contributed 33 to 2367 mol c SO 2− 4 ha −1 yr −1 from north to south along the gradient, equal to 3.7 to 71.7% of annual cation outputs. Excess H + deposition accounted for 8 to 34% of annual cation outputs from north to south, and as much as 72% at one site with coarse‐textured soils. Elevated cation losses related to H + and SO 2− 4 deposition indicate that cation depletion remains a plausible consequence of pollutant deposition to forest ecosystems with poorly buffered soils.