Effects of Acidic and Neutral Sulfate Salt Solutions on Forest Floor Arthropods

Acidic and neutral SO₄²⁻ solutions were applied to a mixed oak (Quercus spp.) forest floor on Walker Branch Watershed in eastern Tennessee to study effects of increased acid deposition on decomposer arthropods and arthropod regulation of P cycling. Treatments consisted of ambient (no applications), 2 × (low), and 10 × (high) annual atmospheric inputs of SO₄²⁻ (as K₂SO₄) and H⁺ plus SO₄²⁻ (as KHSO₄). Arthropods were sampled monthly in litter and mineral soil. Soils were analyzed for Bray no. 2 P and for salt pH, NH₄Cl exchangeable Al³⁺ and K⁺, and water-extractable SO₄²⁻. Over a 14-month period, the number of forest floor macroarthropods averaged 19% lower in the high SO₄²⁻ salt treatment compared with the control. Macroarthropod fungivores also were adversely affected by the high SO₄²⁻ salt treatment and by the high and the low KHSO₄ treatments. The effects of the high SO₄²⁻ treatment on macroarthropods may have resulted from a salt effect caused by the concentrated K₂SO₄ applications (112 mmol K⁺ m⁻² per application). Microarthropod numbers increased significantly in response to the low K₂SO₄ additions and to the high and low KHSO₄ treatments. Microarthropod fungivores, predators, and detritivores responded similarly. Comparison of microarthropod densities and soil K⁺ concentrations suggested that the response resulted from a direct fertilizer effect of K⁺ on arthropods or indirectly from an effect on microorganisms. Mineral soil pH, exchangeable Al³⁺, and Bray P were unaffected by the treatments. Soil pH (0–15 cm) was unaffected by the KHSO₄ applications because most of the H⁺ addition was neutralized by the forest floor. Analysis of litter leachate during a single treatment interval (5 weeks) indicated that only 31% of the H⁺ applied in the high KHSO₄ treatment reached the mineral soil. Although increased inputs of H⁺ appear to have little effect on decomposer arthropods, effects of nonacidic SO₄²⁻ warrant further study.