Soil solution response to acidic deposition in a northern hardwood forest

An intensive plot-scale acidification experiment evaluated the effects of H 2SO 4, HNO 3, and combined H 2SO 4 and HNO 3 treatments on the chemistry of soils and soil solutions in a northern hardwood forest. Treatments were delivered to 18 plots (each of 15m × 15m, three plots per treatment) during 20-week field seasons by a hill-slope irrigation system and consisted of two levels of H 2SO 4 (∼2000 and 4000 eq ha −1 year −1), two levels of HNO 3 (∼2000 and 4000 eq ha −1 year −1), one level of combined H 2SO 4/HNO 3 (∼2000 eq ha −1 year −1 each of SO 4 2− and NO 3 −), and a control (water only). Soil solutions responded rapidly to all treatments with increased leaching of SO 4 2− and/or NO 3 − (depending on treatment) accompanied by increased leaching of Ca 2+ and Mg 2+ from the upper B horizon. As solutions passed through the upper 25 cm of soil, mean SO 4 2− concentrations decreased by 50–86% of initial values in all S treatments and mean NO 3 − concentrations decreased by 71–93% of initial values in the low nitrogen treatments. This reflected the importance of anion adsorption for SO 4 2− and biological immobilization for NO 3 − in these forested Spodosols. Mean NO 3 − concentrations, however, remained constant with depth in the high nitrogen treatment, suggesting that this high rate of NO 3 − input exceeded the plant/soil capacity to immobilize N. During the autumn and winter following the treatment period, soil solution SO 4 2− and NO 3 − concentrations were greater for treated soils than for those of the control for 2–7 months. This suggests that anion desorption from soil exchange sites occurred in response to lowered soil solution concentrations of these strong acid anions.