FOREST SOIL RESPONSE TO ACID AND SALT ADDITIONS OF SULFATE .2. ALUMINUM AND BASE CATIONS

Reconstructed Spodosol and intact Alfisol soil columns were used to examine the effects of 52 weeks of additions of various simulated throughfall solutions on base cation, Al, acid neutralizing capacity, and pH levels in soil leachates. Our purpose was to determine the effects of acid and salt additions of SO4(2-) on cation leaching in two forest soils as well as to investigate the influence of episodic events of "seasalt" and pCO2. For leachates collected from the forest floor after H2SO4 additions, H+ exchange for Ca2+ provided the major buffering mechanism of acid inputs. Salt inputs (Na2SO4) increased leachate pH in Spodosol columns with mineral soil due to specific adsorption of added SO4(2-) and release of OH- (e.g., Forest floor + 40 cm B column leachates had pH values of 5.15 and 4.70 at week 52 in salt and control treatments, respectively). Increases in leachate SO4(2-) and other anion concentrations from all H2SO4 columns were generally balanced by increased Ca2+ and Mg2+ concentrations (e.g., FF + 40 cm B column leachates had 154 and 85-mu-eq Ca2+ L-1 at week 52 in acid and control treatments, respectively), whereas Na2SO4 leachate anions were balanced by Na+. Large additions (1000-mu-eq.L-1) of salt (NaCl) decreased soil pH and acid neutralizing capacity in short-term experiments (1 day) and interacted with pCO2 levels to control closed and degassed pH as well as acid neutralizing capacity. Our work illustrates the importance of soil cation exchange (especially in the forest floor), anion concentrations, and pCO2 levels in controlling the leachate chemistry in response to acidic and "seasalt" deposition events.