Long-term trends in soil solution and stream water chemistry at the Hubbard Brook Experimental Forest: Relationship with landscape position

In acid-sensitive watersheds of the northeastern US, decreases in SO ₂ emissions and atmospheric deposition of sulfur have not been accompanied by marked changes in pH and acid neutralizing capacity (ANC). To better understand this phenomenon, we investigated the long-term trends in soil solution (1984-1998) and stream water (1982-2000) chemistry along a natural soil catena at the Hubbard Brook Experimental Forest, New Hampshire, USA. Significant declines in strong acid anion concentrations were accompanied by declines in base cation concentrations in soil solutions draining the Oa and Bs soil horizons at all elevations. The magnitude of change varied with position in the landscape. Recovery, as indicated by increasing ANC (mean 2.38 μEq L^-1 year^-1) and decreasing concentrations of inorganic monomeric Al (mean 1.03 μmol L^-1 year^-1), was confined to solutions draining the Bs horizon at mid-to-higher elevations. However, persistently low Ca²⁺/Al_i ratios (<1) in Bs soil solutions at these sites may be evidence of continuing Al stress to trees. In Bs soil solution at a lower elevation site and in Oa soil solutions at all sites, declines in base cations (mean 3.71 μEq L^-1 year^-1) were either similar to or exceeded declines in strong acid anions (mean 3.25 μEq L^-1 year^-1) resulting in no change in ANC. Changes in the chemistry of stream water reflected changes in soil solutions, with the greatest improvement in ANC occurring at high elevation and the rate of increase decreasing with decreases in elevation. The pH of soil solutions and stream waters either declined or did not change significantly. Therefore pH-buffering processes, including hydrolysis of Al and possibly the deprotonation of organic acids, have prevented increases in drainage water pH despite considerable reductions in inputs of strong acids.