Long-term trends in soil solution and stream water chemistry at the Hubbard Brook Experimental Forest: relationship with landscape position SHEILA M. PALMER*, CHARLES T. DRISCOLL and CHRIS E. JOHNSON Department of Civil and Environmental Engineering, Syracuse University, 220 Hinds Hall, Syracuse, NY 13244, USA; *Author for correspondence (e-mail: shpalmer@syr.edu; phone: 1þ315-443-4287; fax: 1þ315-443-1243) Received 6 February 2003; accepted in revised form 11 August 2003 Key words: Acid rain, Aluminum, Calcium, Dissolved organic carbon, Landscape position, Sulfate Abstract. In acid-sensitive watersheds of the northeastern US, decreases in SO 2 emissions and atmo- spheric 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 con- centrations 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 mEq L 1 year 1 ) and decreasing concentrations of inorganic monomeric Al (mean 1.03 mmol L 1 year 1 ), was confined to solutions draining the Bs horizon at mid-to-higher elevations. However, persistently low Ca 2+ /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 mEq L 1 year 1 ) were either similar to or exceeded declines in strong acid anions (mean 3.25 mEq 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 im- provement 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. Introduction Atmospheric deposition of sulfur (S) has been declining for the past 30 years in response to SO 2 emission controls in many parts of Europe and North America, resulting in decreases in sulfate (SO 2 4 ) concentrations in surface waters (Likens et al. 1996; Stoddard et al. 1999; Driscoll et al. 2001). However, in acid-sensitive watersheds of the northeastern US, decreases in strong acid anions in stream water have not been accompanied by widespread increases in pH or acid neutralizing capacity (ANC: Stoddard et al. 1999). This pattern is largely because atmospheric inputs of base cations have also declined and exchangeable base cation pools in soil are low, possibly due to depletion by acidic deposition (Likens et al. 1996, 1998; Stoddard et al. 1999; Driscoll et al. 2001; Gbondo-Tugbawa and Driscoll 2003). Atmospheric deposition of base cations have been declining in the northern and # 2004 Kluwer Academic Publishers. Printed in the Netherlands. Biogeochemistry 51–70, 2004. 68: