Factors controlling soil water and stream water aluminum concentrations after a clearcut in a forested watershed with calcium-poor soils

The 24 ha Dry Creek watershed in the Catskill Mountains of southeastern New York State USA was clearcut during the winter of 1996–1997. The interactions among acidity, nitrate (NO₃⁻), aluminum (Al), and calcium (Ca²⁺) in streamwater, soil water, and groundwater were evaluated to determine how they affected the speciation, solubility, and concentrations of Al after the harvest. Watershed soils were characterized by low base saturation, high exchangeable Al concentrations, and low exchangeable base cation concentrations prior to the harvest. Mean streamwater NO₃⁻ concentration was about 20 μmol l⁻¹ for the 3 years before the harvest, increased sharply after the harvest, and peaked at 1,309 μmol l⁻¹ about 5 months after the harvest. Nitrate and inorganic monomeric aluminum (Al_im) export increased by 4−fold during the first year after the harvest. Al_im mobilization is of concern because it is toxic to some fish species and can inhibit the uptake of Ca²⁺ by tree roots. Organic complexation appeared to control Al solubility in the O horizon while ion exchange and possibly equilibrium with imogolite appeared to control Al solubility in the B horizon. Al_im and NO₃⁻ concentrations were strongly correlated in B-horizon soil water after the clearcut (r ² = 0.96), especially at NO₃⁻ concentrations greater than 100 μmol l⁻¹. Groundwater entering the stream from perennial springs contained high concentrations of base cations and low concentrations of NO₃⁻ which mixed with acidic, high Al_im soil water and decreased the concentration of Al_im in streamwater after the harvest. Five years after the harvest soil water NO₃⁻ concentrations had dropped below preharvest levels as the demand for nitrogen by regenerating vegetation increased, but groundwater NO₃⁻ concentrations remained elevated because groundwater has a longer residence time. As a result streamwater NO₃⁻ concentrations had not fallen below preharvest levels, even during the growing season, 5 years after the harvest because of the contribution of groundwater to the stream. Streamwater NO₃⁻ and Al_im concentrations increased more than reported in previous forest harvesting studies and the recovery was slower likely because the watershed has experienced several decades of acid deposition that has depleted initially base-poor soils of exchangeable base cations and caused long-term acidification of the soil.