Spatial and temporal patterns in the biogeochemistry of aluminum at the Hubbard Brook Experimental Forest, New Hampshire

This study describes: (i) the long-term trends in the chemistry and speciation of aluminum (Al) in soil solutions and stream waters; (ii) the landscape controls on the concentration of Al species in streams; and (iii) Al biogeochemistry in forest floor soils in the Hubbard Brook (HB) Valley in New Hampshire. Long-term changes in drainage water chemistry varied with position in the landscape. In Bs soil solutions at mid- to higher elevations, declines in the sum of concentrations of strong acid anions (C A : 2[sulfate (SO 4 2- )] + [chloride (Cl - )] + [nitrate (NO 3 - )]) and smaller declines in the sum of concentrations of base cations (C B : 2[calcium (Ca 2+ )] + 2[magnesium (Mg 2+ )] + [sodium (Na + )] + [potassium (K + )]) were accompanied by decreases in concentrations of inorganic monomeric Al (Al i ) with little change in pH. These changes probably reflect the release of C B and Al i through exchange mechanisms in response to changing inputs of mobile acid anions. In most streams sampled in this study, there was evidence of Al(OH) 3 control on Al solubility. Exceptions to this mechanism occurred at stream sites where dissolved organic carbon (DOC) concentrations were elevated. For all streams, forest floor depth and drainage area together explained much of the variation in concentrations of total monomeric Al. There was a strong relationship between organic monomeric aluminum (Al o ) and DOC concentrations in (i) the long-term record for soil solutions and (ii) stream water sampled across the HB valley. Furthermore, there was strong evidence of organic matter control on Al solubility in the forest floor. Modeling scenarios indicated that organic binding sites in the forest floor are far from saturated with Al. Furthermore, Al does not appear to limit the retention of increased Ca inputs to the forest floor, which has important implications for the remedial application of Ca to forest soils. Despite these findings, there may be continuing stress to forest biota from elevated Al concentrations. This is particularly true at mid-to higher elevations, where Al concentrations in fine root tissues are highest and where Ca 2+ /Al i ratios in Bs soil solutions are persistently low (molar ratio \u3c1)