Aluminum Leaching by Mineral Acids in Forest Soils: II. Role of the Forest Floor

Abstract

AbstractTo simulate the effects of forest floor organic horizons on leaching of labile and nonlabile forms of Al from underlying mineral soil horizons by acid precipitation, albic (E), ochric (A), and spodic (Bs) mineral horizons were leached in the laboratory with 1.0 mM HNO3, 0.5 mM H2SO4, or NH4NO3 control solutions at ionic strength 0.01 with or without an Oi‐Oe‐Oa forest floor horizon sequence (FF) above each mineral horizon. Adding 2 and 0.5 cmolc (H+)/kg to the organic horizons and mineral soils, respectively, simulated snowmelt additions to the soils in mountainous regions of the northeastern USA, where lake acidification has been attributed to acid precipitation leaching of forest soils. Total Al concentrations in leachates from soil columns containing the E or A horizon below the FF were greater than the sum of the concentrations in leachates from the FF and mineral horizon when leached separately. This positive synergistic behavior of the FF‐mineral horizon sequences was also observed in the FF‐Bs horizon sequence when leached with the control solution, but the synergism was negative for both forms of Al when leached with the acids. Sulfuric acid leached less Al from the Bs horizon than did HNO3, regardless of the presence of an FF, but HNO3, H2SO4, and control solutions leached similar concentrations of Al from the E and A horizons. The FF effects on the mineral soil leachates were attributed to effects of Ca, SO4, NO3, and dissolved organic C leached from the FF to the mineral horizons since the FF removed nearly all added H+.