Aluminum solubility and mobility in relation to organic carbon in surface soils affected by six tree species of the northeastern United States

We compared Al solubility and mobility in surface soils among six tree species (sugar maple [Acer saccharum], white ash [Fraxinus americana], red maple [Acer rubrum, L.], American beech [Fagus grandifolia, Ehrh.], red oak [Quercus rubra, L.], and hemlock [Tsuga canadensis, Carr.]) in a mixed hardwood forest in northwestern Connecticut. We analyzed forest floors and mineral soils at 0¿5- and 10¿20-cm depths for exchangeable cations, pyrophosphate extractable Al (Alpyr; presumably solid organically bound Al), and total carbon, and solutions (from tension lysimeters) at 0- and 20-cm depths for pH, total organic C (TOC), and Al fractions. Forest floors beneath red maple, beech, and red oak had the highest exchangeable and extractable Al contents (0.4¿0.6 and 2.9¿3.2 mol m¿2, respectively), and the forest floor beneath sugar maple the lowest (0.1 and 0.8 mol m¿2, respectively). High concentrations of exchangeable H (under hemlock) and exchangeable Ca (under sugar maple) appear to depress exchangeable Al in forest floors. For soil solutions at 20-cm depth during the spring season, we found a strong and significant relationship between dissolved organic Al complexes (Alorg) and TOC. The TOC under all species appeared to have a similar capacity to bind Alorg. The capacity to bind dissolved Al was not saturated at high TOC concentrations in the forest floors of hemlock and red oak. Particularly under hemlock, low pH (3.5¿3.7) and high concentrations of TOC (40¿70 mg l¿1) percolating from the forest floor helped to dissolve Al from soil minerals, and more Alorg moved to deeper soil layers under hemlock than under the hardwood species. Despite pronounced differences in dissolved Al among tree species, no significant differences were found in the pyrophosphate extractable Al pools in mineral soil among tree species at different depths. The intensity, with which these trees influence Al migration throughout their life span, was probably too small to have caused pronounced effects in Al redistribution in the soil