Aluminum release rates from selected Spodosol Bs horizons: Effect of pH and solid-phase aluminum pools
Spodosol Bs horizon soils from Hubbard Brook, New Hampshire, Bear Brook watershed, Maine and Findley Lake watershed, Washington, were examined to determine the effect of pH and solid-phase Al pools on Al release rates. The clay fraction of all soils was dominated by hydroxy-Al interlayered 2:1 layer silicates and Al-humus complexes; the Washington soil also contained high concentrations of imogolite. Aluminum release rates were determined by reacting the <250-μm soil fraction with an acetate buffer (10 -3M) adjusted to pH 3.3, 3.9, or 4.2 in a stirred, flow-through reaction vessel. Aluminum release rates at t = 0 ranged from 11-29 nmol/g/s(0.23-0.47 nmol/m 2/s) at pH 3.3 to 5.8-14.5 nmol/ g/s (0.12-0.25 nmol/m 2/s) at pH 4.2. The order of the reaction with respect to hydrogen ion concentration was 0.29-0.32 for initial rates and 0.36-0.57 after 60 min. Selective dissolution techniques using KCl, Na-pyrophosphate and acid oxalate were evaluated as a method for discriminating the source of dissolved Al. Treatment with KCl increased Al release rates for all soils and may result from precipitation of Al displaced by the KCl treatment. The Maine and New Hampshire soils showed a large reduction in Al release rates following treatment with pyrophosphate, indicating the possible role of Al-humus complexes in regulating release rates. In contrast, release rates in the Washington soil showed a large reduction in the initial rates following pyrophosphate treatment and a further reduction for the entire dissolution period following removal of imogolite with the acid oxalate treatment.