Aluminum solubility and release rates from soil horizons dominated by aluminum-humes complexes

Abstract

Aluminum solubility and release rates were studied for A horizons of three nonallophanic Andosols and for a Bhs horizon of one Spodosol that had active Al fractions dominated by Al-humus complexes. The A horizon of one allophanic Andosol was also used for comparison. Aluminum solubility relationships over the pH range 3–5 were determined in a 30-d equilibration study. Aluminum release rates were measured using a stirred, flow-through reaction vessel with a 10^-3 M acetate buffer adjusted to pH 3.5, 4.0, and 4.5. The effect of ionic strength on Al release was determined using different concentrations of CaCl₂ adjusted to pH 4.0. To determine the source of the dissolved Al, soils were pretreated with KC1, Na-pyrophosphate, and acid oxalate to remove various solid-phase pools of active Al. The nonallophanic Andosol and Spodosol samples displayed pAl versus pH slopes of 2.0–2.4 and solubilities considerably lower than those of common A1(OH)₃ phases. In contrast, the allophanic Andosol exhibited a pAl versus pH slope of 2.9 and a solubility virtually equal to that of synthetic gibbsite. All the soils showed very rapid Al release rates at t = 0 ranging from 16–44 nmol g^-1 s^-1 at pH 3.5 to 4–8 nmol g^-1 s^-1 at pH 4.5. The H⁺-dependence at t = 0 ranged between 0.64–0.86 for the nonallophanic Andosols and Spodosol to 0.46 for the allophanic Andosol. The ionic strength dependence was generally greater for the nonallophanic Andosols and Spodosol than for the allophanic Andosol. Pyrophosphate treatment of the nonallophanic Andosols and Spodosol reduced Al release rates to very low levels indicating that Al-humus complexes were the primary source of dissolved Al For the allophanic Andosol, both pyrophosphate and acid oxalate treatments resulted in reductions of Al release rates indicating that the dissolved Al fraction was derived from both Al-humus complexes (approximately 20% of the total dissolved Al) and allophanic materials (80%).