Relationships among pH, aluminum solubility and aluminum complexation with organic matter in acid forest soils of the Northeastern United States

• pH, Al solubility and Al–SOM complexes control acid–base chemistry in forest soils.
• Two Al binding/solubility models were tested with data from the northeastern USA.
• The effects of Al saturation must be considered in the Al–SOM complexation models.
• Al–H competition controls pH and Al solubility in Oa and upper mineral horizons.
• Base cations compete with exchangeable H ions to regulate pH in Oi/Oe horizons.

We investigated the interactions among pH, Al solubility and Al–soil organic matter (SOM) complexation to test the hypothesis that competition between Al and H ions for cation binding sites on soil organic matter (SOM) determines soil pH and Al solubility in acidic forest soils in the northeastern U.S. Samples from five soil horizons in 39 forested watersheds across the northeastern U.S. were used with two Al complexation models to test the hypothesis. Our results indicated that Al solubility increases with soil depth when pH is less than 4.5 and the ratio of the fractions of organically bound Al and H (NAl/NH) or the ratio of organically bound Al to total organic carbon (Alorg/C) remains roughly constant within each horizon. Both the NAl/NH and the Alorg/C ratios are effective measures of the ratio of sites bound to Al and H in Al–SOM complexation models. The log-linear relationship pAlKCl - p(Alorg/C) = 0.75 × pHKCl - 1.56 (r = 0.86, P < . 0001) can be used to describe Al–pH relationships in Oie, Oa, Bh and Bs1 horizon samples from across the region. The competition between H+ and Al3 + for binding to SOM affects soil pH and Al solubility to a greater degree in Oa and Bs1 horizons than in Oie and E horizons. Our data are consistent with the conceptual model that portrays the Al–pH relationship in acid forest soils as a balance between organic acidity and alkalinity produced through the weathering of aluminosilicate minerals.