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

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 (N_Al/N_H) or the ratio of organically bound Al to total organic carbon (Al_org/C) remains roughly constant within each horizon. Both the N_Al/N_H and the Al_org/C ratios are effective measures of the ratio of sites bound to Al and H in Al-SOM complexation models. The log-linear relationship pAl_KCl-p(Al_org/C)=0.75×pH_KCl-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.