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Do the Competitions among Calcium, Aluminum and Hydrogen Ion for Organic Binding Sites Determine Soil pH and Aluminum Solubility in Adirondack Forest Soils?

Wei Li^1*, Chris E. Johnson¹ and April M. Melvin²

Long-term acid deposition has resulted in a decrease of soil pH, the depletion of labile calcium (Ca) and the mobilization of aluminum (Al) in Adirondack forest ecosystems. In acidic soils, the labile Al³⁺ is highly toxic to most organisms, affects the growth of plants, and can displace base cations (such as Ca²⁺, Mg²⁺, K⁺, Na⁺) on the exchange sites of soil organic matter (SOM). Liming is generally applied to mitigate the soil acidity and to improve the soil base status. After liming, the acid cations (Al³⁺, H⁺) may be neutralized by base cations (Ca²⁺), and the soil pH may be improved, suggesting that the competition between Ca²⁺ and Al³⁺ binding on SOM is the key process controlling soil pH and Al solubility. However, some researchers have noted that the fraction of exchangeable base cations and Al to cation exchange capacity (CEC) might present a strong correlation with soil pH, and it has been suggested that Al³⁺ might act as a base cation. Furthermore, researchers have hypothesized that the composition of hydrogen ions (H⁺) and Al³⁺ adsorbed to the SOM determines the soil pH and Al solubility. The goal of this project is to explore the relationships between Al³⁺, Ca²⁺ and H⁺ bound to the organic exchange sites of SOM, and to improve understanding of the key acid-base processes determining the soil pH and Al solubility in the acidic forest ecosystem. We have conducted experiments to determine the acid-base chemistry of thirty-six samples from three horizons (Oe, Oa and mineral) of two limed subcatchments (II and IV) and two control subcatchments (III and V) at the Woods Lake watershed in the Adirondack Region. Our study indicated that the soil pH and the base saturation (the ratio of exchangeable base cations to CEC) have experienced a large increase in organic horizons of treatment sites comparing reference sites, and the soil Al concentrations are opposite. The model of cation exchange reactions among H⁺, Al³⁺ and Ca²⁺ and the pH-dependent solubility of Al hydroxide should be both considered while determining soil pH and Al solubility. The cation exchange reaction works more effectively for Oa horizon than for Oe and mineral horizons. Later on, we will optimize the WinHumicV model by using the batch titration data of reference samples and calculate equilibrium concentrations of cations in solution with changing soil Al and Ca concentrations. The model simulation results will be used to compare the liming effects of the treated subcatchments with the measured results, and furthermore test the relationships between Al³⁺, Ca²⁺ and H⁺ on organic binding sites.

¹ Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13078, USA, and
² Department of Ecology and Evolutionary Biology, Cornell University, Ithaca, NY 14853, USA
^* Email: ; Phone: 315-882-8664;
Present address: Department of Civil and Environmental Engineering, Syracuse University, Syracuse, NY 13078, USA