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SOIL & WATER MANAGEMENT & CONSERVATION

Relationship between Aluminum in Soils and Soil Water in Mineral Horizons of a Range of Acid Forest Soils

^a Dep. of Forest Soil Science, Agricultural Univ. of Krakow, al 29 Listopada 46, PL-31-425 Krakow, Poland
^b Dep. of Plant and Environmental Sciences, Norwegian Univ. of Life Sciences, N-1432 Aas, Norway

^* Corresponding author (rlgruba{at}cyf-kr.edu.pl).

Previous studies suggest that the solubility of Al in acid forest soils may be expressed as the log Al³⁺ activity = –n pH + log K_H-Al + log Al_p/C_p concentration ratio, with Al_p and C_p representing pyrophosphate-extractable Al and C, respectively, and K_H-Al is the binding constant of Al to protonated complexation sites on soil organic matter (SOM). We show that a slightly modified model, in which Al_p is replaced by Al_org (= Al_p – KCl-extractable Al), successfully describes the concentration of Al in A and B horizons of a wide range of acid forest soils (Typic Dystochrepts) developed in different parent materials from a large number of spruce [Picea abies (L.) Karst.] and mixed forest stands in southern Poland. Analysis of the data suggests a reaction stoichiometry of n = 1.27 and log K_H-Al = 1.98 with R² = 0.76. Furthermore, statistical analysis of a large set of published data, in addition to our own data from the Polish soils, indicates that log[Al_org/C_p] is significantly correlated with soil pH measured in 1 mol L^–1 KCl (pH_KCl), suggesting that the Al binding to SOM is less at small pH_KCl due to increased competition with H⁺: log[Al_org/C_p] = 0.82 pH_KCl – 4.16 (R² = 0.77, n = 276). Combining the two equations gives a simple Al solubility model for mineral horizons in acid forest soils: log Al = –1.27pH + 0.82pH_KCl – 2.18 (R² = 0.82, n = 127). In this equation, pH_KCl relates to chemical soil properties, whereas the pH of soil water also depends on solution characteristics like ionic strength. The equation may be applied to acid forest soils having a soil solution pH in the range of 3.0 to 4.2. Such conditions are rather common in surface horizons (A, E, and upper B) of acid forest soils of the temperate and boreal zones.

Abbreviations: CEC, cation exchange capacity • CEC_e, effective cation exchange capacity • SOM, soil organic matter