Relating sulfate sorption in forest soils to lithological classes, as defined to calculate Critical Loads of Acidity

Weathering rates, as required for the determination of Critical Loads of Ac idity, can be approximated by using lithological classes defined based on t he acid buffering capacity of the different parent materials. Sulfate sorpt ion capacity of soils may also be required for the determination of Critica l Loads of Acidity, especially in acid soils rich in Al and Fe oxy-hydroxid es. The objective of this study was to determine if a relationship between SO42- sorption in soils and the different lithological classes from which t hey developed could be established. Seventy-one soils from Galicia (MY Spai n) derived from a variety of parent materials were sampled. Total annual pr ecipitation ranges from 600 to > 2000 mm yr(-1) and mean annual temperature is 12 degrees C. Sulfate sorption was studied by adding either 0.4 or 1.6 mM SO42- solutions to soils. Sorption increased from soils derived from qua rtzitic rocks (class 1) to those derived from granitic rocks and slates (cl ass 2), and reached a maximum in soils developed from basic materials, in w hich andic properties were common (class 3). In soils derived from these th ree lithological classes, SO42- sorption was primarily governed by the cont ent of oxalate-extractable Al. On the other hand, SO42- sorption in soils d eveloped from class 4 rocks (mostly derived from serpentinites) fell in bet ween lithological classes 2 and 3, in spite of them having high contents of Fe oxy-hydroxides. The relatively high pH of these soils, compared to soil s derived from more acidic rocks, probably negatively affected SO42- sorpti on. The only soil derived from limestones, also included in class 4 rocks, had a very low SO42- sorption capacity related to its alkaline pH. The rela tionship found in this study between SO42- sorption in forest soils from no rthwestern Spain and the different lithological classes from which they dev eloped could be of interest for calculating Critical Loads of Acidity in th ese ecosystems, given the general common SO42- sorption pattern followed by soils within each lithological class. (C) 1999 Elsevier Science B.V. All r ights reserved.