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

Abstract

Weathering rates, as required for the determination of Critical Loads of Acidity, can be approximated by using lithological classes defined based on the acid buffering capacity of the different parent materials. Sulfate sorption capacity of soils may also be required for the determination of Critical Loads of Acidity, especially in acid soils rich in Al and Fe oxy-hydroxides. The objective of this study was to determine if a relationship between SO 4 2− sorption in soils and the different lithological classes from which they developed could be established. Seventy-one soils from Galicia (NW Spain) derived from a variety of parent materials were sampled. Total annual precipitation ranges from 600 to >2000 mm yr −1 and mean annual temperature is 12°C. Sulfate sorption was studied by adding either 0.4 or 1.6 mM SO 4 2− solutions to soils. Sorption increased from soils derived from quartzitic rocks (class 1) to those derived from granitic rocks and slates (class 2), and reached a maximum in soils developed from basic materials, in which andic properties were common (class 3). In soils derived from these three lithological classes, SO 4 2− sorption was primarily governed by the content of oxalate-extractable Al. On the other hand, SO 4 2− sorption in soils developed from class 4 rocks (mostly derived from serpentinites) fell in between 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 soils derived from more acidic rocks, probably negatively affected SO 4 2− sorption. The only soil derived from limestones, also included in class 4 rocks, had a very low SO 4 2− sorption capacity related to its alkaline pH. The relationship found in this study between SO 4 2− sorption in forest soils from northwestern Spain and the different lithological classes from which they developed could be of interest for calculating Critical Loads of Acidity in these ecosystems, given the general common SO 4 2− sorption pattern followed by soils within each lithological class.