Parent Material Influence on Sulfate Sorption in Forest Soils from Northwestern Spain

Abstract

Sulfate sorption by forest soils decreases the potential detrimental effect of S deposition on cation leaching. Twenty‐four soils from the 3416‐km2 area surrounding two lignite‐fueled power‐generating plants in NW Spain were studied to relate SO2−4 sorption to soil properties and ultimately to the parent material. The area contains a variety of parent materials, and has a wide range of acidic soils (Haplumbrepts, Fulvudands, and Kanhaplohumults). Total annual precipitation ranges from 1100 to 1900 mm yr−1 and mean annual temperature is 12°C. Sulfate sorption was studied by adding either 0.4 or 1.6 mM SO2−4 solutions to soils. Sorption decreased in the following order: amphibolite > biotitic schist > granite > phyllite and was positively related to Al extractable with sodium hydroxide, oxalate, and pyrophosphate, and to Fe extractable with dithionite‐citrate and oxalate, which were high in soils derived from basic materials. Organic matter apparently had counteracting effects on SO2−4 sorption. First, sorption was important in surface horizons with pH‐NaF ≥9.7, mainly due to the presence of Al–humus complexes, which often give these soils an andic character. Second, sorption was specially low in surface horizons of soils derived from acidic materials with pH‐NaF <8.0, which may be attributed to competition between SO2−4 and organic acids for sorption sites, in addition to the low content of reactive surfaces. Sorption was positively related to soil pH due to the positive relation between Fe and Al oxy‐hydroxides and basic materials. These results suggest the need to take into account the influence of parent material on SO2−4 sorption in assessing the sensitivity of soils to S deposition.