Impact of forest fire ash on surface charge characteristics of Andisols

Forest fires are a common occurrence in central southern Chile where soils are mostly Andisols. The ash deposits may alter the chemical properties of the soils in the burned areas and nearby agricultural lands. In a column experiment, three Andisols found in this region were treated with 2-cm-thick ash deposit on the surface and leached with 12 pore volumes of water for 3 months. At the end of the leaching, the ash-treated and their respective control soils were sectioned, and the surface charge characteristics of each soil section were determined by electrophoretic mobility test and potentiometric titration. Chemical constituents released from the ash affected mainly the surface 0- to 5-cm section of the soils. The ash treatment and leaching caused the soil pH of the 0- to 5-cm section to increase from 2 to 3 pH units. Consequently, the negative surface charge and cation exchange capacity of affected soils both increased. The ash treatment changed the isoelectric point when the isoelectric point of respective control soil section was >3. The point of zero salt effect and the point of zero net proton charge of the ash-treated soils were increased in the 0- to 5-cm section and the influences of the ash leachates gradually diminished with soil depths. The alkaline leachates dissolved and mobilized the soil organic carbon and associated soil constituents. The effects observed on the surface charge and dissociation constants are consistent with anion and/or dissolved organic carbon adsorption. The surface dissociation constants, estimated by the constant capacitance model, showed that the ash treatment caused modifications of the active sites on the soil particles. Although ash leachates increase the cation retention capacity, the adsorption of other cations may be adversely affected by the ash deposit that is rich in Ca and K.