Buffer Mechanisms and their Kinetics in Variable Charge Soils Affected by Acid Deposition in Southeast Mexico.
Lucy Mora Palomino1, Christina Siebe Grabach1, Andrea Herre2, Martin Kaupenjohann2, and Karl Stahr3. (1) Instituto de Geología, Universidad Nacional Autónoma de México, Circuito Exterior, Ciudad Universitaria, s/n, México, Mexico, (2) Institut für Ökologie, Technische Universität Berlin, Germany, 12587 Berlin, Salzufer 12, Deutschland, Berlin, Germany, (3) Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Straße 27, 70599 Stuttgart, Germany
Acid deposition produced by industrial facilities that emit sulfur dioxide and nitrous oxides affects vegetation, soil and aquatic systems in their surroundings. Its effects have been widely studied in temperate regions of industrialized countries, but little is known about its magnitude and behavior in tropical regions. In Mexico the petroleum extracting and processing industry is a very important source of atmospheric SO2 and NOx. Many petroleum facilities are located in the southeastern part of the country. Dominant soils in this region are Acrisols, which are acid soils of variable charge and are considered vulnerable to acid deposition. We studied the acid buffer mechanisms and their kinetics in Acrisols at the surroundings of a gas processing facility called Cactus located in northern Chiapas. Dry acid deposition was quantified and soil profiles were described and sampled along a transect and at different distances from the facility. Also 3 profiles were sampled along a catena (crest, slope and valley) at the site of largest deposition. With samples from Ah and Bt horizons of the different profiles, batch experiments were conducted, in which the pH was maintained constant using ion exchange resins and samples were shaken during different lengths of time (0.5 hours and up to 6 months). Released cations and anions at the different time intervals (Fig. 1) were analyzed as well as dissolved organic carbon. It could be shown that short term buffering is controlled by cation exchange and dissociation of organic matter. Medium term buffering is controlled by dissolution of colloidal aluminum and iron oxides which cover primary minerals, and long term buffering by dissolution of primary minerals like volcanic glass, calcium feldspars and hornblende introduced to the soils by the latest eruption of El Chichón volcano. All reactions release Al 3+ ions to the solution, which does not represent a real buffer mechanism, since this element takes part in the total soil acidity, and it also is toxic for plant roots and many soil organisms. The exchangeable Al3+ contents are positively related to the amount of clay in the soils, and since the soils nearby to the Cactus facility are the most clayey ones, these soils have the largest potential to release Al3+. Fig. 1: Ca2+ release as a function of time at the soil profile close to the facility.