Saturday, 15 July 2006

Variable Charge Soils Identified in a Cloud Forest.

Maria Guadalupe Tenorio Arvide1, Joe B. Dixon2, Otilio A. Acevedo Sandoval3, Miguel A. Valera Perez1, and Gladys Linares Fleites4. (1) Benemerita Univ Autonoma de Puebla, 14 Sur 6303, Jardines de San Manuel. Puebla., Pue, Mexico, (2) Texas A&M Univ, 370 Olsen Blvd., College Station, TX 77843-2474, (3) Univ Autonoma del Estado de Hidalgo, Pachuca, Hidalgo, Mexico, (4) Benemerita Univ Autonoma de Puebla, 14 Sur 6301.72470, Puebla, Mexico

Variable charge soils occupy vast areas in the world, and cover a wide range of landscapes even if they have different morphological, mineralogical, chemical, biologic and genetic properties. They have a common property, the surface charge is variable in magnitude and sign. This variation is due to differences in pH, ionic strength and soil solution composition. This research was conducted in Sierra Norte de Puebla, Mexico, with the vegetation in a cloud forest and soils derived from volcanic ash. The environmental conditions of these soils are severely affected by the high humidity from the Gulf of Mexico that condenses into the annual precipitation of 2000 to 4000 mm (regime perudic, thermic soils). The composition of the volcanic ash is variable, being of intermediate and basic types emitted by a group of complex volcanic cinder cones including the enormous calderas of Teziutlan formed during the Pleistocene. Representative soil profiles were described, georeferenced, and sampled by horizon. The physical and chemical properties of the samples were characterized and then classified according to World Reference Bank (WRB, 2000). On the other hand the pH was determined according to Uehara and Gillman (1981) this parameter is an accepted criterion for variable charge soils. Also, it was necessary to quantify the mineralogical fraction that contributed to the variable charge properties such as aluminum, iron, and silicon in the form of organic-mineral complexes and short-range-order minerals. Thus, it was done by selective dissolution of different samples with acid-oxalate and pyrophosphate solutions. Following dissolution, the extracted Al, Fe, and Si were determined by inductively coupled plasma (ICP). Organic mater was determinate by Walkley-Black method. The ERN06 profile had primary minerals, physical-chemical properties, and environmental conditions of a Pachic Andosol according to WRB. Special characteristics of this profile are its high amount in organic matter (30% for the epipedon), the advanced weathering, and volcanic ash parental material that caused the high amount of soluble Al, Fe and Si extracted with pyrophosphate solution (2.20%; 0.78%, and 0.34%, respectively). This was considered a variable charge soil and the delta-pH > -0.5. In contrast, the ERN09 profile (an Eutric Cambisol) contained abundant organic matter (15% in the epipedon) too, the parent material contains calcium carbonate gives a high concentration in exchange bases and soluble bases, fundamentally ionic calcium (250% base saturation) occasionally low concentration of Al, Fe, and Si (0.05%; 0.04%; and 0.8% respectively). This soil has predominantly permanent charge delta-pH < -0.5. In summary, of the results from the characterization of representative profiles, soils classified by WRB as Humic Acrisol, Pachic Andosol and Haplic Luvisol are variable charge soils and the profiles classified Eutric Cambisol contain a permanent charge, according to the proposal by Uehara and Gillman (1981) who reported a relationship between inorganic constituents and soil order. Also, we conclude that the soils contain pyroclastic materials that stabilize the organic colloids that impede the mineralization of the organic matter by aluminum. For the Luvisol, Ca ions substitute for Al in the organic colloid, so the synergisms between these elements impedes the presence of the other element, in this case Al. The amount of Fe is low in general for all the profiles studied. The percentage extractable Al and Si with oxalate and the relation Al/(Al+Si) indicated that allophane is present (Dahlgren, 1994).

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