DISTRIBUCIÓN DE ALGUNOS ELEMENTOS MAYORES (K, Ca, Ti, Fe) Y TRAZA (Ga, Rb, Sr, Y, Zr, Br) EN DOS SUELOS POLICÍCLICOS
PODSÓLICOS.

E. PEITEADO VARELA, R. PIÑEIRO REBOLO, A. MARTÍNEZ CORTIZAS

Departamento de Edafología y Química Agrícola, Universidad de Santiago de Compostela, Campus Sur s/n, 15782 Santiago de Compostela


Abstract

Two polycyclic podzolic soils from northwestern Spain were analyzed for some major and trace elements. Pena Cebral (PC) is located in Serra dos Ancares and is composed by a basal mixed coluvium of shales and phylites, buried by a quartzitic coluvium; while Curro Vello II (CVII) is situated in the Serra do Xistral and developed on a granodioritic coluvium. The vertical distribution of four major (K, Ca, Ti, Fe) and 6 trace elements (Ga, Rb, Sr, Y, Zr y Br) in the fine earth (FE, <2mm) and the silt+clay fraction (SC, <50 µm) was studied using detailed concentrations profiles, concentration ratios between fractions and enrichment factors. The concentrations obtained for the fine earth of PC were: K 1.9-4.0%; Ca is below the detection limit (<0.01%) in all samples except for the upper most one (O horizon); Ti 0.3-0.5%; Fe 1.4-4.1%; Ga 9.7-18.3 mg kg-1; Rb 87.3-139.6 mg kg-1; Sr 20.8-48 mg kg-1; Y 20-50.8 mg kg-1; Zr 271.4-630 mg kg-1; Br 2.8-30 mg kg-1. In CVII the concentrations determined ranged: K 1.3-3.3%; Ca 0.14-0.25%; Ti 0.08-0.25%; Fe 1.0-3.1%; Ga 11.1-21.2 mg kg-1; Rb 84.9-132.4 mg kg-1; Sr 41-52 mg kg-1; Y 9.6-53.9 mg kg-1; Zr 75.7-205 mg kg-1; Br 42.1-362.9 mg kg-1. Both soils have lower average concentrations of Ca and Sr and higher Fe in CVII and Zr in PC than those reported by other researchers for the parent materials of the areas where they are located. All the elements, exception made of the K, Ca and Sr in CVII, showed higher concentrations in the SC than in the FE and than those estimated for the sands (up to 4.5 times in PC a67nd 2.5 times in CVII).
Both the distribution profiles and the fractionation of elements seems to be the result of the interplay of several factors. The influence of the parent material mineralogy is expresed in higher Sr concentrations in CVII than in PC while the opposite is true for K, Ti, Fe, Zr, Y and the proportion of fine material (igneous
material versus metamorphic material). The higher concentrations of the elements in the SC, particularly for the more mobile ones (K, Ca, Sr), in PC soil indicate a lower degree of evolution. In CVII these elements are depleted in the SC indicating a greater degree of chemical evolution, showing in both cases the effect of the geochemical environment (weathering and pedogenesis). Results also indicate that Fe and Br, and to some extent Ti, are mobile in these soils due to podzolitation. The accumulation of Br in the spodic horizons suggests that the element is incorporated into the soil organic matter in the surface horizons and transported down in the profile by mobile organic compounds, something which is consistent with recent work on the halogenation of the soil organic matter. Thus, Br seems to be a good indicator of the intensity of the podzolisation, although further research is necessary to elucidate the mechanisms. On the other hand,this element is subjected to a pronounced source effect, with concentrations 15 times higher in CVII than in PC, as it should be expected given the proximity to the ocean of the former soil.

Key words: polycyclic soils, podzolisation, major elements, trace elements, bromine.