Effect of geo-pedological conditions on the distribution and chemical speciation of selected trace elements in forest soils of western Alborz, Iran

Geologic and pedologic controls have an important influence on the abundance, distribution and behavior of elements in natural soil environments. In order to assess the role of these factors on distribution and chemical speciation of selected trace elements in soil, soils on six parent materials including phyllite, tonalite, peridotite, dolerite, shale and limestone were sampled in western Alborz in Iran. Soil genesis and development of representative residual pedons were studied for each parent material and the total content of Si, Al, Fe, Ca, Mg, Co, Cr, Cu, Mn, Ni, Pb, V and Zn were compared. Enrichment/depletion patterns of trace elements were assessed using Ti as the reference element and lowest horizon as the reference layer. A modified BCR (European Community Bureau of Reference) sequential extraction procedure was used to fractionate the elements into four operationally defined species. The total contents of Co, Cr, Ni and V are highest in soils derived from peridotite (53, 984, 285, and 204 mg kg− 1, respectively) and dolerite (39, 1023, 176, and 185 mg kg− 1, respectively). Lead and Zn contents are highest in soils derived from shale (27 and 106 mg kg− 1, respectively), which is probably due to the high sorption capacity of clay. Different elements exhibited very different enrichment/depletion patterns in a given pedon. Moreover, depth distribution patterns differed also for a given element among soils with different parent material and even showed considerable variation among soils with similar lithology. Lead, Zn, Cu and Mn have been generally enriched in most pedons, except in some acidic and strongly leached soils, whereas Co, Cr, Fe, Ni and V have been leached, especially from Dystrudepts and Eutrudepts. The latter elements, however, showed enrichment trend in Hapludalfs and Argiudolls parallel to the development of illuvial B horizons. Iron, Cr, Cu, Ni, V and Zn are present primarily in the residual fraction, independent of soil and parent material type, whereas Mn, Pb and Co were mainly present in the non-residual fractions of most pedons. Trace element concentrations in the soils of the study area are related primarily to the parent material type while pedogenic processes appear to have played a minor role.