Soil geochemistry and pedological processes. The case study of the quaternary soils of the montagnola senese (central Italy)

The role of soil as an environmental filter is widely acknowledged, although not fully understood in all the processes involved. Unfortunately in the monitoring of some environmental parameters indicative of soil quality, such as heavy metals, we can observe a general tendency to simplify the issue. In fact, data refer only to a part of soil, i.e. the uppermost part, or the plow layer, while characteristics and processes which occur in the lower parts of the profile are neglected. On the other hand many soils, like Paleosols, which are quite widespread in Italy, have a very thick and complex profile, in which a significant elemental concentration can take place as a result of natural, pedological processes. This stresses the importance of in-depth investigation when the object of laboratory analysis is to provide advice for specific land uses. Aim of this work was to study the role played by soil forming processes in addressing element behaviour in some soils of the Montagnola Senese territory. Results of this work show an accumulation of many elements with respect to parent material. However this trend was not uniform in all cases, pointing out that their re-distribution in soil horizons can be related to different pedogenetic processes. The accumulation of some elements in soils can be to some extent related to organic matter content, pH and cation exchange capacity, but mainly in the upper horizons, while clay richness seems to play a more important role in determining the element concentration in all soil horizons: correlation coefficients with high level of significance have been found between clay and Ti, K and Cr, but also Fe, Zn and Pb are correlated with clay content, with the exception of those horizons, which are affected by element redistribution caused by oxidative-reductive processes. Several elements show a time dependent concentration process. Ti, K, Na and Mn seem to increase through time from the Holocene, to the Upper and Middle Pleistocene; Cr, Pb and Zn, similarly to Fe, from Holocene up to the Lower Pleistocene. The accumulation process proceeds along with clay neo-genesis and illuviation, but it can be affected by clay impoverishment, due to ferrolysis, together with the element mobilisation produced by reducing conditions. If clay impoverishment is characteristic of eluvial horizons and bleached streaks of fragipan and glossic horizons, mobilisation of Fe, Zn and Pb is manifested in the reduced parts of almost all the horizons with bad drainage.