Features of Accumulation and Distribution of Trace Elements in the Profiles of Chernozems on Different Parent Rocks in the Rostov Region

The study of the elemental composition of soils developing under minimal influence of anthropogenic activity (background reference soils) is of key importance for an objective assessment of the geochemical state of landscapes. In this paper, we consider the features of accumulation and vertical distribution patterns of a wide range of trace elements (V, Cr, Co, Ni, Cu, Zn, As, Sr, Pb), as well as manganese (in the form of MnO), in the profiles of chernozems within the specially protected natural areas of the Rostov region. The content of all the studied trace elements in the genetic horizons does not exceed the environmental norms (maximum and tentative permissible concentrations), and their average concentrations in the humus-accumulative horizon correspond to the regional geochemical background. The leading factor in geochemical differentiation of soils is the lithological features of parent rocks: statistically significant differences between chernozems on loesslike loams and on the eluvium of hard carbonate rocks are seen for 7 of 10 elements (p < 0.05), which is confirmed by the principal component analysis (50.3% of the total variance). Two types of element distribution in the vertical soil profiles have been identified: (a) gradual, with a smooth gradient (on loesslike loams) and (b) contrasting, with distinct two layers (on carbonate rocks). Using multivariate statistical analysis, four geochemical associations of elements are distinguished based on their accumulation mechanisms: lithogenic silicate (V–Cr–Ni–Zn–MnO), carbonate (Sr–Co), pedogenic (Cu), and aerogenic (Pb–As). The resulting differentiated values are recommended as a regional reference background. It is shown that the use of a single background (reference) value without taking into account the specificity of the parent rocks leads to systematic distortion of the pollution indices. It is suggested that data on the content of Pb and As contents in the upper soil layer (0–10 cm) can be used as the most sensitive indicator of the long-term aerogenic input of the elements; the reference base should be updated with a frequency of at least once every 10–15 years.