Geochemical partition of chemical elements in Kastanozems and Solonetz in a local catchment within a semiarid landscape of SW Russia

Abstract The vertical and lateral migrations of chemical elements (ChEs) under semiarid conditions were studied in a soil catena within the uppermost local catchment (2 ha) in the NE part of the Ergeni Upland (the NW Kalmykia, SW Russia). The data were interpreted using the key soil properties typically influencing the mobilisation and accumulation capacity of ChEs: pH, electrical conductivity and soil organic and inorganic carbon content as well as clay, silt and sand fractions. The patterns of the vertical distributions of ChEs were similar in all soils (Kastanozems and Solonetz) irrespective of their topographic location, but the depth of translocation was larger in the gully bottom. The content of mobile fractions of Ca, Mn and, to a lesser extent, P was higher than a residual fraction, which means that these elements have the highest potential mobility in soils (considering the sum of the three mobile fractions: weakly bound acid-soluble, bound with complexes and (hydr)oxides of Fe and Mn). In Kastanozems and Solonetz, weakly bound acid-soluble forms of the readily soluble ChEs (B, Ca, K, Na and Sr), and acid-soluble compounds (of Cd, Fe, K, Na and P) as well as both acid-soluble compounds and complexes for biogenic heavy metals (Co, Cu and Mn) are the major actors of geochemical migration. In the gully catchment, spatial differentiation is governed by the availability of water and the presence of readily soluble compounds of B, Cl, Li, Na and S. Within the topsoil of Kastanozems and Solonetz, the mobility of ChEs decreases in the following order: Mn, Ca > Pb, Co, Cu > V, Ba, Zn > Fe, Al > K, Cr > Si and Ti. In Kastanozems and Solonetz, mobile B, Ca, K, Na, Sr and, to a lesser extent, Mg (in the whole soil profiles) as well as Ba, Be, Mo and S (in subsoil and parent materials) are weakly bound by carbonate and exchangeable compounds. Mobile Co, Cu and Mn are held by soil organic and inorganic complexes. Mobile Cd and Fe occurred mostly within compounds extracted by 1 M nitric acid. The content of elements differed from the parent materials more strongly in the topsoil of Kastanozems due to the accumulation of substances at the biogeochemical barrier in the A-horizon and in the subsoil of Solonetz due to the accumulation of substances at the sorption barrier in the natric horizon. The ‘Solonetz at microhighs–Kastanozems at microdepressions’ is a translocation-solutional microcatena due to the migration of the most mobile chemicals in the soil solution in spring (downward movement) and the dry season (upward movement). Furthermore, the 'Kastanozems at the interfluve–Kastanozems at the gully bottom' is a leaching catena due to the downward movement of less mobile chemicals with melted snow and meteoric water.