The nature of alkalinity in virgin and anthropogenically modified solonetzes of northern Kalmykia

Abstract

The alkalinity of virgin solonetzes and its changes under the impact of 50-year-long afforestation and agricultural loads were investigated. It was found that the maximum alkalinity in both virgin and anthropogenically modified solonetzes is typical of the gypsum-free carbonate horizons containing exchangeable sodium. In these horizons of the virgin (B2, BCca) and anthropogenically modified (BMK, BCca) solonetzes, sodium carbonates (Na2CO3 and NaHCO3) are present because of the in situ ion exchange reaction between sodium in the exchange complex and calcium of calcium carbonates in the solution (the Gedroits reaction). The boundaries of the occurrence of soda in the soil profile are controlled by calcium compounds: the upper boundary of soda formation is specified by the presence of calcium carbonates, whereas the lower boundary is specified by the appearance of calcium sulfates (gypsum). In other horizons, including the solonetzic (B1) horizon, in which calcium carbonates are absent, soda is not formed, and the soil pH and carbonate alkalinity are controlled by the carbonate-calcium equilibria.