Relationship Between Total Soluble Salts (TSS) and Ion Composition in Sodic Soils of the Karabakh Steppe: Implications for Ecosystem Services

Salinity, and particularly sodicity, represents one of the key constraints on agricultural productivity and the functioning of soil ecosystem services in the semi-arid regions of Azerbaijan, including the Karabakh Steppe of Azerbaijan. A reliable assessment of salinity conditions, together with ecosystem restoration and the design of effective reclamation strategies, requires a clear understanding of the relationships between total soluble salts (UHD), electrical conductivity (EC), and the concentrations of individual ions. In this study, the interactions between UHD and major ions were examined in sodic soils characterized by HCO₃⁻–SO₄²⁻ and SO₄²⁻–HCO₃⁻ salinity types. The analysis was based on detailed ion composition measurements and integrated soil profile investigations, covering both the plant root zone and the deeper vadose zone. The results indicate consistently strong correlations between UHD and EC throughout the soil profiles (R > 0.8). In contrast, the relationships between UHD and individual base ions showed greater variability, with corre+lation coefficients generally remaining below 0.90. This pattern suggests that ionic composition, rather than total salt concentration alone, plays a dominant role in regulating EC values.The salinity regime was defined by a cation dominance sequence of Na⁺ > Ca²⁺ > Mg²⁺, while the anion composition in both the root zone and vadose zone followed the order HCO₃⁻ > SO₄²⁻ > Cl⁻. This distribution is typical of long-term irrigated sodic soils in semi-desert environments. Surface horizons exhibited moderately low selective permeability; however, permeability became increasingly variable with depth and reached maximum values below the root zone, particularly at depths of 100–150 cm. This vertical pattern reflects the downward leaching of salts from upper horizons driven by irrigation and precipitation, followed by their accumulation in deeper vadose layers under conditions of restricted drainage and upward capillary flux from shallow sodic groundwater. These results demonstrate that EC alone is insufficient to characterize sodicity hazards. Integrated evaluation of TSS, ion composition, and depth-dependent processes provides a robust framework for salinity monitoring, soil reclamation, sustainable soil health, and ecosystem management in semi-arid irrigated systems.