Ion association in water solution of soil and vadose zone of chestnut saline solonetz as a driver of terrestrial carbon sink

Abstract. The assessment of soil and vadose zone as the drains for carbon sink and proper modeling of the effects and extremes of biogeochemical cycles in the terrestrial biosphere are the key components to understanding the carbon cycle, global climate system, and aquatic and terrestrial system uncertainties. Calcium carbonate equilibrium causes saturation of solution with CaCO₃, and it determines its material composition, migration and accumulation of salts. In a solution electrically neutral ion pairs are formed: CaCO₃⁰, CaSO₄⁰, MgCO₃⁰, and MgSO₄⁰, as well as charged ion pairs CaHCO₃⁺, MgHCO₃⁺, NaCO₃⁻, NaSO₄⁻, CaOH⁺, and MgOH⁺. The calcium carbonate equilibrium algorithm, mathematical model and original software to calculate the real equilibrium forms of ions and to determine the nature of calcium carbonate balance in a solution were developed. This approach conducts the quantitative assessment of real ion forms of solution in solonetz soil and vadose zone of dry steppe taking into account the ion association at high ionic strength of saline soil solution. The concentrations of free and associated ion form were calculated according to analytical ion concentration in real solution. In the iteration procedure, the equations were used to find the following: ion material balance, a linear interpolation of equilibrium constants, a method of ionic pairs, the laws of initial concentration preservation, operating masses of equilibrium system, and the concentration constants of ion pair dissociation. The coefficient of ion association γ_e was determined as the ratio of ions free form to analytical content of ion γ_e = C_ass∕C_an. Depending on soil and vadose zone layer, concentration and composition of solution in the ionic pair's form are 11–52 % Ca²⁺; 22.2–54.6 % Mg²⁺; 1.1–10.5 % Na⁺; 3.7–23.8 HCO₃⁻, 23.3–61.6 % SO₄²⁻, and up to 85.7 % CO₃²⁻. The carbonate system of soil and vadose zone water solution helps to explain the evolution of salted soils, vadose and saturation zones, and landscape. It also helps to improve the soil maintenance, plant nutrition and irrigation.

The association of ions in soil solutions is one of the drivers promoting transformation of solution, excessive fluxes of carbon in the soil, and loss of carbon from soil through vadose zone.