Release kinetics and distribution of boron in different fractions in some calcareous soils

The aim of this study was to evaluate the release kinetics, speciation, and fractionation of boron (B) in some calcareous soils of western Iran. Ten surface soil samples were incubated with 100 mg B kg^?1 for a week at field capacity moisture. After air drying of samples, the trend of B release was experimented using sequential extraction with 10 mM CaCl₂. B speciation in soil solution was calculated for the first and the last steps of extraction by the visual MINTEQ program. The distribution of B among five fractions including exchangeable (F1), specially adsorbed (F2), bound by Fe–Mn oxides (F3), organically bound (F4) and residual (F5), was determined in control and spiked soils. The results indicated that the release rates were initially rapid followed by a slower reaction and the main proportion of the added B was extracted by CaCl₂. The release kinetics of B was described well with Elovich, parabolic diffusion, power function, and first-order equations. The speciation results revealed that the uncharged boric acid (H₃BO ₃ ⁰ ) was the dominant species in soil solutions. In control soils, B concentration in different fractions decreased in the following order: F5 > F1 > F2 > F3 > F4. In spiked soils, however, the largest and the smallest fractions were exchangeable and residual, respectively. This implies that B transformation from soluble to less mobile and non-labile forms is not a rapid process and requires more than a week. The significant relationship observed between kinetic parameters of power and parabolic equations and organically bound B fraction and OM content indicated that organic matter played an important role in B adsorption and release in calcareous soils.