Abstract: It is an important research topic how to improve the phytoavailability and slow down the release of phosphorus fertilizer in salinized soil. Poly-glutamic acid (γ-PGA) is considered as a good fertilizer with nutrient preserving capability and slow release of fertilizer.However,the mechanisms involved in these functions are still not well clarified.This study aimed at the underlying mechanisms by simulated experiments of vaporization and crystallization of salt solution containing various concentrations of γ-PGA.The solutions pH and EC (electric conductivity) were recorded in real time,thermodynamic interactions of γ-PGA with salt components were quantified by ITC (Isothermal Titration Calorimetry),and the precipitation products were analyzed by XRD (X-ray diffraction) techniques.The results showed that the pH of salt solution was generally increased at first and then decreased with the vaporization time.The presence of γ-PGA postponed significantly the increase or decrease inflection point of the pH change curves,which indicated that γ-PGA inhibited substantially the precipitation of calcium precipitates,such as HAP and calcite.This inhibitory effect was closely related to the complexation capacity of γ-PGA with the anions and ions in the salt solution.Although HPO^2-₄-Ca²⁺ complex has a much higher stability than the γ-PGA-HPO^2-₄ and γ-PGA-Ca²⁺complexes,the number of binding sites for γ-PGA-Ca²⁺ was two orders higher than those of HPO^2-₄-Ca²⁺.This means that most of Ca²⁺ was bound to γ-PGA and thus inhibited the interaction of HPO^2-₄-Ca²⁺ and subsequent precipitation due to vaporization.However,γ-PGA had little effect on the kinds of the precipitates and crystals.This study revealed that γ-PGA had great binding capacity for Ca²⁺ to compete against precipitation of phosphate with calcium and thus enhanced the phytoavailability of phosphorus in salinized soil and slowed down the release of phosphorus due to the strong complexation of γ-PGA and phosphate.

Key words: γ-PGA, phosphate, vaporization, ITC