如何提高盐碱土中磷养分的植物有效性是盐碱地农业的重要研究课题。聚谷氨酸(γ-PGA)被认为具有良好的保肥性能和缓释肥性能,但是其内在机理并不清晰。本文通过模拟盐溶液蒸发结晶实验,应用盐溶液pH和电导率(EC)的在线测定、盐溶液各组分之间及各组分与γ-PGA相互作用的热动力学参数测量,研究了聚谷氨酸抑制盐溶液中钙质磷酸盐沉淀的机理。实验表明,随着蒸发过程,盐溶液的pH总体上为先持续上升然后持续下降。当γ-PGA存在时,pH从上升变为下降的拐点所需的时间明显延长,这说明γ-PGA显著延缓了钙质矿物(如HAP和碳酸钙)的沉淀生成。 这种延缓作用主要与γ-PGA和各种盐分离子之间络合作用的显著差异相关。虽然HPO2-4-Ca2+络合物的稳定性大于γ-PGA-HPO2-4及γ-PGA-Ca2+,但γ-PGA可以为Ca2+提供的结合位点数高出HPO2-4-Ca2+两个数量级,盐溶液中大量的Ca2+被γ-PGA络合,从而抑制了HPO2-4-Ca2+的反应及随后磷酸钙类矿物的沉淀。虽然γ-PGA可延缓钙质矿物的沉淀形成,但对溶液完全蒸发后产生的结晶产物没有明显影响。实验结果表明,γ-PGA可通过抑制磷酸盐与Ca2+形成沉淀,而提高盐碱土中植物对P养分的利用效率,通过与HPO2-4形成高稳定的络合物起到P肥缓释剂的作用。γ-PGA在实际土壤溶液和土壤中的应用还需要进一步研究。
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 HPO2-4-Ca2+ complex has a much higher stability than the γ-PGA-HPO2-4 and γ-PGA-Ca2+complexes,the number of binding sites for γ-PGA-Ca2+ was two orders higher than those of HPO2-4-Ca2+.This means that most of Ca2+ was bound to γ-PGA and thus inhibited the interaction of HPO2-4-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.
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