Phytoremediation of secondary saline soil by halophytes with the enhancement of γ-polyglutamic acid.

Soil salinization has severely affected the quality of tillage land in China, and most greenhouse soils in Shanghai suburb suffer from secondary salinization with high salinity levels dominated by Ca2+, Mg2+ and NO3-. In this work, a sandy loam soil (Calcaric Fluvisols) contaminated by the above ions was selected as research object, and the binding conditions and abilities of γ-polyglutamic acid (γ-PGA) to water-soluble Ca2+ and Mg2+ in the soil were examined, and then pot experiments were conducted to remove Ca2+, Mg2+ and NO3- by two halophytes (Sedum aizoon L., Sesbania cannabina Pers.) integrated with γ-PGA application. The results showed that under the conditions of adding 1000 mg L-1γ-PGA (pH 7) and 25 °C, the binding efficiencies of Ca2+ and Mg2+ were 51.59% and 68.03%, respectively. Compared with Sesbania cannabina Pers., Sedum aizoon L. displayed better remediation performance for the soil with γ-PGA application in pot experiments, and the removal efficiencies of Ca2+, Mg2+ and NO3- reached 93.25%, 94.78% and 84.26% after applying 1000 mg L-1γ-PGA for 56 d, respectively. Moreover, γ-PGA application could mitigate salt stress and promote plant growth, and activate antioxidant defense systems. Compared with the control, 1000 mg L-1γ-PGA application significantly increased plant height and fresh weight of Sedum aizoon L., and the removal efficiencies of Ca2+, Mg2+ and NO3- increased by 45.48%, 82.62% and 69.91%, respectively. In the future, more in-depth mechanism of joint effect and field-scale investigation need to be further studied.