Study on different nitrogen forms and combined application of chitosan for enhancing phytoremediation of Cd-contaminated soil by Hylotelephium spectabile

Objectives Hylotelephium spectabile is one of the most potential plants used for remediating Cd-contaminated soil, while further measures still needed to improve its phytoextraction efficiency, and thereby increase the practical value in phytoremediation of Cd-contaminated field. This study compared the effect of different nitrogen forms and chelator on Cd absorption and translocation of H. spectabile in Cd-contaminated soils as well as the control mechanism.

Methods A pot experiment of H. spectabile was conducted using two typical Cd-contaminated soils in southern China, waterloggogenic paddy soil (pH 7.07) was collected from Mianzhu City, Sichuan Province and brown rendzina (pH 6.0) from Yangshuo County, Guangxi Region. Under the same fertilizer rates of nitrogen, phosphorus and potassium, application treatments of physiological neutral N (NH₄HCO₃), physiological acid N (NH₄Cl), physiological alkaline N (NH₃·H₂O) and combined application of NH₄HCO₃+chitosan (NH₄HCO₃+chitosan) were setup. After the H. spectabile was transplanted and cultured for 20 days, the dry weight and Cd concentrations of roots, stems and leaves were investigated. The soil pH, available Cd content and the contents of different Cd forms in rhizosphere soil were measured.

Results The pH in rhizosphere of two tested soils was significantly decreased, while the available Cd concentrations were significantly increased with addition of NH₄Cl in comparison with NH₄HCO₃. In addition, combined application of NH₄HCO₃+chitosan possessed more superior ability in acidization of rhizosphere soils than NH₄Cl, which significantly increased the available Cd concentration in two rhizosphere soils by 18.6% and 15.5%, respectively. Compared with NH₄HCO₃, addition of NH₄Cl also significantly increased the water soluble and acid extractable Cd contents of two tested soils. However, although combined application of NH₄HCO₃+chitosan increased the water-soluble Cd concentration, no significantly differences were found in acid extractable Cd when compared to NH₄HCO₃ treatment. The results may be attributed to that chitosan led to formation of relatively stable complex with Cd in soils, and promoted growth and Cd absorption of acid extractable Cd in rhizosphere soils. Application of three nitrogen forms showed no obvious differences in promoting growth and Cd absorption and translocation of H. spectabile, while combined application of NH₄HCO₃+chitosan significantly increased the dry biomass of different tissues of H. spectabile by 29.6%–79.8% compared with NH₄HCO₃. Furthermore, the promotion effects of NH₄HCO₃+chitosan treatment on Cd absorption and translocation of H. spectabile varied with soil types, pH, and contamination levels. In brown rendzina, the Cd concentrations in different tissues of H. spectabile, the bioconcentration factors and translocation factors were significantly increased by 16.0%–143.4%, 45.8%–137.5%, and 25.3%–81.6% respectively, compared with NH₄HCO₃, while in water loggogenic paddy soil the promotion effects were not significant.

Conclusions Application of physiological acid nitrogen is beneficial to Cd absorption by H. spectabile under the condition of same nitrogen fertilizer rates, and the combined application of nitrogen fertilizer and chitosan may be an effective measure for enhancing phytoremediation of Cd-contaminated soils using H. spectabile, which is more effective in neutral or alkaline soil than in acid one.