Water-soluble carboxymethyl chitosan and rhamnolipids promote the remediation of Cd-contaminated soil by mediating the growth of Hylotelephium spectabile and regulating the rhizospheric ecological environment

The application of biodegradable chelating agents in phytoremediation is a promising approach. This study aimed to investigate the effects and roles of underlying mechanisms of water-soluble carboxymethyl chitosan (WSCC) and rhamnolipids (RLs) on the remediation of Cd-contaminated soil by Hylotelephium spectabile. WSCC and RLs mediated the growth of H. spectabile by increasing chlorophyll content and the activity of antioxidant enzymes as well as promoted the conversion of water-extractable Cd to HAc-extractable Cd in leaves. WSCC and RLs promoted the secretion of malic acid, acetic acid, and succinic acid by the roots; decreased soil pH; increased the number of functional groups, such as hydroxyl, amino, and carboxyl groups, in the soil; and changed the diversity and structure of bacterial communities in the soil, thereby improving the bioavailability of Cd in the soil and creating a good ecological environment of the rhizosphere. The combined application of WSCC and RLs had a better auxiliary effect than single application of either, especially under CR2 treatment (1.5 g·kg^-1 WSCC + 0.2 g·kg^-1 RLs), where the accumulation of Cd in plants significantly increased by 159.86 % compared with the control. These findings indicated that WSCC and RLs enhanced the remediation efficiency of H. spectabile by regulating both plant growth and the ecological environment of the rhizosphere.