Transformation of metal fractions in the rhizosphere of elsholtzia splendens in mining and smelter-contaminated soils: Contribution of fulvic-metal complex

Abstract

Rhizosphere is generally enriched with organic matter which may affect the speciation of heavy metals by the formation of organ-metallic complex. This study investigated the transformation of Cu, Zn, Fe, and Mn fractions, notably organically bound metal fractions, in the rhizosphere of E. splendens using a nine-step SSE method. Compared to the non-rhizosphere, Cu/Zn-fulvic complex significantly increased in the rhizosphere of E. splendens both in the smelter-contaminated soil (SS) and the mining soil (MS), with an enhanced amount of 110.5 and 30.8 mg kg-1 for Cu and 184.5 and 911.7 mg kg-1 for Zn, respectively. Furthermore, organic Cu/Zn and adsorbed Mn significantly enhanced in the rhizosphere as compared to the non-rhizosphere of E. splendens in the MS, while amorphous Cu/Zn, reducible Cu/Zn, Fe-fulvic complex, and organic Mn significantly changed between the rhizosphere and non-rhizosphere of E. splendens in the SS. Moreover, significant accumulation of exchangeable Cu also occurred in the rhizosphere of E. splendens in both the SS and MS. These results demonstrated that the cultivation of E. splendens enhanced the phytoavailability of Cu and Zn in soils by the formation of metal-fulvic complex in the rhizosphere. In addition, the transformation of the fractions of Cu, Zn, Fe, and Mn in the rhizosphere of E. splendens also depended greatly on the soil properties.