Labile zinc concentration and free copper ion activity in the rhizosphere of forest soils

Water-soluble and acid-extractable Cu and Zn, water-soluble organic carbon (WSOC), pH, differential pulse anodic stripping voltammetry-labile Zn (Zn-L), Zn2+ activity (Windemere humic aqueous model [WHAM]; http://chess.ensmp.fr/ chemsites.html), and Cu2+ activity with an ion-selective electrode were compared between the rhizosphere and the bulk components of nine acidic forest sites from southeastern Canada. At all sites, the WSOC contents were higher in the rhizosphere than in the bulk component. Acidity was also higher in the rhizosphere, although pH differences were significant at only five sites. The concentrations of Zn in water extracts and ZnL contents (at six sites) were higher in the rhizosphere, whereas acid-extractable Zn was only marginally increased in the rhizosphere. Calculations with WHAM indicated that free Zn2+ ion activities were higher in the rhizosphere than in the bulk soil but that the fraction of total dissolved Zn in water extracts that is present as free Zn2+ did not differ significantly between the two components. The concentration of Cu in the water extract was higher in the rhizosphere for all sites, but acid-extractable Cu levels did not differ. The fraction of water-soluble Cu present as Cu2+ was higher in the bulk soil, although Cu2+ activities did not significantly vary with proximity to roots. These results showed that the processes acting in the rhizosphere of forest soils strongly affected the concentrations of dissolved Zn and Cu and that this microenvironment should be considered when estimating the bioavailability and the ecological risks of metals in soils.