No abstract

Though soil-plant transfer is the first step by which radiocesium enters the food chain, it has been scarcely studied in the rhizosphere. Forty-seven soil horizons from 17 pedons with widely varying properties were contaminated with carrier-free 137 Cs + and placed into close contact with an active macroscopic rhizosphere of ryegrass for 4 days. The 137 Cs rhizospheric mobilization was strongly correlated with the sodium tetraphenylboron-extractable 137 Cs (r ) 0.94), supporting that K depletion in the rhizosphere is a capital driving force in 137 Cs uptake. The 137 Cs soilplant transfer factor varied from 0.02 to 3.69 g g -1 between soil materials and was strongly negatively correlated to the radiocesium interception potential (RIP) (r ) -0.88), a common Cs binding characteristic in soil. RIP largely differed between soil materials (13-4861 µmol g -1 ) and was directly related with the soil vermiculite content (r ) 0.70). Our results, validated in a wide variety of soils, show that both vermiculitic minerals and plant roots act as competitive sinks for 137 Cs + in the rhizosphere. They further support that many 137 Cs-polluted soils in semi-natural environments can act as a potential source for long-term contamination of the above standing vegetation because they have low K availability.

No abstract

Water-soluble and acid-extractable Cu and Zn, water-soluble organic carbon (WSOC), pH, differential pulse anodic stripping voltammetry-labile Zn (ZnL), 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.