Distribution and speciation of Cu and Zn near spring barley (Hordeum vulgare) roots in digested sewage sludge-amended soil

Abstract

Risk management for agricultural use of digested sewage sludge requires better understanding of the behaviour and fate of contaminant metals in the plant root zone. A study employing rhizo-pot and plug-tray experiments was conducted to identify the zone near spring barley roots ( Hordeum vulgare ) where concentration and speciation of Cu and Zn are affected. Cu and Zn bonding environments in the root epidermis/cortex and vascular tissue were also identified. In the digested sludge-amended soil, spring barley absorbed Cu only from the immediate vicinity of the roots (<< 1 mm), but Zn was taken up from further afield (> 1 mm). In the rhizosphere Cu was predominately present as Cu(I) oxides or as Cu(II) absorbed/bonded to phosphate, whereas Zn was present as Zn(II) in inner-sphere complexes with metal oxide surfaces, as Zn(II) sulphides or Zn(II) bonded to/incorporated into carbonates. Cu taken-up by spring barley roots was largely sequestered in the root epidermis and/or cortex predominately in the coordination environments similar to those seen in the rhizosphere. Only a small proportion of the Cu was translocated into the vascular tissue (where it is in the same two bonding environments). Zn taken-up by spring barley roots was present as Zn(II) sulphides, Zn(II) absorbed to/incorporated into carbonates, or Zn(II)-organic complexes. Zn was readily translocated from roots to shoots. Better understanding of these differences in the mobility and uptake of Cu and Zn in sludge-amended agricultural soils could be used to undertake element specific risk assessments.

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