Influence of soil properties and aging on arsenic phytotoxicity

Bioavailability of As varies among soils, and this needs to be taken into account during environmental risk assessment. Using a standardized barley root elongation assay, we investigated the phytotoxicity of arsenate added to 16 European soils that varied widely in their physicochemical properties. The effective concentrations of As causing 10% (EC10) and 50% (EC50) inhibition were estimated based on the concentrations of total added As or As extracted with 0.05 M (NH4)2SO4 or 0.05 M NH4H2PO4. In addition, four soils were used to evaluate changes in arsenate phytotoxicity over a three-month period. The EC10s and EC50s of added As varied from 4.2 to 206.7 mg/kg and from 26.6 to 458.2 mg/kg, respectively. Multiple-regression analysis showed that the variability in the EC10 and EC50 was largely (>89%) explained by the contents of amorphous Mn oxide and clay and, to a lesser extent, Fe oxide, indicating that arsenate adsorption was a key factor controlling its bioavailability. Neither (NH4)2SO4 nor NH4H2PO4 extraction could explain arsenate phytotoxicity independently of soil properties. Furthermore, arsenate phytotoxicity decreased significantly after aging for three months, although the extent of aging differed among soils. This aging effect should be taken into account during the risk assessment.