Can Anions Applied with Metals to Soil Lead to Overestimation of EC50s?

Abstract

When assessing cationic metal toxicity in soils metals are often added to soil as the chloride, nitrate or sulphate salt. In many studies the effects of these anions are ignored or discounted; rarely are appropriate controls included. This may be appropriate at low metal concentrations. However, if assessing toxicity in soils with high metal buffer capacities or for less toxic metal cations, salinity effects from applied anions may contribute significantly to the measured toxicity. Our study used five soils varying in pH, clay content and organic matter. Ten rates of Pb or Zn nitrate were added to 360 g subsamples of soil. After drying, the samples were split and one half was leached until the leachate electrical conductivity was less than 1.0 dS m-1 (leached) and the other half wetted to water holding capacity (non-leached). Four Lactuca sativa Iceberg c.v. Marksman (Lettuce) plants were grown in each pot for 31 days, using a complete nutrient solution to ensure healthy growth. Plant dry weights were used to assess phytotoxic responses by comparison of the EC50 values between leached and non-leached treatments. Leached and non-leached control plants dry weights were not significantly different. In four of the five soils there was no effect of leaching on EC50 values, but in one soil leaching increased the EC50 for Zn and Pb by 2 fold and 3 fold, respectively. Removal of metal by leaching did not explain the effect of leaching on EC50 values. There was no obvious link between soil characteristics (e.g. soil pH, electrical conductivity, carbon or clay content) and the changes in EC50 values and further analysis is being conducted. However, these data highlight the possibility of overestimating EC50 values or other toxicity measures for particular test species due to the toxic contribution of anions applied with metals to soils. Funded by ILZRO.