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TECHNICAL REPORT
HEAVY METALS IN THE ENVIRONMENT

Cupric Ion Activity in Peat Soil as a Toxicity Indicator for Maize

Department of Crop and Soil Sciences, Cornell Univ., Ithaca NY 14853

Corresponding author (mbm7{at}cornell.edu)

Received for publication April 17, 2000. Copper phytotoxicity in soils is difficult to assess because Cu accumulates at and damages roots, and is not readily transferred to shoots. Soil chemical properties strongly influence Cu speciation, so that total soil Cu alone is not a broadly useful indicator of potential toxicity to plants. The present study measured free Cu²⁺ activity in Cu-enriched peat soils using the ion selective electrode. The soil Cu²⁺ activity was related to the severity of phytotoxicity as measured by several indicators in a maize (Zea mays L.) bioassay, including leaf chlorosis, root stunting, and reduced shoot growth and Fe concentration. A soil Cu²⁺ activity of 10^-7.0 to 10^-7.5, corresponding to total Cu of about 275 mg/kg in the peat soil, caused phytotoxicity in maize seedlings. It is proposed that Cu²⁺ activity is more directly related to phytotoxic effects than other soil tests, such as extractions with strong acids or chelating agents, because it is the free Cu²⁺ in soil solution that has the most direct toxic effects on roots. There was very limited uptake of Cu into maize shoots, and even when Cu²⁺ activity and total soil Cu were raised into the extreme toxicity range of 10^-5 and 4000 mg/kg, respectively, shoot Cu remained less than 35 mg/kg. These results indicate the inadequacy of the USEPA risk assessment of potential for Cu toxicity to crops amended with sewage sludge, which assumed a no-effect level of maize shoot Cu of 40 mg/kg.

Abbreviations: NOAEL, no observable adverse effect level • ISE, ion-selective electrode • K_d, partition or distribution coefficient, the ratio of total to dissolved metal • pCu, negative logarithm of the free Cu²⁺ ion molar concentration in soil solution

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