Cupric ion activity in peat soil as a toxicity indicator for maize

Area of Science:

• Environmental Chemistry
• Plant Science
• Soil Science

Background:

• Copper (Cu) phytotoxicity is challenging to assess due to root accumulation and limited shoot transfer.
• Total soil Cu is an unreliable indicator of plant toxicity because soil properties affect Cu speciation.

Purpose of the Study:

• To measure free Cu2+ activity in Cu-enriched peat soils.
• To correlate Cu2+ activity with phytotoxicity indicators in maize (Zea mays L.).
• To evaluate the adequacy of current risk assessment methods for Cu toxicity.

Main Methods:

• Ion-selective electrode used to measure free Cu2+ activity in peat soils.
• Maize bioassay to assess phytotoxicity (leaf chlorosis, root stunting, shoot growth, Fe concentration).

Main Results:

• A soil Cu2+ activity of 10(-7.0) to 10(-7.5) (approx. 275 mg/kg total Cu) caused maize phytotoxicity.
• Free Cu2+ activity showed a stronger correlation with phytotoxicity than acid or chelating agent extractions.
• Maize shoot Cu remained below 35 mg/kg even at extreme toxicity levels (10(-5) Cu2+ activity, 4000 mg/kg total Cu).

Conclusions:

• Free Cu2+ activity is a more direct measure of copper phytotoxicity than total soil copper.
• Current USEPA risk assessments for copper toxicity in crops may be inadequate due to focusing on shoot copper concentrations.