The effects of copper on microbial activity and the degradation of atrazine and indoxacarb in a New Zealand soil

Copper is present in a range of fungicides as well as in some animal manures and biosolids that are applied to agricultural soils as fertilisers. Elevated and increasing levels of copper in agricultural soils are of worldwide concern. Copper is toxic to soil microorganisms and has been reported to reduce the ability of soil microorganisms to degrade pesticides. A glasshouse study was undertaken to determine if copper inhibited the degradation of atrazine and indoxacarb in soil. A fine sandy loam agricultural soil was fortified with copper at five concentrations over a concentration range of 0–1000 mg/kg copper, then field-aged for 6 months prior to treatment with either indoxacarb or atrazine at a rate of 2 mg/kg. The soils were sampled twice at intervals based on published half-lives. The samples were analysed for a range of parameters including total and bioavailable copper, urease and phosphatase activity, ergosterol and either indoxacarb or atrazine and its degradation products. The soil microbial biomass and enzyme activities decreased with increasing copper concentration (p < 0.05). There were no significant differences in soil atrazine and indoxacarb concentrations between the copper levels. At sampling time two, the concentrations of hydroxyatrazine in treatments containing the three highest copper concentrations were significantly greater (p < 0.05) than for the control soil. Our results indicate that copper does not inhibit the first step of indoxacarb and atrazine degradation, but may affect degradation of secondary metabolites like hydroxyatrazine in soil.