The Effects of Copper on the Degradation of Atrazine and Indoxacarb in a New Zealand Soil (2010)
AuthorsDewey, Katrina Anneshow all
Pesticides are an important component of New Zealand’s primary production sectors. Infestation of pests and diseases can affect crop yield, crop value and damage the country’s export reputation, resulting in economic losses. Repeat applications of pesticides, however, can result in contamination of land and water. Therefore, it is important to understand the fate of pesticides in the environment. Factors which can affect pesticide persistence include soil properties (pH, SOM, CEC), leaching and run-off, volatilisation and co-contamination with heavy metals. Many soils in New Zealand contain high levels of copper from historical applications of copper-based pesticides. Co-contamination of soils may lead to the persistence of some synthetic organic pesticides. An investigation was undertaken to determine the effects of co-contamination with copper on the biodegradation of atrazine and indoxacarb in a New Zealand soil. A Templeton sandy loam soil was spiked with CuSO₄ to achieve concentrations of 0, 100, 250, 500 and 1000 mg kg⁻¹ Cu. The spiked soils were field aged for six months prior to pesticide spiking with either atrazine or indoxacarb. The aged Cu-spiked soils were spiked with either atrazine or indoxacarb at a rate of 2 mg kg⁻¹. A glasshouse study was conducted to determine if copper inhibited the degradation of the pesticides. The pesticide-spiked soils were sampled at the time of spiking (t₀), at the estimated half-lives (t₁) and at twice the estimated half-lives (t₂) of the individual pesticides. The estimated half-lives were based on literature values. The bioavailability and subsequent adverse effects of copper on the soil microbial community was investigated. Total and bioavailable copper concentrations, phosphatase and urease enzyme activities, microbial biomass, and pesticide residue concentrations were all measured in the experimental soil. Methods were developed for the extraction of atrazine, atrazine metabolite and indoxacarb residues from the experimental soil. Total copper concentrations extracted ranged from 4–1060 mg kg⁻¹ in the experimental soils and were consistent throughout the pesticide degradation studies. The bioavailability of copper was a maximum of 2% of the total copper concentration. Bioavailable copper concentrations were positively correlated to total copper (p<0.01). Soil biological properties were investigated to determine the effects of copper on the soil microbial community. Phosphatase and urease enzyme activities, as well as microbial biomass concentrations, were negatively correlated with total copper (p<0.05). Total copper was a better indicator of effects on microorganisms than bioavailable copper. The soil biological properties began showing adverse effects above a total copper concentration of 100 mg kg⁻¹. This concentration also corresponds to New Zealand’s copper limit in biosolids, which is protective of human, plant and microorganism health. Phosphate buffer extraction methods were developed for the analysis of atrazine and indoxacarb residues in the experimental soil by HPLC-UV. Elevated copper concentrations did not inhibit the degradation of atrazine or indoxacarb in the experimental soil. The half-lives of both atrazine (≤19.4 d) and indoxacarb (≤18.8 d) were lower in the spiked experimental soils than the means reported in previous New Zealand and international studies, but were within the reported ranges. This study provided the first data on the fate of indoxacarb in New Zealand. Hydroxyatrazine was the only metabolite detected in the atrazine-spiked experimental soils. Significant differences between the control (Cu-1) and copper levels above 100 mg kg⁻¹ were observed for hydroxyatrazine at t₂. Significant negative correlations were observed between hydroxyatrazine and the microbiomass at t₁ and phosphatase activity at t₂ (p<0.05). These significant relationships suggest that elevated copper concentrations may alter the degradation of this metabolite in the experimental soils due copper toxicity of the soil microbial community. The results of this thesis indicate that elevated levels of copper above 100 mg kg⁻¹ negatively impact the soil microbial community and may reduce the overall health of the soil. Biodegradation is a key mechanism for the degradation of atrazine and indoxacarb in the soil, so it is important that the health of the soil microbial community is maintained. Therefore, it is recommended that atrazine and indoxacarb are only applied to soils with a total copper concentration less than 100 mg kg⁻¹. This will protect the health of the soil microbial community and prevent the potential adverse effects of copper on the degradation of pesticide metabolites in the soil.