Effects of Temperature and Moisture on the Ecotoxicity of Metal-Based Fungicides Towards Earthworms

Abstract

Climate change is a major threat to biotic and abiotic ecosystems worldwide and might potentially affect the mobility and bioavailability of metals in soil. Anthropogenic activities such as mining, agriculture and the use of pesticides and fungicides all contribute to soil pollution. These activities coupled with climate change could have a greater effect on soil organisms. The aim of this study was to determine if varying soil moisture contents and different temperatures will influence the toxicity of single and binary mixtures of the fungicides, copper oxychloride and mancozeb, (in different concentrations) towards earthworms (Eisenia andrei) as bioindicators of soil quality. Tests were done using artificial soil in accordance with the OECD guidelines under two different temperatures viz. 20°C and 25°C and two different moisture contents viz. 30% and 50%. The concentrations for copper oxychloride (CuOx) were 200, 500 and 1000 mg/kg and mancozeb (MnZn) 44, 850 and 1250 mg/kg in single and binary mixtures. Endpoints measured included earthworm growth, reproduction, avoidance-behaviour, body metal analyses, comet assays (DNA analysis) and soil enzymatic analyses. The most significant results in mortality manifested in the 25°C 30% exposure group which had a 73% and 70% mortality rate in the CuOx1000 and CuOx1000 + MnZn1250 mg/kg treatments respectively. In terms of growth and reproduction a decrease in both biomass and reproductive viability was observed with increasing concentrations. In the copper oxychloride treatment, there was a significant difference (p<0.05) between 20°C 30% and 20°C 50%, and between 25°C 30% and 25°C 50%, which implies that soil moisture had a significant effect. Mancozeb treatments had significant differences between the 25°C 30% exposure and the 20°C 30%, 20°C 50%, 25°C 50% exposures. Binary treatments showed a significant difference only at the CuOx1000 + MnZn1250 mg/kg (20°C 50%) exposure when compared to the other temperatures and moistures 20°C 30%; 25°C 30%; 25°C 50%. The avoidance test indicated that earthworms avoided soils spiked with (CuOx500, CuOx1000, MnZn850, MnZn1250, CuOx500 + MnZn850, CuOx1000 + MnZn1250 mg/kg) under all the different temperatures and soil moistures (20°C 30; 20°C 50%; 25°C 30%; 25°C 50%). DNA damage increased as the concentrations of the treatments increased. Significant differences (p<0.05) were found between the different temperatures and soil moistures although the different experiments produced different results. Body metal analysis test corresponded with the comet assay test viz. an increase in body metal concentration as the concentrations of the treatments increased. Significant differences (p<0.05) were found in the copper and manganese concentrations although no significant differences were found in the zinc concentrations. Soil enzymatic analysis indicated significant differences in the alkaline phosphatase and glucosidase tests for the single and binary (copper oxychloride and mancozeb) treatments and for the 25°50% exposure it was the most significant. It is evident that fluctuating temperatures and soil moistures had varying effects on the single and binary mixtures of copper oxychloride and mancozeb.

It was concluded that varying temperatures and soil moistures would have a significantly different result on the effects of metal-based fungicides towards earthworms based on the title of this study. Increasing the temperature and lowering the soil moisture had the most detrimental ecotoxicity effects on earthworms exposed to metal-based fungicides.