Assessing the effects of biochar as an ameliorant for biopesticide polluted soils utilising earthworms
Abstract
Two plant biopesticides, Bidens pilosa L. and Tagetes minuta L., were investigated for possible effects on earthworms and soil enzymatic activity, compared to the conventional pesticide, Fenamiphos. The efficacy of biochar as an ameliorant was also investigated to determine if the possible effects of plant biopesticides could be mitigated.
The growth and reproduction of earthworms were monitored at sublethal endpoints and the genotoxicity of the plant biopesticide extracts was evaluated using the comet assay. The influence of the test substance in soil on earthworm behaviour was evaluated with an avoidance behaviour assay. The relative growth rate (%RGR) was determined by the change in biomass of the earthworms at 28 days of the experiment. Fecundity success was measured by counting the number of cocoons and the number of juveniles on day 56. The DNA damage in earthworm coelomocytes was evaluated using single-cell gel electrophoresis, by scoring the tail length of the comet formed because of the deoxyribonucleic acid (DNA) strand breaks in the cell. The soil enzyme activities were evaluated by utilising the alkaline phosphatase, ꞵ-glucosidase and urease assay, and analysed using the spectrophotometer method. Artificial soil used for all treatments was prepared according to the OECD standard guidelines.
The soil treated with Fenamiphos had a greater negative effect on earthworm relative growth rate (%RGR) and reproduction than both plant biopesticides (B. pilosa and T. minuta). Biochar amended treatments, had the greatest effect on earthworm %RGR. However, there was no significant difference in earthworm reproduction in the biochar supplemented treatments compared to treatments without biochar.
Mixed results were obtained in the DNA damage of the earthworms caused by the plant biopesticides (B. pilosa and T. minuta). Overall, B. pilosa exposures induced more DNA strand breaks than T. minuta exposures, except for the 1000 mg/kg T. minuta concentration, which surprisingly has very high DNA strand breaks, with an average tail length of 49 μm. Fenamiphos induced more DNA strand breaks at much lower concentrations (12.5 – 100 mg/kg) than the plant biopesticides. All the treatments supplemented with biochar showed no DNA damage and had no significant difference (p<0.05) from the control, suggesting biochar prevented DNA damage.
There were no statistical differences among all treatments (B. pilosa, T. minuta, and Fenamiphos), including the control of the alkaline phosphatase and urease activities in the soil. There is an increase in p-nitrophenol released in Fenamiphos-treated soil (25 and 100 mg/kg) and, surprisingly, T. minuta (250 mg/kg) indicating an increase in ꞵ-glucosidase activity. All treatments (B. pilosa, T. minuta, and Fenamiphos) supplemented with biochar have no significantly different soil enzymatic activity (alkaline phosphatase, ꞵ-glucosidase, and urease activity) compared to the control. In conclusion, Fenamiphos reported more distinct negative effects on soil organisms than the plant biopesticides used in this study, and biochar does not act as an efficient ameliorant for the plant biopesticides (B. pilosa and T. minuta), and the chemical pesticide (Fenamiphos).