Abstract: As a persistent organic pollutant (POP), benzo(a)pyrene (B[a]P) is gradually accumulating in the soil, posing a serious threat to the soil environment quality. The routine approach to environmental risk assessment of B[a]P is only based on one-time pollution, i.e. mixing the soil with B[a]P only once, which deviates from the step-by-step procedure of B[a]P entering the soil. In this paper, a toxic bioassay was conducted to investigate toxic effects of B[a]P on superoxide dismutase (SOD) and peroxidase (POD) activities and malondialdehyde (MDA) levels in coelomocytes of earthworm (Eisenia fetida), using the multiple-time superimposed and one-time pollution methods. Results show that with the exposure to B[a]P going on, SOD and POD activities and MDA content in coelomocytes of earthworm dropped rapidly in the first 14 days (1-14 d), and then declined slowly in the 42 days to follow (14-56 d) under both the multiple-time superimposed and one-time pollution methods. In the first 14 days, the SOD activity and MDA content in earthworms found in the 0-20 cm soil layer under superimposed pollution was 20.97% and 15.96% lower, respectively, while POD activity was 20.44% higher than that under one-time pollution. However, the SOD and POD activities, as well as the MDA content in earthworms found in the >20-40 cm soil layer under superimposed pollution was 52.89%, 18.00% and 70.60% lower than that under one-time pollution, respectively. All the above findings suggest that toxicity of multiple-time superimposed B[a]P pollution to earthworms was lower than that of one-time pollution. The study provides crucial data for evaluating ecological toxicity of cumulative B[a]P pollution in the soil.

Key words: B[a]P, superimposed pollution, Eisenia foetida, coelomocyte, SOD, POD, MDA