A Comparative Study Of Rac-and S-metolachlor Toxicity To Earthworm

Soils serve as a reservoir for pesticides, hence continuous application ofpesticedes may present risks to induce soil pollution and affect soil fauna. Earthwormsare one of the most important components in decomposer communities and contributesignificantly to the organic matter decomposition, nutrient cycling and soil formation.The earthworm Eisenia fitida is considered as a suitable biomonitor animal todetermine the ecological hazard of heavy metals and pesticides contaminated soil. Theeffects of pesticides on the earthworm have been studied in China and abroad, butenantioselective toxicity of chiral pesticides to the earthworm is little known.Racemic product of metolachlor contains two R-enantiomers and twoS-enantiomers. S-metolachlor is now widely used instead of rac-metolachlor becausethe former is more effective in herbicidal activity than the latter. A comparativetoxicity assessment of rac-and S-metolachlor was discussed through their effects onthe survival rate, avoidance behavior, weight change and some enzyme activities ofthe earthworm Eiseniafitida. The results showed that there were no huge differencesbetween the LC₅₀ of rac-metolachlor and that of S-enantiomers, both of which werelow toxicant based on the classification standard of toxicity.In the avoidance response test, the responses of the earthworms treated withrac-metolachlor were stronger than those of S-metolachlor treatment. In the weightchange test, the no-observed-adverse-effect levels (NOELs) of the earthworm exposedto rac-metolachlor was much lower than that of S-metolachlor exposure. Mostly,when treated with a certern concentration, the weight increment of earthworms treatedwith rac-metolachlor was larger than that of S-metolachlor treatment, this wassignificant after 2 days exposure to 64 mg·kg^-1. Both results above implied thatrac-metolachlor had induced more toxicity on earthworm than S-motolachlor did.The results of the chronic toxicity test indicated that the rac-metolachlor andS-metolachlor had different effects on enzyme activities of earthworm. In the artificialsoils, both rac-metolachlor and S-metolachlor had negative effect on the cellulaseactivity and catalase (CAT) activity, but the persistent time of negative effect of S-metolachlor was shorter than that of rac-metolachlor. The POD activity was activedby both pesticides at the end of the test and no significant differences exsited betweenthem. In the natural soils, the activities of cellulase and catalase (CAT) were inhibitedby both herbicides, while S-metolachlor was more toxic than rac-metolachlor toearthworm. The POD activity was actived after 2 and 7d exposure, while inhibitedafter 14d exposure by the two herbicides, and the negative effects disappeared after28d exposure. It suggested that the two herbicides had no significant differences to thePOD activity.Compared the parameters measured in the artificial soil with those in the naturalsoil, the change of measured parameters except POD activity indicated that thetoxicity of herbicides to the earthworm in the artificial soil was much higher than thatin the natural soil.