Characteristics, Soil Ecological Effects, And Their Minimization Of Chlorpyrifos Residues In Greenhouse Soil And Vegetables

Greenhouse production of vegetables has been developed rapidly in China during the past decades. Compared to field conditions, greenhouse provides more favorable climate for fast reproduction of pests and diseases that result in extensive and frequent applications of pesticides. Chlorpyrifos is a broad spectrum organophosphorus insecticide and acaricide widely used for insect pest control on greenhouse vegetables. For the evaluation on the safety of chlorpyrifos application in the greenhouse, studies were conducted on dissipation of chlorpyrifos on vegetable and in soil in the greenhouse and open field, the influences of chlorpyrifos alone and in combination with other commonly used pesticides on soil microbial diversity. One fungal strain capble of degrading chlorpyrifos was isolated from soil. The fungal degradation of chlorpyrifos and its mechanism in pure cultures, and bioaugmentation of chlorpyrifos degradation on vegetable and in soil by the fungal strain preparation and its cell-free extracts were also investigated. The results were summarized as follows:Dissipation of chlorpyrifos on pakchoi and in soil was fitted to the first-order and bi-exponential models, respectively. The hermetic environment of the greenhouse and changes of seasons alter dissipation behavior of chlorpyrifos on pakchoi and in soil. The dissipation rates of chlorpyrifos on pakchoi and in soil in the greenhouse were lower than those in the open field, and the rates in the autumn were lower than those in the summer. Compared to the open field, the half-lives of degradation (DT₅₀) for chlorpyrifos at the recommended and double dosages on greenhouse pakchoi were extended by 26.0% and 10.7% in the summer, and 6.6% and 15.7% in the autumn, the corresponding DT₅₀ on greenhouse soil were extended 44.4% and 140.0% in the summer, and 16.2% and 63.1% in the autumn, respectively. Chlorpyrifos residues at pre-harvest time in the greenhouse were higher than those in the open field by almost 50%.Degradation of chlorpyrifos at levels of 4.0, 8.0, and 12.0 mg kg^-1 in soil were all fitted to the first-order function, its DT₅₀ were measured to be 14.32 d, 16.70 d, and 18.00 d, respectively. The DT₅₀ of chlorpyrifos in soil was extended with the concentration of chlorpyrifos. In all treatments, soil microbial diversity was inhibited significantly at 7 d after chlorpyrifos treatment, and the inhibitory effect was increased with the concentration of chlorpyrifos, but disappeared gradually with the time, and soil microbial diversity in all treatments was recovered to the level of the control at 21 d after treatment.Degradation of chlorpyrifos, triadimefon, and butachlor alone and in combination after repeated treatments in soil were all fitted to the first-order function. The DT₅₀ of pesticides in all treatments were shortened gradually with the increasing of application times, except that the DT₅₀ of chlorpyrifos was slightly extended in the combined treatment of chlorpyrifos and triadimefon. In all treatments, soil microbial diversity was inhibited significantly at 3 d after pesticides application, but the inhibitory effect disappeared gradually with the time, and soil microbial diversity recovered to or exceeded the level of the control at 21 d. The inhibitory effect of pesticides on soil microbial diversity disappeared gradually with the increasing of application times.A fungal strain DSP capable of utilizing chlorpyrifos as sole carbon and energy sources was isolated. Based on its morphological characteristics and 18S rDNA sequence analysis, the isolated strain was identified as Verticillium sp.The ability of the fungal strain DSP to degrade chlorpyrifos in pure cultures depends on pesticide concentration, pH, and temperature. Degradation of chlorpyrifos by the fungal strain DSP was fitted to the first-order function. The DT50 of chlorpyrifos were measured to be 2.03 d, 2.93 d, and 3.49 d at concentrations of 1, 10, and 100 mg l^-1, 2.03 d, 1.93 d, and 2.11 d at pH 5.0, 7.0, and 9.0, and 3.31 d, 2.03 d, and 1.88 d at 15, 25, and 35°C, respectively. Fungal degradation was inhibited slightly by chlorpyrifos at high concentration of 100 mg l^-1. The degradation rates of chlorpyrifos by the fungal strain DSP were effected by pH and temperature following an order of pH 7.0>pH 5.0>pH 9.0, and of 35°C >25°C> 15°C, respectively. TCP was identified as a main degradation product of chlorpyrifos by GC-MS analysis, and it was likely that TCP was further transferred to be TMP or dechlorination. According to the capability of the fungal strain DSP to utilize chlorpyrifos as sole carbon and energy sources, it is reasonable to propose that chlorpyrifos is mineralized by the fungal strain DSP.Degradation of chlorpyrifos in soil and on vegetable was significantly accelerated by the addition of the fungal strain DSP. Degradation of chlorpyrifos in sterilized soil, previously chlorpyrifos-untreated soil, and previously chlorpyrifos-treated soil were all fitted to the first-order function. Compared to the un-inoculated controls, the DT₅₀ of chlorpyrifos in three inoculated soils were shorted by 81.2%, 59.1%, and 24.4%, respectively. In contrast to the controls, the DT₅₀ of chlorpyrifos in the greenhouse and open field were shortened by 10.9% and 17.6% on pakchoi treated with the strain DSP preparation, and 12.0% and 37.1% in inoculated soils, respectively.Degradation of chlorpyrifos on pakchoi, water spinach, Malabar spinach, haricot beans, and pepper was enhanced significantly by cell-free extracts. Degradation of chlorpyrifos on five vegetables were all fitted to the first-order function, and the degradation rates of chlorpyrifos on E (1:10) treated vegetables were higher than those on E (1:20) treated vegetables. Compared with the controls, the DT50 of chlorpyrifos were shorten by 29.7%, 34.4%, 19.4%, 19.4%, and 13.9%, and by 46.2%, 56.8%, 34.0%, 45.7%, and 32.3% on E (1:20) and E (1:10) treated pakchoi, water spinach, Malabar spinach, haricot beans, and pepper, respectively. The results indicate cell-free extracts is a promising method for the detoxification of pesticide residues on vegetables.