| Quinalphos as an organic phosphorus pesticide is widely used in cotton, rice, fruit, vegetables and other crops to control a variety of insect pests, mainly for the prevention of gall midge, cnaphalocrocis medinalis, helicoverpa armigera, citrus leafminer, small green leafhopper and pieris rapae. However, the relevant research reports about quinalphos degradation characteristics mainly focus on hydrolysis and photolysis, and its environment behavior is rarely reported. This work aims to elucidate degradation dynamics of quinalphos in cotton field and associated degradation factors. Main contents include:(1) degradation dynamics of quinalphos in cotton field; (2) degradation factors of quinalphos in representative provinces (Zhejiang, Hubei, Henan) of China in cotton, including light factor, water factor, microbial factor, soil organic matter; (3) the formation mechanism of 2-hydroxyquinoxaline through quinalphos; (4) screening and identification for degrading bacteria of quinalphos; (5) degradation pathway in different degradation factors of quinalphos. The main results are as follows:First, the half-life of quinalphos in cotton leaf was 4.2 days, the half-life of quinalphos in soil was 4.5 days. The residues of quinalphos on cotton leaf after application was 0.040 mg·kg-1, and the residues of quinalphos in soil after application was 0.132 mg·kg-1. The degradation rate of quinalphos in cotton field environment was relatively high, and was thus classified as an easily-degradable compound.In addition, we found that the half-lives of quinalphos in buffer solution with pH=4, pH=6, pH=7, pH=8, pH=9 were 69.3 days,72.2 days,74.5 days,56.4 days,38.1 days, respectively. The photolysis half-life of quinalphos in cotton leaf in light group was 9.4 days and the photolysis half-life in control group was 14.8 days. The photolysis half-lives of quinalphos on the surface of cotton field soils from three provinces in light group were 6.4 days,5.8 days and 3.8 days, respectively, and the photolysis half-lives in the control group were 26.9 days,32.2 days and 30.0 days, respectively. The half-lives of quinalphos in soil solution from three provinces were 28.9 days,25.1 days and 22.3 days, respectively. The half-lives of quinalphos in sterilized soils from three provinces were 18.3 days,26.4 days and 35.9 days, respectively. The half-lives of quinalphos in non-sterile soil from three provinces were 7.8 days,12.1 days and 21.2 days, respectively. Thus we infered that major degradation factor of quinalphos was microbial factor.The monitoring of quinalphos metabolites 2-hydroxyquinoxaline in cotton leaf and soil found that the initial concentration of 2-hydroxyquinoxaline were 4.704 mg·kg-1 and 6.356 mg·kg-1, respectively. The highest concentration of quinalphos in cotton leaf was at 0 days, and the concentration was 4.704 mg·kg-1, the highest concentration of quinalphos in soil was at 1 days, and the concentration was 9.165 mg·kg-1. In the research of water factor, light factor, microbial factor and organic matter content factor, the concentration of 2-hydroxyquinoxaline distributed between 0-0.197 mg·kg-1, and 2-hydroxyquinoxaline was not obviously accumulated.In addition, seven representative bacterial isolates for degradation of quinalphos were screened from Zhejiang, Hubei and Henan provinces and identified according to 16S rRNA genes. Four isolates were Bacillus sp., one was Pseudomonas sp., one belonged to Citrobacter sp., and the last one remained to be identified, which suggested that the main degradation strain for quinalphos in the cotton field was Bacillus genus.Finally, through GC-MS/MS scan, the degradation ways of quinalphos in different degradation factors were deduced. The main degradation product in water factor was 2-hydroxyquinoxaline, while the main degradation products attributed to light factor were 2-hydroxyquinoxaline and C7H9N2O2PS. The main degradation products attributed to microbial factor were 2-hydroxyquinoxaline, C4H11O3PS and C8H6N2O2S. |
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