Residual Characteristics And Ecological Effects Of Chlorothalonil After Its Repeated Applications In Soil

The fungicide, chlorothalonil (2,4,5,6-tetrachlorobenzene-1,3-dicarbonitrile), is a non-systemic and broad-spectrum benzene fungicide widely used for the control of plant diseases on vegetables, fruits and cash crops. Repeated chlorothalonil applications on vegetables under greenhouse conditions may result in potential pollution of soil. To evaluate the safety and ecological risk of repeated chlorothalonil applications, degradation and accumulation of chlorothalonil in soils and the effects on soil respiration activity, soil enzyme activities and diversity of soil microbial community were investigated under laboratory and greenhouse conditions. The results were summarized as follows:1. Dissipation pattern of chlorothalonil after four successive chlorothalonil applications at levels of5mg kg-1and10mg kg-1, corresponding to the recommendation dose and double the recommendation dose, respectively, was fitted to the first-order model in soil under laboratory condition. The half-lives (DT50) at a concentration of5mg kg-1after the first, second, third, and fourth applications were2.99,3.03,3.10, and2.99d, respectively. The DT50at a level of10mg kg1after four successive introductions to soil were6.14,7.75,7.00, and6.05d, respectively. Approximately42.2%,22.0%,13.1%, and9.1%dissipated within15days after the first, second, third, and fourth treatments with25mg kg-1of chlorothalonil, respectively and a significant accumulation of chlorothalonil in soil was observed with its repeated applications at25mg kg-1. The results obtained in this study indicated that multiple successive applications of chlorothalonil to soil exhibited a concentration dependent impact on the persistence of chlorothalonil.2. Microbial functional diversity in soil after repeated chlorothalonil applications were estimated with BIOLOG Ecoplates. An obvious inhibitory effect was observed in microbial utilization of carbon substrates after four successive chlorothalonil applications at levels of10and25mg kg-1and exhibited a dosage-dependent process. A Significant decrease of Shannon, McIntosh, and Simpson indexes was found after repeated chlorothalonil applications at both the recommendation dose and five times the recommendation dose. A recovery trend in Shannon, McIntosh, and Simpson indexes was found as Soil incubation time increased.3. Soil enzyme activities were investigated in soil after repeated chlorothalonil applications under laboratory condition. No significant change on soil catalase and invertase activities at levels of5,10, and25mg kg-1was observed after four repeated chlorothalonil applications. Soil dehydrogenase and neutral phosphatase activities were inhibited significantly on day15after four successive treatment with chlorothalonil at concentrations of10and25mg kg-1and the inhibitory effect exhibited a dosage-dependent relationship. As the application frequency of chlorothalonil increased, a recovery trend in dehydrogenase activity was found and a profound inhibition in neutral phosphatase activity was observed. Repeated chlorothalonil applications at levels of10and25mg kg-1in soil resulted in an inhibition of soil urease activity.4. The effects of repeated applications of chlorothalonil on soil respiration as well as soil genotoxicity were studied under laboratory condition using substrate induced respiration and Vwia faba micronucleus test, respectively. The results showed soil respiration for7h (SR7) was inhibited after repeated chlorothalonil applications at5,10, and25mg kg-1, and the inhibitory effect was increased with the concentration of chlorothalonil, and a recovery trend in SR7was found with increase of application frequency. Compared with the control, micronucleus frequency of Vicia faba root tip was significantly enhanced after the four introductions of chlorothalonil. The micronucleus frequency increased rapidly with chlorothalonil dosages and frequency. These results indicate that repeated chlorothalonil applications may lead to soil genotoxicity.5. Soil bacterial community structure was evaluated15days after repeated chlorothalonil applications using temperature gradient gel electrophoresis (TGGE) and partial sequence analysis of PCR-amplified16S rDNA genes. The results showed that the bacterial community structure was altered by repeated chlorothalonil applications at double recommended dose and the five times the recommended dose in soil. Bands appearing to be either enhanced or inhibited as a result of the repeated chlorothalonil treatments were excised and sequenced. Sequencing of excised TGGE bands indicated the repeated applications of chlorothalonil had an obvious impact on several bacteria taxa of Bacillales,proteobacteria, Bacteroidetes, and Actinobacteria.6. The effect of successive chlorothalonil applications on its persistence, soil respiration activity, and enzyme activity, and bacterial community structure in soil was investigated under greenhouse conditions. Accumulation of chlorothalonil and its metabolite,4-hydroxychlorothalonil, was observed after repeated chlorothalonil applications in soil, and the accumulation level was closely correlated to application dosage and its frequency. No significant change in soil respiration and dehydrogenase activities was found15days after repeated applications at a level of2kg a.i./ha, whereas soil respiration and dehydrogenase activities was significantly inhibited after four repeated applications at10kg a.i./ha. TGGE patterns demonstrated that no significant shifts in soil bacterial community structure were induced by the four successive chlorothalonil applications under greenhouse conditions.