Rice fields, diazinon-treated

Diazinon persisted for about 15 days in a flooded soil (pH 6,6) that had been treated previously with the insecticide but, in a flooded soil that had never been exposed to diazinon, it persisted for about 60 days. Similarly, water from a diazinon-treated rice field inactivated the insecticide within 5 days after incubation. Microorganisms that developed in response to insecticide application accelerated its hydrolysis and the subsequent mineralization of the hydrolysis product, 2-isopropyl-6-methyl-4-hydroxy pyrimidine, to COg. A Fla-vobacterium sp., isolated from water of a treated rice field, had exceptionally high capability to metabolize diazinon as sole carbon source. This provides unequivocal evidence that microbes are involved in the rapid inactivation of diazinon in rice fields. 

Analyses of residues in the soils after incubation showed that the persistence of diazinon was considerably shorter in the previously treated soil than in the untreated soil. The half-life value for diazinon in previously treated soil was 1.7 days while in the untreated soil it was 9.9 days. Most of the insecticide added to the previously treated soil was lost within 10 days. Paddy water from the same fields were tested also for diazinon-degrading activity (17). Again water from a rice field treated previously with diazinon inactivated the insecticide more rapidly than did the water from an untreated field. In the water from the previously treated field the insecticide dissipated completely within 3-5 days of incubation after an initial lag of 1-2 days (17, 18). Table II summarizes the results of the study on the stability of diazinon in soil and paddy water. The data indicated clearly that a factor capable of degrading diazinon developed in rice fields of the Institute farm after insecticide applications. The diazinon-degrading factor, found in the diazinon-treated rice fields in the Institute farm, was noticed also in three other locations in the Philippines (19). 

Biological. Sethimathan and Yoshida (1973a) isolated a Flavobacterium sp. (ATCC 27551) from rice paddy water that metabolized diazinon as the sole caibon source. Diazinonwas readily hydrolyzed to 2-isopropyl-4-methyl-6-ltydroxypyrimidine under aerobic conditions but less rapidly under anaerobic conditions. This bacterium as well as enrichment cultures isolated from a diazinon-treated rice field mineralized the hydrolysis product to carbon dioxide (Sethunathan and Pathak, 1971 Sethunathan and Yoshida, 1973). Rosenberg and Alexander (1979) demonstrated that two strains of Pseudomonas grew on diazinon and produced diethyl phosphorothioate as the major end product. The rate of microbial degradation increased in the presence of an enzyme (parathion hydrolase), produced by a mixed culture of Pseudomonas sp. (Honeycutt et al., 1984). 

Isotope studies were conducted to determine the biochemical pathway of enhanced diazinon metabolism observed in paddy water of treated fields. In these studies, diazinon labelled at 4-position on the pyrimidine ring was incubated with paddy water from diazinon-treated and untreated fields as described earlier (17). The enclosed C02-free system was utilized to measure the C02 production, and the radioactivity in the evolved C02 was assayed by liquid scintillation counting (17). Diazinon residues were analyzed by gas-liquid chromatography after extraction with hexane. The results are summarized in Figure 1. Water from treated rice fields was capable of metabolizing 4-carbon atom on the pyrimdine ring, releasing more than 66% of the added radioactivity as C02 within five days of incubation. Such rapid metabolism was not observed with water from untreated field. 

The persistence of diazinon in soil from a rice field that had never been exposed to diazinon before and in soil from a previously treated field was studied under flooded conditions. The treated fields received granular applications of diazinon (17) at rates and intervals recommended for rice pest control viz, 2 kilogram/ha active ingredient every 20 days. Soils (Maahas clay, pH 6.6) were collected from three treated and three untreated fields 12 days after the third application. Soils were air-dried and screened (2mm) 20 gram samples of these soils were placed in test tubes and flooded with 25 ml of aqueous diazinon solution. The insecticide residues from the soils incubated at 30 °C were extracted periodically with hexane-acetone (2) and analyzed in a gas chromatograph fitted with a cesium bromide detector as described earlier (18). 

In attempts to characterize the factor that developed in the rice fields of the Institute farm, we enriched paddy water from treated fields with specific diazinon-degrading microorganisms (17). The insecticide in aqueous solution was incubated with paddy water from a treated field at a one to one ratio. When the insecticide disappeared from this mixture, 5 ml of this incubation mixture was incubated again with 5 ml of aqueous diazinon solution. During a 10 day incubation period, five transfers were made. Following the first transfer, the insecticide disappeared from the incubation mixture between 96-120 hours, but in less than 6 hours after the fifth transfer (17, 22). Evidently, the factor originally... 

Bacteria were isolated also from paddy water of diazinon-treated fields in Maligaya Rice Research and Training Center, Philippines, following the procedures used in the isolation of bacteria from the Institute rice fields. The most active isolate, Corynebacterium sp., decomposed diazinon in mineral solution only in the presence of ethyl alcohol or glucose (25). 

Figure 1. Diazinon degradation after its incubation with water from a rice field treated previously with diazinon and from an untreated rice field (17)...

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