Influence and persistence of phorate and carbofuran insecticides on microorganisms in rice field

An experiment was conducted in microplots (4 m x 4 m) with two insecticides, phorate and carbofuran at rates of 1.5 and 1.0 kga.i.ha(-1) respectively, to investigate its effect on the population and distribution of bacteria, actinomycetes and fungi as well as the persistence of the insecticidal residues in rhizosphere soils of rice (Oryza sativa L., variety IR-50). Application of the insecticides stimulated the population of bacteria, actinomycetes and fungi in the rhizosphere soils, and the stimulation was more pronounced with phorate as compared to carbofuran. Both the insecticides did not have marked effect on the numbers of Streptomyces and Nocardia in the rhizosphere soils. However, the growth of Bacillus, Escherichia, Flavobacterium, Micromonospora, Penicillium, Aspergillus and Trichoderma with phorate and that of Bacillus, Corynebacterium, Flavobacterium, Aspergillus and Phytophthora with carbofuran were increased. On the other hand, the numbers of Staphylococcus, Micrococcus, Fusarium, Humicola and Rhizopus under phorate and Pseudomonas, Staphylococcus, Micrococcus, Klebsiella, Fusarium, Humicola and Rhizopus under carbofuran were inhibited. Both the insecticides persisted in the rhizosphere soil for a short period of time and the rate of dissipation of carbofuran was higher than that of phorate in the soil depicting the half-life (T1/2) 9.1 and 10.4 days, respectively.     

Photolysis of 14C-sulfadiazine in water and manure

Photolysis of 14C-sulfadiazine in aqueous solution under simulated sunlight followed first-order kinetics. The impact of H2O2, humic acid, fulvic acid and acetone to enhance the photodegradation of sulfadiazine (SDZ) was studied. Six photoproducts, 4-OH-SDZ, 5-OH-SDZ, N-formyl-SDZ, 4-[2-iminopyrimidine-1(2H)-yl] aniline, 2-aminopyrimidine, and aniline were identified. Extrusion of SO2 was found to be the main degradation process during irradiation. These photoproducts can occur in water and soil upon sunlight exposure, when soil is treated with SDZ contained in manure. Due to photodegradation the experimental half-life of the SDZ in water was 32h and in the presence of photosensitizers the half-life values were 19.3-31.4h, 17.2-31.4h, 12.6-29.8h, and 3.8-30.7h for H2O2, humic acid, fulvic acid, and acetone, respectively depending on the concentration of the photosensitizers. The presence of photosensitizers markedly reduced SDZ persistence, indicating that indirect photolytic processes are important factors governing the photodegradation of SDZ in aqueous environments. Investigation revealed further persistence behavior of SDZ in manure. The half-life value of SDZ in manure was 158h.     

A comparative study on the dissipation and microbial metabolism of organophosphate and carbamate insecticides in orchaqualf and fluvaquent soils of West Bengal

An experiment has been conducted under laboratory conditions to investigate the effect of phorate (an organophosphate insecticide) and carbofuran (a carbamate insecticide) at their recommended field rates (1.5 and 1.0 kga.i.ha-1, respectively) on the growth and multiplication of microorganisms as well as rate of dissipation and persistence of the insecticidal residues including their metabolites in laterite (typic orchaqualf) and alluvial (typic fluvaquent) soils of West Bengal. Application of phorate and carbofuran in general, induced growth and development of bacteria, actinomycetes, fungi, N2-fixing bacteria and phosphate solubilizing microorganisms in both the soils and the stimulation was more pronounced with phorate as compared to carbofuran. Application of phorate recorded highest stimulation of fungi in laterite and actinomycetes in alluvial soil. Carbofuran on the other hand, augmented fungi and N2-fixing bacteria in laterite and actinomycetes in alluvial soil. Bacterial population was inhibited due to the application of carbofuran in alluvial soil. Phorate sulfoxide and phorate sulfone, the two metabolites of phorate and 3-hydroxycarbofuran and 3-ketocarbofuran, the two metabolites of carbofuran isolated were less persistent in both the soils. Phorate persisted in laterite and alluvial soils up to 45 and 60 days, respectively depicting the half-life (T1/2) 9.7 and 11.5 days, respectively while the T1/2 of carbofuran for the said soils were 16.9 and 8.8 days, respectively. No metabolite of carbofuran was detected in soils after 30 days of incubation while phorate sulfone persisted in alluvial soil even after 60 days of application of the insecticide.     

The effect of land use on soil health indicators in peri-urban agriculture in the humid forest zone of southern cameroon

The objective of this study was to identify the effect of different land uses in peri-urban agriculture on the soil properties. Soil health indicators were evaluated in the top 10 cm at five tilled agricultural sites involving different cropping systems and use of agrochemicals within the peri-urban agricultural areas of Yaounde, Cameroon, and compared with a native forest land. The experimental data showed that the selected indicators were sensitive to cropping practice. Most cropped land had significantly higher total C, available N and P concentrations, soil pH, electrical conductivity, salinity, biomass C and P, dehydrogenase, beta-glucosidase, and acid phosphatase activities. Land producing corn (Zea mays L.) and sugarcane (Saccharum officinarum L.) differed from that producing tomatoes (Lycopersicon esculentum Mill.), but cultivation of these crops has significantly impacted native soil quality. However, phenol oxidase, microbal biomass C/organic C (C(mic)/C(org)), and microbial biomass C/microbial biomass P (C(mic)/P(mic)) were negatively affected. These appeared to be more consistent indicators of negative management causing changes to soil health and may be suitable for an early appraisal of soil health.     

Dissipation and metabolism of C-spiroxamine in soil under laboratory condition

Spiroxamine [SPX] belongs to a spiroketalamine group of substances. The biodegradation of [1,3-dioxolane-4-¹⁴C]-SPX has been examined in 2 soils of different physicochemical properties. The total recovery of radioactivity from soils was 98.6-103.5% of that applied. The total amount of extractable radioactivity declined with a simultaneous increase in non-extractable radioactivity. Volatile organics were detected at lower levels; however, mineralization played a marked effect on the route of SPX dissipation. The half-life ranges between 37 and 44 d. SPX does not undergo any enantioselective degradation. 4 metabolites: despropyl-SPX, desethyl-SPX, SPX N-oxide and SPX acid were identified, applying mass spectrometric technique. Sorption-desorption data fitted well with a Freundlich model in log form (r², 0.99). K_Dsorp ranged between 44 and 230, suggesting SPX ought to be considered as a substance with low leaching potential [groundwater ubiquity score (GUS), <1.8]. Furthermore, an overall low desorption of 1-11% indicates firm retention of SPX by the soils.     

Evaluation of sorption-desorption processes for metalaxyl in natural and artificial soils

The main process controlling soil-pesticide interaction is the sorption-desorption as influenced by active soil surfaces. The sorption phenomena can influence translocation, volatility, persistence and bioactivity of a pesticide in soil. The present investigation was conducted on natural and artificial soils in order to enumerate the effect of soil components such as montmorillonite and ferrihydrite on the sorption behaviour of the fungicide metalaxyl and if sorption-desorption of the chiral pesticide affects the enantiomeric ratio. The sorption-desorption characteristics of metalaxyl were investigated by batch equilibration technique in a natural soil, two artificial soils, and in pure montmorillonite and ferrihydrite. After extraction, pesticide residues were analyzed by conventional and chiral chromatography using tandem mass spectrometry. A K_dSorp (2.3–6.5) suggests low level sorption of metalaxyl with an appreciable risk of run-off and leaching. Thus, metalaxyl poses a threat to surface and ground water contamination. Furthermore, desorption tests revealed a hysteretic effect (H ≤ 0.8) in natural and artificial soils. Significant amount of metalaxyl was found tightly bound to the adsorbents without desorbing readily after desorption cycle. Desorption of 22–56% of the total amount of the retained metalaxyl was determined. This study reveals that an artificial soil derived from different soil constituents can be used to assess their influence on sorption/desorption processes. The present investigation showed that both montmorillonite and ferrihydrite play a significant role in the sorption of metalaxyl. The sorption doesn't influence the enantiomeric ratio of racemic metalaxyl.     

Influence of biotic and abiotic factors on dissipating metalaxyl in soil.

Under laboratory condition, dissipation of metalaxyl in sterile and non-sterile soils, its sorption behaviour and fate in presence of light have been studied. The half-life value of metalaxyl was found in the range of 36-73 d in non-sterile soil. 5.3-14.7% dissipation was observed due to abiotic factors other than light. Metalaxyl was found photostable in soil showing half-life of 188- 502 h under simulated sunlight. In adsorption study, a non-linear relationship between concentration of metalaxyl and its adsorption into soils was observed. Estimated koc value increased as organic carbon content decreased. Adsorption and desorption kD values ranged between 53.5 and 151.1.     

Sorption and desorption of sulfadiazine in soil and soil-manure systems

Sulfadiazine is a widely used veterinary medicine that has high potential to enter the environment, especially the soil compartment by the application of manure on agricultural land and grass land or by the deposition of dung pats on pasture. Once it reaches the soil environments, it may enter into surface and ground water. Therefore, sorption-desorption behavior of sulfadiazine was studied under laboratory conditions in five different soils varying in their physicochemical properties. A batch equilibration technique was used with initial aqueous solution concentration of sulfadiazine at 5, 0.5, 0.05, and 0.005 microg mL(-1). Sorption-desorption data in soils with and without manure were well fitted with Freundlich model in log form (r(2), 0.99). A sorption-desorption hysteresis effect was apparent in all soils. A significant amount of sulfadiazine was found tightly bound to the soil particles and did not desorb after the desorption process. Moreover, presence of manure enhanced hysteresis effect. Hysteresis coefficient (H) value from soils in absence of manure (0.9-1.0) increased to the soils in presence of manure (0.9-1.8). Soils in the absence of manure showed low level of K(D Sorp.) values ranging from 0.1 to 24.3, suggesting low level sorption of sulfadiazine with appreciable risk of run-off and leaching, and in turn, surface and ground water contamination. However, presence of manure increased the sorption tendency of sulfadiazine significantly (K(D Sorp.), 6.9-40.2). K(D) values pertaining to desorption cycle increased from 1.2-90.4 to 10.4-167.3 in absence and presence of manure, respectively.     

Metalaxyl—its persistence and metabolism in soil

Different biotic and abiotic factors were found to play a vital role in attenuating metalaxyl residues in soil. In addition to metalaxyl many other products were found by HPLC and GC‐MS analysis while studying its soil metabolism in presence of natural sunlight. Three compounds were identified and characterized: 2,6‐dimethylaniline, 2,6‐dimethyl‐N‐ethylacetanilide and N‐(2,6‐dimethyl phenyl) alanine methyl ester.