Fate of a phenylpyrazole in vegetation and soil under tropical field conditions

The fate of fipronil, a phenylpyrazole insecticide, and its metabolites under tropical conditions was studied in soil and in vegetation after treatment for locust control. Two different plots were treated with a formulation of fipronil at doses of 5 and 10 g of active ingredient ha(-)(1), respectively. Vegetation and soil at depths of 0-5 and 5-20 cm were sampled for up to 2 months after treatment. After extraction and purification on fipronil immunoaffinity cartridges, residues were analyzed by gas chromatography using electron capture and mass detectors. In soil, a rapid initial decrease of fipronil was observed with a rapid formation of the sulfone and the photodegradate; the amide and the sulfide were not detected. In vegetation, a rapid initial decrease of fipronil was also observed with a rapid formation of mostly the sulfone; the photodegradate and the sulfide were also detected but at much lower concentrations. The metabolites resulting from the degradation of fipronil were similar in both soil and vegetation, but their relative concentrations were different.     

Degradation and sorption of commonly detected PPCPs in wetland sediments under aerobic and anaerobic conditions

Purpose

Wetlands are a popular tool to treat/polish wastewater by reducing nutrient loading into the environment. In addition to nutrients, organic contaminants, such as pharmaceuticals and personal care products (PPCPs), are commonly detected in treated wastewater. Treatment wetlands may reduce concentrations of PPCPs before the treated effluent enters rivers and streams. Oxygen status may greatly affect the attenuation of PPCPs in wetland sediments by influencing microbial makeup and activity. An understanding of the effect of redox conditions on the degradation of PPCPs and the factors influencing PPCP sorption to wetland sediments is needed to maximize PPCP removal in treatment wetlands.

Materials and methods

Three wetland sediments from the San Diego Creek and Newport Bay watershed in Southern California, USA, were incubated under aerobic and anaerobic conditions to assess the degradation of several regularly occurring PPCPs and their phase distribution as a function of time.

Results and discussion

Under aerobic conditions, ibuprofen, N,N-diethyl-meta-toluamide (DEET), and gemfibrozil generally had half-life values around 20?days, while the half-life of carbamazepine was substantially longer (between 165 and 264?days). The anaerobic half-lives of gemfibrozil and ibuprofen increased by factors of 11?C34 and carbamazepine increased by factors of 1.5?C2.5. There was no detectable anaerobic degradation of DEET. The apparent phase distribution coefficient increased over time for DEET, carbamazepine and gemfibrozil, indicating that sorption of PPCPs to wetland sediments may be more limited than that predicted using equilibrium sorption coefficient values.

Conclusions

Knowledge of the capacity of wetland sediments for degrading and sorbing PPCPs is vital to the design of treatment wetlands. Degradation of the selected PPCPs was enhanced under aerobic conditions as compared to anaerobic conditions. Sorption to sediments increased with contact time, indicating that longer hydraulic retention will increase wetland capabilities for removing PPCPs.     

Hydrolysis kinetics of fenthion and its metabolites in buffered aqueous media

This study investigates the hydrolysis kinetics of fenthion and its five oxidation metabolites in pH 7 and pH 9 buffered aqueous media at 25, 50, and 65 degrees C. Five metabolites and three hydrolysis products were synthesized and purified. The reactant and the corresponding hydrolysis products were determined by HPLC. Rate constant and half-life studies revealed that fenthion and its metabolites were relatively stable in neutral media, and their stability decreased as pH increased. The half-lives at 25 degrees C ranged from 59.0 days for fenthion to 16.5 days for fenoxon sulfone at pH 7, and from 55.5 days for fenthion to 9.50 days for fenoxon sulfone at pH 9; half-lives were greatly reduced at elevated temperatures. The activation energy (E(a)) was found to range from 16.7 to 22.1 kcal/mol for the compounds investigated. The phenol hydrolysis product of fenthion and fenoxon, 3-methyl-4-methylthiophenol was not stable in pH 7 and pH 9 buffered solutions at 50 degrees C, whereas 3-methyl-4-methylsulfonylphenol and 3-methyl-4-methylsulfinylphenol were relatively stable under the same conditions. At pH 9, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were combination of hydroxide ion and neutral water molecule attacking on the P atom to form corresponding phenol derivatives. Under neutral conditions, the primary hydrolysis mechanisms of fenthion and its oxidation metabolites were assumed to be the combination of water molecule attacking on the P atom to form phenol derivatives and on the C atom to yield dealkylation products.     

Phenylpyrazole insecticide photochemistry, metabolism, and GABAergic action: ethiprole compared with fipronil

Ethiprole differs from fipronil, the major phenylpyrazole insecticide, only in an ethylsulfinyl substituent replacing the trifluoromethylsulfinyl moiety. This study compares their photochemistry, metabolism, action at the gamma-aminobutyric acid (GABA) receptor, and insecticidal potency. On exposure to sunlight as a thin film, ethiprole undergoes oxidation (major), reduction, and desethylsulfinylation but not desulfinylation whereas the major photoreaction for fipronil is desulfinylation. Metabolic sulfone formation is more rapid with ethiprole than fipronil in human expressed CYP3A4 in vitro and mouse brain and liver in vivo. High biological activity is observed for the sulfide, sulfoxide, sulfone, and desulfinyl derivatives in both the ethiprole and the fipronil series in GABA receptor assays (human recombinant beta3 homomer and house fly head membranes) with [(3)H]EBOB and in topical toxicity to house flies with and without the P450-inhibiting synergist piperonyl butoxide. On an overall basis, the ethiprole series is very similar in potency to the fipronil series.     

Uptake and xylem transport of fipronil in sunflower

The phenylpyrazole insecticide, fipronil, is used in seed coating against Agriotes larvae, which infest mainly corn and sunflower. Coating the seeds of the cultivated plants with fipronil has proven its effectiveness against Agriotes populations. In the case of sunflower or even corn, the possible root uptake of this insecticide may lead to a toxic effect against pollinators such as honeybees. In the present report, the uptake and transport of fipronil inside the sunflower seedling was studied in the laboratory. In a first study, sunflower was cultivated on an aqueous medium containing fipronil. An intense root uptake of fipronil occurred, leading to a transport into leaves depending upon transpiration. In a second study, plants were cultivated on a soil in which fipronil was uniformly distributed. Under our soil conditions (20% organic carbon), the partition coefficient between soil and water (K(d)) was found to be equal to 386 +/- 30. The average rate of fipronil transfer from soil water to seedlings was from 2 to 2.6 times lower than water transfer. During the 3 week experiment, 55% of recovered labeled compounds was in the parent form and 35% had been converted to lipophilic metabolites, with either a 4-CF(3)-SO(2) or 4-CF(3)-S substituant, which are also very potent lipophilic insecticides. This paper suggests that the possible uptake of fipronil by sunflower seedlings under agronomic conditions is mainly controlled by the physicochemical characteristics of the seed-coating mixture.     

Microbially Mediated Degradation of Common Pharmaceuticals and Personal Care Products in Soil Under Aerobic and Reduced Oxygen Conditions

Biological degradation rates of estrogen compounds and common pharmaceutical and personal care products (PPCPs) were examined in soils with a long history of exposure to these compounds through wastewater effluent and in soil not previously exposed. Biological degradation rates over 14 days were compared under aerobic and anaerobic conditions. Estrogen compounds including estrone, 17??-estradiol, estriol, and 17??-ethinylestradiol exhibited rapid degradation by soil microorganisms in both aerobic and anaerobic conditions. Rapid degradation rates for estrone, estriol, and 17??-ethinylestradiol occurred in pre-exposed soil under aerobic conditions; half-lives calculated under these conditions were 0.6, 0.7, and 0.8 day, respectively. Unexposed soil showed similar or slightly longer half-lives than pre-exposed soil under aerobic conditions. The exception was 17??-estradiol; in all treatments, degradation in unexposed soil resulted in a shorter half-life (2.1 versus 2.3 days). Anaerobic soils exhibited high biological degradation of estrogens as well. Half-lives of all estrogens ranged from 0.7 to 6.3 days in anaerobic soils. Triclosan degraded faster under aerobic conditions with half-lives of 5.9 and 8.9 days in exposed and unexposed soil. Under anaerobic conditions, triclosan half-lives were 15.3 days in unexposed and 28.8 days in exposed soil. Ibuprofen showed the least propensity toward biological degradation than other chemicals tested. Biological degradation of ibuprofen was only observed in unexposed soil; a half-life of 41.2 days was determined under anaerobic conditions and 121.9 days under aerobic conditions. Interestingly, unexposed soil exhibited a greater ability under anaerobic conditions to biologically degrade tested compounds than previously exposed soil.     

[噻唑基-2-14C]-毒氟磷在水-沉积物系统中的降解与残留转化研究

毒氟磷是我国自主创制的新型植物抗病毒剂,研究毒氟磷的环境行为和归趋对科学评价其环境风险具有重要意义。本试验采用¹⁴C同位素示踪技术,以[噻唑基-2-¹⁴C]-毒氟磷为示踪剂,研究了毒氟磷在A和B两种水-沉积物系统中的好氧降解动态以及残留转化规律。结果表明,毒氟磷母体在两种水-沉积物系统中的水相消散和降解均符合一级动力学模型,其水相消散半减期分别为7.23 d (A)和13.86 d (B),在水-沉积物整个系统中的母体降解半减期则分别为121.60 d (A)和65.39 d (B)。毒氟磷在供试水-沉积物系统中的残留转化规律存在显著差异,培养结束时(培养时间为100 d),可提态残留在A和B系统中分别占放射性引入量的77.41%和43.71%,矿化量分别占放射性引入量的1.11%和2.83%,结合态残留分别占放射性引入量的24.11%和49.25%;较高的降解率和阳离子交换量是毒氟磷及其代谢产物在B系统中更易形成结合残留的主要原因。本研究结果为科学评价毒氟磷在水-沉积物系统中的安全性提供了基础数据和科学依据。     

Degradation and binding of atrazine in surface and subsurface soils

Understanding the dissipation rates of chemicals in unsaturated and saturated zones of subsurface soils will help determine if reductions of concentrations to acceptable levels will occur. Chemical properties and microbial biomass and activity were determined for the surface (0-15 cm), lower root (50-105 cm), and vadose (175-220 cm) zones in a Huntington silty clay loam (Fluventic Hapludoll) collected from an agricultural field near Piketon, OH. The rates of sorption, mineralization, and transformation (formation of bound residues and metabolites) of atrazine were determined. Microbial activity was estimated from the mineralization of (14)C-benzoate. We observed decreased levels of nutrients (total organic carbon, N, and P) and microbial biomass with depth, while activity as measured with benzoate metabolism was higher in the vadose zone than in either the surface or the root zones. Sorption coefficients (K(f)) declined from 8.17 in the surface to 3.31 in the vadose zone. Sorption was positively correlated with organic C content. Rates of atrazine mineralization and bound residues formation were, respectively, 12-2.3-fold lower in the vadose than in the surface soil. Estimated half-lives of atrazine ranged from 77 to 101 days in the surface soil, but increased to over 900 days in the subsurface soils. The decreased dissipation of atrazine with increasing depth in the profile is the result of decreased microbial activity toward atrazine, measured either as total biomass or as populations of atrazine-degrading microorganisms. The combination of reduced dissipation and low sorption indicates that there is potential for atrazine movement in the subsurface soils.     

氧化还原条件对土壤磷素固定与释放的影响

为探讨水稻土由氧化环境转为还原条件时对土壤磷素固定与释放的影响,选择18种水稻土样品进行室内模拟实验,通过测定不同条件下磷的等温吸附曲线和采用氧化铁试纸测定法进行多次提取以对比淹水前后土壤磷的累计解吸量,发现大部分供试样品的固磷能力在淹水条件下有了显著的提高,全部样品在淹水后磷的释放量都有不同程度的减少。进一步的研究表明淹水条件下土壤对磷的固定与释放的变化主要与淹水后土壤Eh的降低和pH的升高而导致的大量无定形铁的形成有关。     

Sorption Behaviors of Sodium Dodecylbenzene Sulfonate (SDBS) on Marine Sediments

The sorption behavior of sodium dodecylbenzene sulfonate (SDBS), an anionic surfactant, on marine sediments was systematically investigated. The experimental results showed that 100 min was required for the adsorption equilibrium. For the sediments treated by HCl and H₂O, sorption behavior of SDBS was fit with linear isotherm very well over the concentration range studied at 298 K. The sorption occurred primarily due to partition function of hydrophobic chains into organic carbon of sediments. Sorption of SDBS on H₂O₂-treated sediments was satisfactorily fit with Freundlich isotherm model and mainly through surface function of clay minerals in the sediment. The sorption was favorably influenced by the increased salinity, deceased pH and decreased temperature of seawater.     

Preferential sorption of phenolic phytotoxins to soil: implications for altering the availability of allelochemicals

Allelopathy, secondary metabolite-mediated plant-to-plant interaction, is gaining application in current agricultural science as well as in invasion ecology. However, the role of sorption to soil in modifying the bioavailability of components in complex allelochemical mixtures is still obscure. Hence, the role of preferential sorption to soil in altering the chemical composition of plant exudates was studied in a silt loam soil using representative mixtures of plant phenolic acids, namely, hydroxybenzoic acid, vanillic acid, coumaric acid, and ferulic acid. The experiment was conducted using a batch equilibration technique, and data were fitted to a Freundlich isotherm. The concentration-dependent sorption coefficient (K(d)) at 10 microg mL(-)(1) was used to assess the sorption affinity of phenolic acids across different systems. Along with solid phase dissolution, all of the phenolic acids exhibited strong site-specific sorption, as evident from their nonlinear isotherms. Removal of organic matter substantially decreased the sorption affinity of all phenolic acids. Direct competition for sorption sites was observed even at low concentrations of phenolic acids. The K(d) of hydroxybenzoic acid was decreased more than 90% in the presence of coumaric acid. About 95% of sorbed vanillic acid was displaced into the soil solution in the presence of ferulic acid. Hydroxybenzoic acid did not affect the sorption affinity of other phenolic acids significantly, whereas ferulic acid showed low displacement by other phenolic acids. The displacement pattern indicated directional sorption of phenolic acids with -OH and -COOH groups. Soil organic matter was associated with preferential sorption. This is the first study to elucidate competitive sorption characteristics of plant secondary metabolites in soil matrix. The results demonstrate that preferential sorption to soil can alter the availability of plant exudates in mixtures and thus may mediate their phytotoxic effects.     

Time-dependent sorption of imidacloprid in two different soils

Time-dependent sorption of imidacloprid [1-[(6-chloro-3-pyridinyl)-methyl]-N-nitro-2-imidazolidinimine] was investigated with two German soils (sandy loam and silt loam). Soil batches containing the active ingredient (0.33 mg/kg) were incubated for 100 days. After selected aging periods, imidacloprid desorbed by 0.01 M CaCl(2) (soluble phase) and by organic solvents (methanol and acetonitrile) and reflux extraction with acidified methanol (sorbed phase) was determined. Calculated sorption coefficients K(d) and K(oc) increased by a factor of 3.2-3.8 during 100 days of aging. Additionally, the time-dependent sorption was verified by a column leaching experiment with the aged soil. The amount of imidacloprid in column eluates (0.01 M CaCl(2)) decreased compared to total recovered by a factor of approximately 2. Sorption of imidacloprid thus increased with residence time in soil, making it more resistant to leaching. These results are further information to explain the low leaching potential of imidacloprid in the field, despite its high water solubility.     

Sorption-desorption of two "aged" sulfonylaminocarbonyltriazolinone herbicide metabolites in soil

Aging (herbicide-soil contact time) has been shown to significantly affect the sorption-desorption characteristics of many herbicides, which in turn can affect the availability of the herbicide for transport, plant uptake, and microbial degradation. In contrast, very little work in this area has been done on herbicide metabolites in soil. The objective of this study was to characterize the sorption-desorption of sulfonylaminocarbonyltriazolinone herbicide metabolites incubated in soils at different soil moisture potentials. A benzenesulfonamide metabolite and a triazolinone metabolite from sulfonylaminocarbonyltriazolinone herbicides were incubated in clay loam and loamy sand soils for up to 12 weeks at -33 kPa and at water contents equivalent to 50 and 75% of that at -33 kPa. Chemicals were extracted sequentially with 0.01 N CaCl(2) and aqueous acetonitrile (solution and sorbed phase concentrations, respectively), and apparent sorption coefficients (K(d,app)) were calculated. Sufficient metabolite remained during the incubation (>55% of applied) to allow determination of the coefficients. The initial aging period (2 weeks after application) significantly increased sorption as indicated by increased K(d,app) values for the chemical remaining, after which they remained relatively constant. After 12 weeks of incubation at -33 kPa, K(d,app) values for benzenesulfonamide and triazolinone increased by a factor of 3.5 in the clay loam soil and by a factor of 5.9 in the loamy sand as compared to freshly treated soils. There was no effect of moisture potential on aged apparent K(d,app) values. These data show the importance of characterization of sorption-desorption in aged herbicide residues, including metabolites, in soil, particularly in the case of prediction of herbicide residue transport in soil. In this case, potential transport of sulfonylaminocarbonyltriazolinone herbicide metabolites would be overpredicted if freshly treated soil K(d) values were used to predict transport.     

滇池沉积物的功能: 磷源还是磷库?

Dianchi Lake is a highly eutrophic lake in southwestern China where phosphorus (P) is the limiting element for eutrophication and where lake sediments play an important role in the P cycle. One hundred and eighteen sites were sampled throughout Dianchi Lake in 2002 to investigate the P loading of the lake sediments, while fresh surface sediments were studied in the laboratory to clarify its role for phosphorus. The results showed that concentrations of total phosphorus (TP) in sediments were very high, with a maximum value of 6.66 g kg^-1, and decreased with sediment depth. P adsorption on surface sediments was rapid with adsorption amounts higher in acidic than in alkaline conditions. The release experiments showed that P release increased as pH rose from 7.0 to 10.5. Additionally, compared with aerobic conditions, P release was much higher under anaerobic: conditions, especially with low P content in the lake water. At present, the sediments in Dianchi Lake still function as a sink for phosphorus at high P levels in lake water. However, if the external P load was reduced and P content in lake water became low, the sediment would have a large potential for P release under anaerobic conditions.     

Lake Sediments from Dianchi Lake： A Phosphorus Sink or Source？

Dianchi Lake is a highly eutrophic lake in southwestern China where phosphorus (P) is the limiting element for eutrophication and where lake sediments play an important role in the P cycle. One hundred and eighteen sites were sampled throughout Dianchi Lake in 2002 to investigate the P loading of the lake sediments, while fresh surface sediments were studied in the laboratory to clarify its role for phosphorus. The results showed that concentrations of total phosphorus(TP) in sediments were very high, with a maximum value of 6.66 g kg^-1, and decreased with sediment depth. P adsorption on surface sediments was rapid with adsorption amounts higher in acidic than in alkaline conditions. The release experiments showed that P release increased as pH rose from 7.0 to 10.5. Additionally, compared with aerobic conditions, P release was much higher under anaerobic conditions, especially with low P content in the lake water. At present, the sediments in Dianchi Lake still function as a sink for phosphorus at high P levels in lake water. However, if the external P load was reduced and P content in lake water became low, the sediment would have a large potential for P release under anaerobic conditions.     

Occurrence of Sexual Hormones in Sediments of Mangrove in Brazil

The presence of sexual hormones (female estrogens) was assessed in sediments of a mangrove located in the urban region of southern Brazil. The estrogens are involved in human sexual reproduction. They act as the chemical messengers, and they are classified as natural and synthetic. The estrogens inputs in the environment are from treated and untreated sewage. The presence of estrogens in sewage is excretion from the female due to natural production and use of contraceptives (synthetic estrogens). With the indiscriminate release of sewage into the environment, estrogens can be found in rivers, lakes, and even in oceans. In this work, the presence of estrone (E1), 17-??-estradiol (E2), and 17-??-ethynilestradiol (EE2) in eight sedimentary stations in Itacorubi mangrove located on Santa Catarina Island, south Brazil, was investigated. Historically, the Itacorubi mangrove has been impacted by anthropogenic activities because the mangrove is inserted in the urban area of the Florianopolis. The estrogen EE2, used as contraceptive, had the highest concentration in mangrove sediment, 129.75?±?3.89 ng/g. E2 was also found, with its concentration ranging from 0.90?±?0.03 to 39.77?±?1.19 ng/g. Following the mechanism, under aerobic or anaerobic conditions, E2 will first be oxidized to E1, which is further oxidized to unknown metabolites and finally to CO₂ and water (mineralized). EE2 is oxidized to unknown metabolites and also finally mineralized. Theoretically, under anaerobic conditions, EE2 can be reduced to E1 even in environments such as mangrove which is essentially anaerobic.     

五氯酚在长期定位施肥土壤中的残留动态

研究好氧和厌氧条件下五氯酚 (PCP) 在长期不施肥 (CK),施无机肥尿素 (N),施有机肥 (OM)和无机有机肥配施 (N+OM) 4 种处理土壤中的残留动态.结果表明,无论在好氧或厌氧条件下,PCP 在土壤中的消解均遵循一级动力学方程.在好氧条件下,PCP 在 CK、N、OM 和 N+OM 4 种处理土壤中半衰期分别为 26.9、27.7、21.5、20.6 天,在厌氧条件下,PCP 的半衰期分别为 30.8、32.2、27.1、25.9 天.表明无论在好氧或厌氧条件下,长期单施有机肥或无机有机肥配施显著加速 PCP 在土壤中的消解,原因可能是长期单施有机肥或无机有机肥配施能显著提高土壤有机质含量和微生物活性,加速 PCP 在土壤中的消解.好氧条件下 PCP 的消解速率显著高于厌氧条件.     

Atrazine and metolachlor degradation in subsoils

Degradation of atrazine [2-chloro-4-etylamino-6-isopropylamino-1,3,5-triazine] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-methoxy-1-methylethyl)-acetamide] in sterile and non-sterile soil samples collected at two different soil depths (0-20 and 80-110 cm) and incubated under aerobic and anaerobic conditions was studied. Under aerobic conditions, the half-life of atrazine in non-sterile surface soil was 49 days. In non-sterile subsoil, the half-life of atrazine (119 days) was increased by 2.5 times compared in surface soils and was not statistically different from half-lives in sterile soils (115 and 110 days in surface soil and subsoil, respectively). Metolachlor degradation occurred only in non-sterile surface soil, with a half-life of 37 days. Under anaerobic conditions, atrazine degradation was markedly slower than under aerobic conditions, with a half-life of 124 and 407 days in non-sterile surface soil and non-sterile subsoil, respectively. No significant difference was found in atrazine degradation in both sterile surface soil (693 days) and subsoil (770 days). Under anaerobic conditions, degradation of metolachlor was observed only in non-sterile surface soil. Results suggest that atrazine degraded both chemically and biologically, while metolachlor degraded only biologically. In addition, observed Eh values of soil samples incubated under anaerobic conditions suggest a significant involvement of soil microorganisms in the overall degradation process of atrazine under anaerobic conditions.     

Linuron sorption-desorption in field-moist soils

Pesticide sorption or binding to soil is traditionally characterized using batch slurry techniques. The objective of this study was to determine linuron sorption in field-moist or unsaturated soils. Experiments were performed using low-density (i.e., 0.25 g mL(-)(1)) supercritical carbon dioxide to remove linuron from the soil water phase, thus allowing calculation of sorption coefficients (K(d)) at low water contents. Both soil water content and temperature influenced sorption. K(d) values increased with increased water content, if less than saturated. K(d) values decreased with increased temperature. K(d) values for linuron sorption on silty clay and sandy loam soils at 12% water content and 40 degrees C were 3.9 and 7.0 mL g(-)(1), respectively. Isosteric heats of sorption (DeltaH(i)) were -41 and -35 kJ mol(-)(1) for the silty clay and sandy loam soils, respectively. The sorption coefficient obtained using the batch method was comparable (K(f) for sandy loam soil = 7. 9 microg(1)(-)(1/)(n)() mL(1/)(n)() g(-)(1)) to that obtained using the SFE technique. On the basis of these results, pesticide sorption as a function of water content must be known to more accurately predict pesticide transport through soils.     

Microbial Degradation of Fipronil in Clay Loam Soil

Fipronil, (±)-5-amino-1-(2,6-dichloro-∝,∝,∝-trifluoro-p-tolyl)-4-trifluoromethysulfinylpyrazole-3-carbonitrile, is used as an effective insecticide for the control of rice pests in China. Although many studies examining the fate of fipronil in the soilenvironment have been conducted, there are no studies on the microbial degradation of fipronil in the soil environment. Fipronil was degradedby microorganisms in the non-sterile clay loam soil, which resulted in the formation of metabolite, MB45950. The degradation of fipronil in non-sterile clay loam soil was mainly influenced by the soil microbes. The half-lives in non-sterile clay loam soil were 9.72 and 8.78 d at 25 and 35 °C, respectively compared to 33.51 and 32.07 d at 25 and 35 °C, respectively in the sterile soil. The microbial viability test showed that non-sterile clay loam soil had viable microorganisms throughout the experiment. Fipronil did not adversely affect the microbes once soil microbes adapted to the presence of fipronil in the clay loam soil.