Metolachlor persistence in laboratory and field soils under Indian tropical conditions

Abstract

Metolachlor [2‐chloro‐N‐(2‐methoxy‐1‐methylethyl)‐2'‐ethyl‐6'‐methyl acetanilide] dissipation under both field and laboratory conditions were studied during summer season in an Indian soil. Metolachlor was found to have moderate persistence with a half‐life of 27 days in field. The herbicide got leached down to 15–30 cm soil layer and residues were found up to harvest day of the sunflower crop in both 0–15 cm and 15–30 cm soil layers. Metolachlor was found to be more persistent in laboratory studies conducted for 190 days. The rate of degradation was faster in soil under flooded partial anaerobic conditions as compared to aerobic soil with a half‐life of 44.3 days. In aerobic soil, metolachlor was very stable with only 49% dissipation in 130 days. Residues remained in both the soils up to the end of the experimental period of 190 days.     

The dissipation of hexazinone in tropical soils under semi-controlled field conditions in Kenya

The dissipation of hexazinone (Velpar) in two tropical soil types in Kenya was studied under field and semi-controlled conditions for a period of 84 days. The dissipation was found to be very rapid and this could be attributed to adverse weather conditions including high initial rainfall as well as to low soil-organic-matter content, volatilization, surface run-off and biodegradation. The DT₅₀ values of dissipation obtained by first order kinetics were 20 days and 21.3 days in clay and loam soil types, respectively. The influence of bargasse compost (1000 μg/g dry soil) was also studied and was found to enhance dissipation to some extent, giving DT₅₀ values of 18 days and 18.3 days in clay and loam soil types, respectively.     

Mathematical prediction of imidacloprid persistence in two Croatian soils with different texture,organic matter content and acidity under laboratory conditions

In the present laboratory study, persistence of imidacloprid (IMI) as a function of initial insecticide concentration and soil properties in two Croatian soils (Krk sandy clay and Istria clay soils) was studied and described mathematically. Upon fitting the obtained experimental data for the higher concentration level (5 mg/kg) to mathematical models, statistical parameters (R ², scaled root mean squared error and χ ² error) indicated that the single first-order kinetics model provided the best prediction of IMI degradation in the Krk sandy clay soil, while in the Istria clay soil biphasic degradation was observed. At the lower concentration level (0.5 mg/kg), the biphasic models Gustafson and Holden models as well as the first-order double exponential model fitted the best experimental data in both soils. The disappearance time (DT₅₀) values estimated by the single first-order double exponential model (from 50 to 132 days) proved that IMI can be categorized as a moderately persistent pesticide. In the Krk sandy clay soil, resulting DT₅₀ values tended to increase with an increase of initial IMI concentration, while in the Istria clay soil, IMI persistence did not depend on the concentration. Organic matter of both experimental soils provided an accelerating effect on the degradation rate. The logistic model demonstrated that the effect of microbial activity was not the most important parameter for the biodegradation of IMI in the Istria clay soil, where IMI degradation could be dominated by chemical processes, such as chemical hydrolysis. The results pointed that mathematical modeling could be considered as the most convenient tool for predicting IMI persistence and contributes to the establishment of adequate monitoring of IMI residues in contaminated soil. Furthermore, IMI usage should be strictly controlled, especially in soils with low organic matter content where the risk of soil and groundwater contamination is much higher due to its longer persistence and consequent leaching and/or moving from soil surface prior to its degradation.     

Behavior of methyl isothiocyanate in soils under field conditions in Morocco

The behavior of methyl isothiocyanate (MITC), active metabolite of metam-sodium (MS), was studied under field conditions in Morocco. MS was applied through drip irrigation in: (i) uncovered soil, (ii) soil covered with transparent polyethylene, and (iii) soil covered with virtual impermeable film. Concentrations of MITC were determined at different soil depths to determine the distribution of MITC and the concentration-time product (CTP). Six hours after MS application, in a sandy soil, MITC reaches the 20-30 cm soil layer, but remains highly concentrated in the upper 10-20 cm soil layer. In a silty clay soil, MITC was concentrated in the upper 0-10 cm soil layer. The dissipation of MITC under different conditions of application was fast and complete after seven days. However, MITC dissipation time (DT(50)) was <24 h in sandy soil treated, but 63 h in silty clay soil. Under these application conditions of MS, the plastic film reduced MITC loss to the atmosphere but the plastic film quality did not affect the behavior of MITC. The use of plastic film maintained high MITC concentrations and appropriate CTP at different soil depths.     

Field dissipation of oxyfluorfen in onion and its dynamics in soil under Indian tropical conditions

Oxyfluorfen, a diphenyl-ether herbicide is being used to control annual and perennial broad-leaved weeds and sedges in a variety of field crops including onion. The present study was aimed to investigate the dynamics and field persistence of oxyfluorfen in onion plant, bulb and soil under Indian tropical conditions. Application of four rates of oxyfluorfen viz., 200, 250, 300 and 400 g AI ha^?1 as pre-emergence gave good weed control in field experiment with onion. The oxyfluorfen residue dissipated faster in plant than in soil respectively, with a mean half-life of 6.1 and 11.2 days. Dissipation followed first-order kinetics. In laboratory column leaching experiments, 17 percent of the applied oxyfluorfen was recovered from the soil and indicates its solubility in water and mobility in sandy clay loam soil was low. A sorption study revealed that the adsorption of oxyfluorfen to the soil was highly influenced by the soil organic carbon with the K_oc value of 5450. The study concludes that the dissipation of oxyfluorfen in soil and onion was dependent on the physico-chemical properties of the soil and environmental conditions.     

Elimination of racemic and enantioenriched metalaxyl based fungicides under tropical conditions in the field

The elimination has been studied of racemic and enantioenriched metalaxyl applied as an emulsifiable concentrate and wettable powder combined with copper in a Cameroonian field site. The kinetics of the degradation/dissipation of metalaxyl and its acid metabolite were investigated using reversed phase HPLC-MS/MS, while the enantiomeric ratios were measured by HPLC-MS/MS using a Chiralcel OD-H HPLC column. Some soil enzymes activities were determined concurrently for 120d. The elimination of racemic metalaxyl was shown to be enantioselective, with the R-enantiomer being degraded more slowly than the S-enantiomer. Dissipation followed approximate square root first-order kinetics (R>0.98) without lag phases. The enantiomers of metalaxyl have different elimination rates, with half-lives ranging from only 0.8 to 1.5 days. After application to soil, the elimination of metalaxyl in the copper containing formulation was slower. The activities of acid phosphatase, alkaline phosphatase, and alkaline glucosidase were monitored throughout the experiments. No significant influence of metalaxyl and copper could be observed on these parameters. The significantly shorter half-life values of all forms of metalaxyl under field conditions, compared to the previously reported laboratory derived ones, may have implications for the plant disease control with these fungicides in tropical rainforest areas.     

Dissipation and mobility of permethrin in the field with repeated applications under tropical conditions

Studies on persistence, mobility and the effect of repeated application of permethrin on its half-life were carried out under field conditions. The half-life of permethrin in the top 20 cm of the soil increased from 11.5 to 23.6 days as the application rates increased from 35 to 140 g ha(-1). Induced by heavier rainfall, more residues moved downward in trial 2 than in trial 1. Repeated applications enhanced degradation rates and mobility of permethrin in the soil. The residue level in the 0-5-cm layer was reduced at day 28 after 17 consecutive applications to a level lower than after 5 applications. The half-life of permethrin was reduced from 15.9 days to 11.2 days after 5 and 17 applications, respectively. The residue reached the 15-20 cm layer much earlier (approximately 3 days after treatment) in soil that received 17 applications as compared to those with two applications.     

Acetochlor mineralization and fate of its two major metabolites in two soils under laboratory conditions

The degradation of the herbicide acetochlor, in a neoluvisol and in a calcosol were studied as a function of depth (0-25cm and 25-50cm) and temperature (25 degrees C and 15 degrees C) under controlled laboratory conditions during 58 and 90 days, respectively. The surface and sub-surface soil samples were respectively spiked with 1 and 0.01mgkg(-1) of 14C-acetochlor, the concentrations observed in previous field monitoring. The half-lives (DT50) varied from 1.4 to 14.9 days depending on the soil, temperature and applied concentration. The maximal mineralization (24%) was observed for the surface calcosol at 25 degrees C. The comparison of results obtained for sterilized and non-sterilized soils, the decrease of DT50 with the increase of temperature, the shape of CO2 emissions and the increase of number of aerobic endogenous microflora through the experiment suggested that biological process are dominant in degradation. A particular attention was paid to the formation and dissipation of metabolites ESA (ethanesulphonic acid) and OA (oxanilic acid) during the whole experiment. At 25 degrees C, ESA and OA were observed after three days, but as ESA concentration decreased over time in surface calcosol, it remained constant in surface neoluvisol. A difference in ESA/OA ratio depends on the soil with a predominance of OA in surface neoluvisol and a disappearance of OA in surface calcosol.     

Interactions of tillage and rainfall on atrazine leaching under field and laboratory conditions

Laboratory studies were conducted to evaluate effects of tillage reversal and rainfall on ¹⁴C-atrazine (2-chloro4-ethylamino-6-isopropylamino- -triazine) leaching patterns. Twelve intact soil cores (16 cm dia x 20 cm deep) were collected from 8-yr no-till (NT) fields. Half the cores were tilled (5 cm deep) prior to ¹⁴C-atrazine treatment (2.7 mg core⁻¹) to all cores. All cores received two rains (27 mm rain in 1.5 h, one day after application followed, two days later, by a 17 mm rain in 2.5 h) and leachate was collected and analyzed for atrazine. These rains simulated the timing, amount and duration of natural rainfall events from a tillage reversal field study. During the first high inte ity rainfall event, a pulse (2.1 μg L-1) of atrazine leached through tilled cores while leaching rate was linear and decreased (1.25 to 0.9 μg L-1) through un-tilled cores. Leaching rate was linear for both the tilled and un-tilled cores during the second rain. Less atrazine was left in the surface 5 cm of tilled soil than un-tilled after the two rains. Results confirmed field observations and suggested that when tillage is reversed on well structured soils, pesticide leaching may increase relative to un-tilled soil but these effects are probably confined to the first rain events after application only.     

Persistence of herbicide fenoxaprop ethyl and its acid metabolite in soil and wheat crop under Indian tropical conditions

The persistence of fenoxaprop ethyl {Ethyl (RS)-2-[4-(6-chloro-1,3-benzoxazol-2-yloxy) phenoxy] propionate} herbicide and its active metabolite fenoxaprop acid was investigated in soil and wheat crop. Fenoxaprop acid was prepared by alkaline hydrolysis of fenoxaprop ethyl. A HPLC method was developed in which fenoxaprop ethyl herbicide and its acid metabolite showed sharp single peak at 6.44 and 2.61 min respectively. The sensitivity of the method for ester and acid was 2 and 1 ng respectively with limit of detection of 0.1 and 0.05 μg mL^?1. The recovery of fenoxaprop ethyl and fenoxaprop acid from soil, wheat straw and grain ranged between 73.8–80.2%. In a field experiment fenoxaprop ethyl (Puma super® 10 EC) when applied to wheat crop at the rate of 120 g and 240 g a.i. ha^-1 as post emergence spray, fenoxaprop ethyl converted to fenoxaprop acid. Residues of fenoxaprop ethyl and acid dissipated in soil with a half-life of 0.5 and 7.3 days, respectively. At harvest no detectable residues of fenoxaprop ethyl or acid were observed in soil, wheat grain and straw samples.     

Assessment of metolachlor and diuron leaching in a tropical soil using undisturbed soil columns under laboratory conditions

In the present study, diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea] and metolachlor [2-chloro-N-(2-ethyl-6-methylphenyl)-N-(2-metoxi-1-methylethyl)acetamide] leaching was studied in undisturbed soil columns collected in a cotton crop area in Mato Grosso State, Brazil. The pesticides were applied to the soil surface in dosages similar to those used in a cotton plantation. To assess the leaching process, soil columns were submitted to simulated rain under laboratory conditions at 25 ± 3°C, in the absence of wind and direct solar radiation. During the rain simulations, leachate solutions were collected and herbicide concentrations were determined. At the end of the experiment, the soil columns were cut into 10 cm sections to determine the remaining herbicide concentrations through the soil profile. Metolachlor was detected in all soil sections, and approximately 4% of the applied mass was leached. Diuron was detected only in the upper two soil sections and was not detected in the leachate. A linear correlation (r > 0.94) between the metolachlor soil concentrations and the organic contents of the soil sections was observed. Mass balance suggests that around 56% of diuron and 40% of metolachlor were degraded during the experiments. Measurements of the water table depth in the area where the samples were collected showed that it varied from 2 to 6 m and is therefore vulnerable to contamination by the studied herbicides, particularly metolachlor, which demonstrated a higher leaching potential.     

Dissipation and translocation of saisenxin in tobacco and soil under conventional field and controlled laboratory conditions

Abstract

The aim of this investigation was to investigate the fate and translocation characteristics of saisenxin (SSX), a novel organic zinc fungicide, in the environment and tobacco plants under conventional field and laboratory conditions. A rapid and sensitive analytical technique based on high-performance liquid chromatography was used for determination of SSX, in soil samples and tobacco leaf, stem and root samples. The method had satisfactiry linearity (R^2?=?0.9999) and the limits of detection and of quantitation of the target compound were 0.06 and 0.20?mg kg^?1, respectively. The average recoveries were in the range of 89.74–94.24% in soil, leaf, stem and root samples, with relative standard deviations of <8%. For conventional field trials, the half-life (t_1/2) of SSX was 5.9–6.5 days in soil and 4.8–5.3 days in tobacco leaves; the corresponding values under controlled laboratory conditions were extended to 7.1 and 7.6?days. The translocation factor (TF) values were in the range of 0–2.25 and 0–0.25 for foliage and root irrigation treatments, respectively. The TFs of SSX in tobacco indicated that tobacco had a high ability to transfer SSX upward.     

Impact of hairy vetch cover crop on herbicide transport under field and laboratory conditions.

This study was conducted to evaluate the effect of hairy vetch cover crop residue on runoff losses of atrazine and metolachlor under both no-till corn field plots and from a laboratory runoff system. A 2-year field study was conducted in which losses of atrazine and metolachlor from vetch and non-vetch field plots were determined from the first runoff event after application (5 and 25 days after application in 1997 and 1998, respectively). A laboratory study was conducted using soil chambers, designed to simulate field soil, water, vegetation, and herbicide treatment conditions, subjected to simulated rain events of 5, 6, 20 and 21 days after application, similar to the rainfall pattern observed in the field study. Atrazine losses ranged from 1.2 to 7.2% and 0.01 to 0.08% and metolachlor losses ranged from 0.7 to 3.1% and 0.01 to 0.1% of the amount applied for the 1997 and 1998 runoff events, respectively. In the laboratory study, atrazine runoff losses ranged from 6.7 to 22.7% and 4.2 to 8.5% and metolachlor losses ranged from 3.6 to 9.8% and 1.1 to 4.7% of the amount applied for the 5-6 and 20-21 day events, respectively. The lower losses from the field study were due to smaller rainfall amounts and a series of small rains prior to the runoff event that likely washed herbicides off crop residue and into soil where adsorption could occur. Runoff losses of both herbicides were slightly higher from non-vetch than vetch field plots. Losses from the laboratory study were related to runoff volume rather than vegetation type.     

Effects of three pesticides on the avoidance behavior of earthworms in laboratory tests performed under temperate and tropical conditions

Little research has been performed on the impact of pesticides on earthworms under tropical conditions. Taking into consideration the often-limited resources in tropical countries, simple screening tests are needed. Therefore, it was investigated whether three pesticides relevant for the Brazilian Amazon (benomyl, carbendazim, lambda-cyhalothrin) affect the avoidance behavior of the earthworm Eisenia fetida. The tests were performed for two days according to ISO guideline 17512 but were adapted to tropical conditions (i.e. test substrate, test organism and temperature). The results indicate that this test gives reproducible and reliable results. Toxicity values (NOEC, EC50) are lower than those determined in 14 day-acute mortality tests and are approximately in the same range such as those found in 56 day-chronic reproduction tests with the same earthworm species, which were performed in parallel. Therefore, the use of the earthworm avoidance tests is recommended as a screening tool for the risk assessment of pesticides.     

Pesticide dispersion by spraying under tropical conditions

Pesticide air pollution by spraying was evaluated under different temperature, humidity and wind climatic conditions in Brazil. Field experiments were performed with application towards the soil and in guava orchards, where spray dispersion was monitored by adding p-aminobenzoic acid (PABA), a fluorescent substance, as a tracer to the water contained in the spray tanks. Samples were collected with filter membranes (Whatman 180025), and the PABA was extracted from the filters by shaking with water in a Petri dish and measured in a spectrofluorometer. A spray aimed towards the soil with filters positioned on the ground and hung at different heights did not show different upward dispersion as observed when lateral pulverization was conducted. In this case, a tractor with a sprayer moved through a 3 m high and 6 m wide frame with filter membranes mounted at 60 cm intervals. Spray dispersion patterns were modified by guava leaf resistance. No influence of temperature and humidity was observed in this short-lived spraying process. Nevertheless, wind drift can occur during airborne dispersion and is an important pesticide pollution source which requires control. Droplets with PABA powered by assisted spraying upwards returned to the ground by gravity and, therefore, did not constitute a vertical source of atmospheric pollution.     

Strategies for reducing airborne pesticides under tropical conditions

Brazil is currently one of the largest pesticide consumers worldwide. However, a lack of scientific information regarding airborne pollution is still an issue, with tragic consequences to human health and the environment. To reduce pollution of the lower air layers, where pesticide spraying occurs, green barriers that filter the air could be an effective mitigation procedure. Modifying pulverization habits, by pulverizing in the late afternoon instead of in the morning could also reduce pesticide volatilization, while other recommendations with the purpose of lowering the pesticide amounts currently applied are likewise pursued. Data obtained about volatilization have demonstrated that, in order to reduce air pollution risks, one of the most effective preventive strategies is to ban products with high vapor pressure. Global/local stakeholders need to assume the responsibility to find the best way to reduce airborne pesticide pollution, which has increasingly shown disastrous effects as major poisons to human health and the environment.     

Behaviour of DDT under laboratory and outdoor conditions in Germany

Abstract

Experiments were conducted on adsorption, volatilization and UV‐degradation of p,p'‐DDT on soil surface, and leaching and degradation in sand columns. p,p'‐DDT was shown to adsorb stronger to soils with higher organic content. UV irradiation at 290 nm for 10 hours mineralized less than 0.1% of DDT in soil.

Results show that only 0.1% of DDT volatilized in a sun‐exposed semi‐closed quartz system. Polar compounds accounted from 1.4% after 55 days. The rate of volatilization and degradation in an open system was much higher; only 15% DDT and 7% DDE were recovered after 6 weeks in the organic extract. p,p'‐DDT was adsorbed to a great extent on the top layers of sand columns; 86% in the top 8 cm.     

Dissipation of oxytetracycline in soils under different redox conditions

This study investigated the dissipation kinetics of oxytetracycline in soils under aerobic and anoxic conditions. Laboratory experiments showed that the dissipation of oxytetracycline in soil followed first-order reaction kinetics and its dissipation rates decreased with increasing concentration. Oxytetracycline dissipated faster in soil under aerobic conditions than under anoxic conditions. The half-lives for oxytetracycline in soil under aerobic conditions ranged between 29 and 56 days for non-sterile treatments and 99-120 days for sterile treatments, while under anoxic conditions the half-lives of oxytetracycline ranged between 43 and 62 days in the non-sterile soil and between 69 and 104 days in the sterile soil. This suggests microbes can degrade oxytetracycline in agricultural soil. Abiotic factors such as strong sorption onto soil components also played a role in the dissipation of oxytetracycline in soil.     

Photochemical nitro-nitrite rearrangement in methyl parathion decay under tropical conditions

This work presents a study of the abiotic degradation of commercially available methyl parathion in aqueous solution at two different concentrations (88 mg/L and 200 μg/L). The effects of solar irradiation and the presence of humic acids were evaluated and revealed a synergistic response between them. The half-life of methyl parathion ranged from 4.9 to 37 days, and the experimental data also show that photochemical processes were the most relevant in this case. The only byproduct found in samples submitted to shadowed conditions was 4-nitrophenol. On the other hand, 4-nitrophenol, methyl paraoxon and a new degradation product (O,O-dimethyl O-p-hydroxyphenyl phosphorothioate) were detected when the samples were exposed directly to sunlight. This newly identified compound was prepared in the laboratory by thiophosphorylation of hydroquinone, and coelution experiments with authentic samples provided unambiguous confirmation of the presence of O,O-dimethyl O-p-hydroxy phenylphosphorothioate in samples.     

Butachlor degradation in tropical soils: effect of application rate, biotic-abiotic interactions and soil conditions

The degradative characteristics of butachlor (N-Butoxymethyl-2-chloro-2',6'-diethyla- cetanilide) were studied under controlled laboratory conditions in clay loam alluvial (AL) soil (Typic udifluvent) and coastal saline (CS) soil (Typic endoaquept) from rice cultivated fields. The application rates included field rate (FR), 2-times FR (2FR) and 10-times FR (10FR). The incubation study was carried out at 30 degrees C with and without decomposed cow manure (DCM) at 60% of maximum water holding capacity (WHC) and waterlogged soil condition. The half-life values depended on the soil types and initial concentrations of butachlor. Butachlor degraded faster in AL soil and in soil amended with DCM under waterlogged condition. Microbial degradation is the major avenue of butachlor degradation from soils.