Soil persistence studies with bromoxynil, propanil and [14C]dicamba in herbicidal mixtures

The persistence of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), [¹⁴C]dicamba (3,6-dichloro-2-methoxybenzoic-7-¹⁴C acid) and propanil [N-(3,4-dichlorophenyl)propionamide] at rates equivalent to 1 kg ha^?1, were studied under laboratory conditions in a clay loam, a heavy clay and a sandy loam at 85% of field capacity and at 20±1°C, both singly and in the presence of herbicides normally applied with these chemicals as tank-mix or split-mix components. The degradation of bromoxynil was rapid with over 90% breakdown occurring within a week in the heavy clay and sandy-loam soils, while in the clay-loam approximately 80% of the bromoxynil had broken down after 7 days. In all three soils degradation was unaffected by the presence of asulam, diclofop-methyl, flamprop-methyl, MCPA, metribuzin or propanil. Propanil underwent rapid degradation in all soil treatments, with over 95% of the applied propanil being dissipated within 7 days. There were no noticeable effects on propanil degradation resulting from applications of asulam, barban, bromoxynil, dicamba, MCPA, MCPB, metribuzin or 2,4-D. The breakdown of [¹⁴C]dicamba in a particular soil was unaffected by being applied alone or in the presence of diclofop-methyl, flampropmethyl, MCPA, metribuzin, propanil or 2,4-D. The times for 50% of the applied dicamba to be degraded were approximately 16 days in both the clay loam and sandy loam, and about 50 days in the heavy clay.     

Perfusion studies with bromoxynil octanoate in soil

The breakdown of bromoxynil octanoate in 5 different soil types has been studied in a soil perfusion apparatus using herbicide labelled with ¹⁴C either in the cyano group or in the aromatic ring. Even when applied at rates equivalent to 5 to 25 times those used commercially, the herbicide was fairly rapidly and extensively degraded at 15°. After 12 to 13 weeks, up to 80% of the radioactivity in the ¹⁴CN group and up to 63 % of the ¹⁴C in the ring were liberated as carbon dioxide. A small proportion (16 to 19%) of the radioactivity from ring-labelled herbicide remained attached to the soil, probably not as the original herbicide, but in a form not readily leached. Only trace quantities of 3,5-dibromo-4-hydroxy-benzamide (0.5%) and 3,5-dibromo-4-hydroxybenzoic acid (0.1%) were detectable during these soil perfusions.     

Biodegradation of the herbicide bromoxynil and its plant cell wall bound residues in an agricultural soil

The mineralization and formation of metabolites and nonextractable residues of the herbicide [¹⁴C]bromoxyniloctanoate ([¹⁴C]3,5-dibromo-4-octanoylbenzonitrile) and the corresponding agent substance [¹⁴C]bromoxynil ([¹⁴C]3,5-dibromo-4-hydroxybenzonitrile) was investigated in a soil from an agricultural site in a model experiment. The mineralization of maize cell wall bound bromoxynil residues was also investigated in the agricultural soil material. The mineralization of [¹⁴C]bromoxynil and [¹⁴C]bromoxyniloctanoate in soil within 60 days amounted up to 42 and 49%, respectively. After the experiments, 52% of the originally applied [¹⁴C]bromoxynil and 44% of the [¹⁴C]bromoxyniloctanoate formed nonextractable residues in soil. Plant cell wall bound [¹⁴C]bromoxynil residues were also mineralized to an extent of about 21% within 70 days; the main portion of 76% persisted as nonextractable residues in the soil. In bacterial enrichment cultures and in soil two polar metabolites were observed; one of it could be identified as 3,5-dibromo-4-hydroxybenzoate and the other could be described tentatively as 3,5-dibromo-4-hydroxybenzamide.     

Soil persistence studies with [14C]MCPA in combination with other herbicides and pesticides

The persistence of [¹⁴C]MCPA at a rate equivalent to 1 kg ha^?1 was studied under laboratory conditions in a clay loam, heavy clay and sandy loam at 85% of field capacity moisture and 20±1°C both alone and in the presence of tri-allate, trifluralin, tri-allate and trifluralin, malathion, Vitaflow DB, malathion and Vitaflow DB, bromoxynil, bromoxynil and asulam, bromoxynil and difenzoquat, dicamba, dicamba and mecoprop, linuron, MCPB, metribuzin, propanil, TCA, benzoylprop-ethyl, diclofop-methyl, and flamprop-methyl. Except in the soils treated with asulam, the half-lives of [¹⁴C]MCPA in all three soil types were similar, being approximately 13±1 days, thus indicating that none of the other chemicals studied adversely affected the soil degradation of MCPA. In the asulam treated soils, the half-lives of the MCPA were about 3 days longer than in non-asulam treated soils; the effect was most marked in the clay loam.     

An analytical procedure for bromoxynil and its octanoate in soils; persistence studies with bromoxynil octanoate in combination with other herbicides in soil

A method is described for the analysis of the herbicide bromoxynil and its octanoate in soils. Following extraction with aqueous acidic acetonitrile, the octanoate was separated from the phenolic bromoxynil by solvent partitioning. The ester and the phenol were assayed by gas-liquid chromatography without further modification or preparation of a derivative. Recoveries in excess of 93% were obtained from soils treated with the phenol and the ester at levels of 0.5 or 0.1 μg g^?1. The persistence of bromoxynil octanoate applied at a rate of 3 μg g^?1 was studied in the laboratory on a heavy clay and a sandy loam at 85% of field capacity moisture and 20°1°C, both alone and in the presence of 2,4-D (2 μg g^?1); MCPA (2 μg g^?1); MCPA+asulam (both at 2 μg g^?1); and MCPA+difenzoquat (both at 2 μg g^?1). In each soil there was a rapid conversion of bromoxynil octanoate to the free phenol, which then underwent a rapid degradation, so that after 7 days, over 90% of the original treatment had disappeared. There appeared to be no effect on bromoxynil breakdown by any of the herbicides added in combination. Small field plots were treated, in early May 1977 and 1978 at two locations in Saskatchewan, with a combination of commercial formulations containing asulam, bromoxynil octanoate, and MCPA at rates of 1 kg ha^?1 each. After 10 weeks the plots were sampled and analysis showed that in all cases, no asulam, bromoxynil, or bromoxynil octanoate could be extracted from the top 10 cm of soil.     

Communication to the Editor Degradation of Bromoxynil,Ioxynil, Dichlobenil and their Mixtures by Agrobacterium radiobacter 8/4

Degradation of three benzonitrile herbicides, bromoxynil (3,5-dibromo-4-hydroxybenzonitrile), ioxynil (3,5-diiodo-4-hydroxybenzonitrile), dichlobenil (2,6-dichlorobenzonitrile), and their mixtures by the soil micro-organism Agrobacterium radiobacter 8/4 was studied in batch cultures. Bromoxynil was found to be most rapidly degraded, while dichlobenil had the lowest toxicity to our strain. All transformations of studied benzonitriles were performed by the nitrile hydratase which has been shown to act on a broad range of substituted aromatic nitriles. © 1997 SCI.     

Demonstrating formation of potentially persistent transformation products from the herbicides bromoxynil and ioxynil using liquid chromatography-tandem mass spectrometry (LC-MS/MS)

It is shown that potentially persistent transformation products can be formed from the herbicides bromoxynil (3,5-dibromo-4-hydroxybenzonitrile) and ioxynil (3,5-diiodo-4-hydroxybenzonitrile), and possible leaching to groundwater is discussed. A similar process to the formation of BAM (2,6-dichlorobenzamide) from the herbicide dichlobenil (2,6-dichlorobenzonitrile) can be anticipated as bromoxynil and ioxynil are analogues of dichlobenil and they are degraded by the enzymes nitrilase, nitrile hydratase and amidase. A biodegradation study using cultured Variovorax sp. DSM 11402, a species commonly found in soil, demonstrated that ioxynil and bromoxynil were fully transformed into their corresponding amides in 2-5 days. These amides were not further degraded within 18 days, and formation of other degradation products was not observed. These results are in agreement with biodegradation experiments with dichlobenil. In soil, dichlobenil is transformed into its only observed degradation product BAM, which is persistent and mobile, and has been found in 19% of 5000 samples of Danish groundwater. Variovorax sp. is known to degrade the non-halogenated analogue benzamide, suggesting that degradation of the three amides may be hindered by the halogenated substituents (meta-Br; meta-I; ortho-Cl). This hypothesis is supported by QSAR modelling of fundamental properties. Using a new optimised liquid chromatography-tandem mass spectrometry (LC-MS/MS) method, the sorption and desorption properties of bromoxynil and ioxynil were characterised in sandy topsoil at four concentration levels. The estimated sorption coefficient K(d) was 1.4 L kg(-1) for bromoxynil and 5.4 L kg(-1) for ioxynil, indicating weak to moderate sorption to topsoil. Desorption of the herbicides showed that they were strongly and irreversible bound to the soil (K(des) > K(d)). The amount of herbicide desorbed depended on the initial concentration level. At low levels, K(des) values were higher, indicating stronger binding than at higher levels. The isocratic LC-MS/MS method developed for simultaneous detection of bromoxynil, ioxynil and their main degradation products is described. Using negative electrospray ionisation (ESI-), the detection limits were 0.4-1.0 microg L(-1), with relative standard deviations of 4-10% (n = 10) using direct injection without clean-up steps. The standard curves showed linearity in the range 5-100 microg L(-1) with r(2) > 0.992.     

Persistence of pronamide in soil

Under field conditions, there was little loss of herbicidal activity following spring application of pronamide when the soil temperature remained below about 13°c, but under normal summer conditions loss was rapid (half-life 2–4 weeks). The rate of loss was retarded when the surface soil became very dry. After autumn application, there was no change in activity during the winter months and assays on samples taken in the following spring showed that little leaching had taken place from the surface 5 cm. In laboratory studies, breakdown was shown to follow first-order kinetics. Half-lives at 10% soil moisture were 29 days at 23°c, 63 days at 15°c and 140 days at 8°c. At 23°c the half-life was extended to 52 days when the soil moisture content was reduced by half.     

Fate of fluroxypyr in soil

Fluroxypyr-MHE (methylheptyl ester) was added to four soils and incubated at 26 ± 1°C and approximately 0.1 MPa moisture. After initial rapid hydrolysis of the ester to fluroxypyr, fluroxypyr degraded with half-lives of 12, 12, 23, and 7 days in Barnes loam, Catlin silt loam, Hanford sandy loam, and Mhoon clay soils, respectively. Two metabolites (4-amino-3,5-dichloro-6-fluoro-pyridin-2-ol and 4-amino-3, 5-dichloro - 6 - fluoro - 2 -methoxypyridine) were identified, with the pyridinol at its maximum concentration after 2 to 4 weeks of incubation, and the methoxypyridine after 8 weeks. Degradation rates of fluroxypyr and its pyridinol were not significantly altered by diurnally varying soil temperature (21°C to 32°C) or moisture, nor by the presence of growing grass. Methoxypyridine dissipation was more rapid under greenhouse conditions, suggesting that laboratory studies underestimated the dissipation rate of this metabolite.     

Simulation of herbicide persistence in soil .III. Propyzamide in different soil types

The effects of soil temperature and soil moisture content on the rate of degradation of propyzamide in five soils were examined under controlled laboratory conditions. Half-lives in soils incubated at field capacity varied from 23 to 42 days at 25°C and from 63 to 112 days at 15°C. The variation in half-life at 25°C and 50% of field capacity was from 56 to 94 days. When the laboratory data were used in conjunction with the relevant meteorological records and soil properties in a computer simulation program, predicted degradation curves for propyzamide in four of the soils in micro-plots were in close agreement with those observed. Use of the program to predict residues of propyzamide in the fifth soil at crop maturity in a series of field experiments concerned with continuity of lettuce production gave values fairly close to those observed when appropriate corrections were made for initial recoveries.     

Simulation of herbicide persistence in soil .I. Simazine and prometryne

The effects of soil temperature and soil moisture content on the rates of degradation of simazine and prometryne were measured under controlled conditions. The time for 50% disappearance of simazine in a sandy loam soil varied from 37 days at 25°C and 13 % soil moisture to 234 days at 15°C and 7% soil moisture. With prometryne, changes in soil moisture content had a greater effect on the rate of loss than similar changes with simazine. The time for 50% disappearance at 25°C was increased from 30 to 590 days with a reduction in soil moisture content from 14 to 5%. With both herbicides, the rate of degradation increased as the initial herbicide concentration decreased and the data suggest that a hyperbolic rate law may be more appropriate than simple first-order kinetics. Degradation curves for three separate field applications of the two herbicides were simulated using the laboratory data and the relevant meteorological records in a computer program. A close fit to the observed pattern of loss of incorporated prometryne was obtained, but prometryne surface-applied was lost rapidly during the first 30–40 days after application. This initial rapid loss could not be predicted by the program. With simazine, the patterns of loss of surface and incorporated treatments were similar, but the simulation model tended to overestimate residue levels. Possible reasons for the discrepancies are discussed.     

Factors affecting degradation rates of five triazole fungicides in two soil types: 1. Laboratory incubations

Triazole fungicides are now widely used commercially and several are known to be persistent in soil. The degradation rates of five such fungicides were measured in laboratory tests with two soils over 720 days, with analysis of soil extracts by high-pressure liquid chromatography. Behaviour in a sandy loam and a clay loam were similar, and incubation of the compounds either singly or in admixture did not influence loss rates except for those of flutriafol which were lower in the latter. Triadimefon was quite rapidly reduced to triadimenol, though traces of the former were always found, indicating a possible redox equilibrium. Flutriafol, epoxiconazole and triadimenol (derived from triadimefon) were very persistent, breakdown following first-order kinetics with half-lives greater than two years at 10 °C and 80% field capacity. Propiconazole was moderately persistent, with a half-life of about 200 days under these conditions. Degradation rates increased about 3-fold as the temperature was increased from 5 to 18 °C, though decreasing soil moisture to 60% field capacity only slightly slowed degradation. The rate constants obtained are used in a companion paper describing field studies on these two soils to compare laboratory-measured degradation rates with losses in the field following commercial sprays. © 1999 Society of Chemical Industry     

Fate of metsulfuron-methyl in soils in relation to pedo-climatic conditions

The dependence of the behaviour of metsulfuron-methyl on soil pH was confirmed during incubations under controlled laboratory conditions with two French soils used for wheat cropping. The fate of [¹⁴C] residues from [triazine-¹⁴C]metsulfuron-methyl was studied by combining different experimen-tal conditions: soil pH (8·1 and 5·2), temperature (28 and 10°C), soil moisture (90 and 50% of soil water holding capacity) and microbial activity (sterile and non-sterile conditions). Metsulfuron-methyl degradation was mainly influenced by soil pH and temperature. The metsulfuron-methyl half-life varied from five days in the acidic soil to 69 days in the alkaline soil. Under sterile conditions, the half-life increased in alkaline soil to 139 days but was not changed in the acidic soil. Metsulfuron-methyl degradation mainly resulted in the formation of the amino-triazine. In the acidic soil, degradation was characterised by rapid hydrolysis giving two specific unidentified metabolites, not detected during incubations in the alkaline soil. Bound residues formation and metsulfuron-methyl mineralisation were highly correlated. The extent of bound residue formation increased when soil water content decreased and was maximal [48 (±4)% of the applied metsulfuron-methyl after 98 incubation days] in the acidic soil at 50% of the water holding capacity and 28°C. Otherwise, bound residues represented between 13 and 32% of the initial radioactivity. © 1998 SCI     

Diuron mineralisation in a Mediterranean vineyard soil: impact of moisture content and temperature

BACKGROUND: The diuron‐mineralising ability of the microbiota of a Mediterranean vineyard soil exposed each year to this herbicide was measured. The impact of soil moisture and temperature on this microbial activity was assessed. RESULTS: The soil microbiota was shown to mineralise diuron. This mineralising activity was positively correlated with soil moisture content, being negligible at 5% and more than 30% at 20% soil moisture content. According to a double Gaussian model applied to fit the dataset, the optimum temperature/soil moisture conditions were 27.9 °C/19.3% for maximum mineralisation rate and 21.9 °C/18.3% for maximum percentage mineralisation. The impact of temperature and soil moisture content variations on diuron mineralisation was estimated. A simulated drought period had a suppressive effect on subsequent diuron mineralisation. This drought effect was more marked when higher temperatures were used to dry (40 °C versus 28 °C) or incubate (28 °C versus 20 °C) the soil. The diuron kinetic parameters measured after drought conditions were no longer in accordance with those estimated by the Gaussian model. CONCLUSION: Although soil microbiota can adapt to diuron mineralisation, its activity is strongly dependent on climatic conditions. It suggests that diuron is not rapidly degraded under Mediterranean climate, and that arable Mediterranean soils are likely to accumulate diuron residues. Copyright © 2010 Society of Chemical Industry     

Photochemistry of halogenated benzene derivatives. Part IX. environmental aquatic phototransformation of bromoxynil (3,5-dibromo-4-hydroxybenzonitrile)

Photochemistry of the herbicide 3,5-dibromo-4-hydroxybenzonitrile (bromoxynil) (I) was investigated by irradiating at approximately 313 nm. Aqueous phase photoreactions of 0.078–7.800 × 10^?5m solutions of I were carried out at different pH values. Quantum yields for the loss of I in buffered solutions were 0.008 (±0.0004), 0.048 (±0.0024), and 0.044 (±0.0022) at pH 2.6, 7.0, and 11.0, respectively. In neutral and basic conditions, I absorbed more strongly at wavelengths > 290 nm, an environmentally significant region. Phototransformation of I was monitored by HPLC and UV-VIS spectrometry. All photoreactions of I gave rise to the generation of two products, 3-bromo-4-hydroxybenzonitrile and 4-hydroxybenzonitrile. The former photoproduct was tentatively identified from its mass spectral data. The photoproducts can be accounted for with a proposed mechanism involving free radicals.     

Ecology of Xiphinema vuittenezi and Xiphinema pachtaicum in vineyards of north-east Croatia1

During a 3-year field study on two vineyards of north-eastern Croatia, the qualitative and quantitative composition as well as the vertical dynamics of Xiphinema spp. were determined each month. The greatest number of fertile X. vuittenezi females was noted in August-September at a soil moisture of 18–20%. The greatest number of larvae of this species was determined in September-October in a temperature range of 14–18°C and soil moisture of 18–22%. The development cycle of X. vuittenezi lasts about 24–33 months under natural conditions and that of its larval stages 3–8 months. The nematodes of this species are susceptible to high temperatures (above 20°C) and drought (under 13%). The greatest number of fertile females of X. pachtaicum was determined in July at a soil temperature of 20–24°C, absolute soil moisture of 16–20%. The greatest number of larvae was noted in September-October at a soil temperature of 16–21°C and soil moisture of 13–23%. The development cycle of X. pachtaicum in field conditions lasts about 12–13 months and that of the larval stages 2–3 months. This species demonstrated reduced activity at soil temperatures under 10°C and at soil moisture under 13%; larvae were less active than females at temperatures over 20°C. On the basis of the results obtained, it is suggested that sampling of vineyards to determine the distribution and population density of the two Xiphinema spp. should be performed at depths down to 50 cm in spring and autumn, which are also the most favourable times for nematicide application.     

DEGRADATION OF DICAMBA IN PRAIRIE SOILS

Summary. Electron-capture gas chromatography was used to detect dicamba residues in three prairie soils, and the breakdown under different moisture conditions was studied. At rates equivalent to 1·1 kg/ha degradation was rapid in all soils at 25°C and at moisture levels in excess of the wilting point, with over 50% of the applied dicamba being dissipated in 2 weeks. Negligible breakdown occurred in the sterile soils, over a 4–week period, indicating that microbial decomposition could be an important factor contributing to dicamba degradation.
The persistence of dicamba was investigated under field conditions at three locations using small plots. Applications equivalent to 1·1 kg/ha were made in October 1971 for analysis in May 1972, and in May 1972 for analysis in August 1972. At both sampling dates no dicamha residues were detected in any of the treatments at the 0–5, 5–10 or 10–15 cm soil levels.
Degradation du dicamba dans les sots de prairies     

Persistence of 2,4,5-T in a heavy clay soil

The persistence of the herbicide 2,4,5-T was studied at different controlled temperatures and moisture levels in Regina heavy clay. Degradation approximated to first-order kinetics and the half-life varied from about 4 days at 35°C and 34% soil moisture to about 60 days at 10°C and 20% soil moisture. The laboratory data were used in conjunction with the appropriate measurements of surface soil temperature and moisture content in the field to simulate the degradation pattern for the herbicide in five separate micro-plot experiments. Satisfactory agreement with the observed patterns of loss was obtained in two of the experiments but in the other three, the model over-estimated rates of loss. It is suggested that the reason for this was the difficulty of obtaining a correct measure of soil moisture content to use in the simulation program.     

Persistence studies with [14C] 2,4-D in soils previously treated with herbicides and pesticides

The persistence of [¹⁴C] 2,4-D at a rate equivalent to 1 kg/ha was compared under laboratory conditions in samples of heavy clay, sandy loam, and clay loam at 85% of field capacity moisture and 20 ± 1°C which had either received no pre-treatment, or had been pre-treated for 7 days at the 2 μg/g level with the herbicides benzoylprop-ethyl, diclofop-methyl, dinitramine, flamprop-methyl, nitrofen, picloram, tri-allate, trifluralin, and a combination of tri-allate and trifluralin. The breakdown of [¹⁴C] 2,4-D was also studied in the same soils that had similarly received pre-treatments of 2 μg/g of the cereal seed dressing Vitaflo-DB, the insecticide, malathion, and a combination of Vitaflo-DB and malathion. In each soil type, the half-life of the 2,4-D was similar regardless of whether the soil had, or had not, received any pre-treatment, indicating that none of the chemicals investigated adversely affected the soil degradation of 2,4-D.     

Degradation and adsorption of 14C-MCPA in soil—influence of concentration, temperature and moisture content on degradation

MCPA was weakly absorbed in soils with 2.4, 3.0 and 2.9% humus. K_d-values were 0.7, 0.9 and 1.0, respectively. In soil, not previously treated with MCPA, the degradation of 0.05 mg kg^?114C-MCPA followed first-order reaction kinetics whereas degradation of 5 mg kg^?1 was only first-order for 2 weeks; exponentially increasing degradation rates followed indicating enrichment of the soil with MCPA decomposers. Degradation was monitored by evolution of ¹⁴CO₂. The influence of temperature on degradation of MCPA (4 mg kg^?1) could initially be described by Q₁₀ values or by the Arrhenius equation. After 1 day of incubation in two field soils Q₁₀ values were 3.3 and 2.9, respectively, between 0°C and 29°C; the activation energies were 87 and 76 kj mol^?1. Exponentially increasing degradation rates followed with doubling times of about 4.0, 1.8, 1.2 and 0.6 days at 6,10, 15 and 21°C, respectively. After 51 days of incubation, at temperatures between 6°C and 29°C, about 60%¹⁴C was evolved in CO₂ and only traces of MCPA were left in the soil. At 0°C and at 40°C only 1% and 10%¹⁴C, respectively, were evolved as CO₂ after 51 days. ¹⁴C-MCPA (4 mg kg^?1) was incubated at moisture contents from that in air-dried soil to 2.3 times field capacity. Optimum for degradation was from 0.6 to 1.2. field capacity. Degradation was very slow where water contents were below the level of wilting point and was nil in air-dried soil. In wet soil degradation was delayed, but even in water-logged soil (2.3 times field capacity) MCPA was decomposed after 4 to 5 weeks at 10°C.