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.     

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.     

Determination of extractable and non-extractable radioactivity from prairie soils treated with carboxyl- and ring-labelled [14C]2,4-D

Ring- and carboxyl-labelled [¹⁴C]2,4-D were incubated under laboratory conditions, at the 2 g/g level, in a heavy clay, sandy loam, and clay loam at 85% of field capacity and 20 1C. The soils were extracted at regular intervals for 35 days with aqaeous acidic acetonitrile, and analysed for [¹⁴C]2,4-D and possible radioactive degradation products. Following solvent extraction, a portion of the soil residues were combusted in oxygen to determine unextracted radioactivity as [¹⁴C]carbon dioxide. The remaining soil residues were then treated with aqueous sodium hydroxide, and the radioactivity associated with the fulvic and humic soil components determined. In all soils there was a rapid decrease in the amounts of extractable radioacitivity, with only 5% of that applied being recoverable after 35 days. All recoverable radioactivity was attributable to [¹⁴C]2,4-D, and no [¹⁴C]-containing degradation products were observed. This loss of extractable radioactivity was accompanied by an increase in non-extractable radioactivity. Approximately 15% of the applied radioactivity, derived from carboxyl-labelled [¹⁴C]2,4-D, and 30% from the ring-labelled [¹⁴C]2,4-D was associated with the soil in a non-extractable form, after 35 days of incubation. After 35 days, less than 5% of the radioactivity from the carboxyl-labelled herbicide, and less than 10% of the ringlabelled material, was associated with the fulvic components derived from the three soils. Less than 5% of the applied radioactivities were identifiable with any of the humic acid components. It was considered that during the incubation [¹⁴C]2,4-D did not become bound or conjugated to soil components, and that non-extractable radioactivity associated with the three soil types resulted from incorporation of radioactive degradation products, such as [¹⁴C]carbon dioxide, into soil organic matter.     

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.     

Studies on the degradation of bis(tributyltin) oxide in soil

The degradation of bis(tri[1-¹⁴C]butyltin) oxide in two soils (1 mg tin kg^?1) has been studied under laboratory conditions. Half of the applied compound disappeared from unsterilised silt loam and sandy loam in approximately 15 and 20 weeks, respectively; it disappeared also from the sterilised soils but to a lesser extent. The formation of small amounts of dibutyltin derivatives was established by thin-layer chromatography both in the unsterilised and sterile soils. The amount of unextractable radioactivity increased with time in the unsterilised and sterile soils. In the unsterilised soils ¹⁴C was released as [¹⁴C]carbon dioxide in amounts equivalent to 20% of the applied radioactivity for silt loam and 10.7% for sandy loam over a period of 42 weeks. Almost no [¹⁴C]carbon dioxide was released from the sterile soils, confirming microbial participation in the degradation of the compound in the unsterilised soils.     

A root bioassay procedure for the determination of chlorsulfuron, diclofop acid and sethoxydim residues in soils

Measurement of the root lengths of pre-ger-minated oat seedlings (Avena sativa L. var. Sioux) grown in the dark in treated soils was used to assay residues of diclofop acid (2-[4-(2,4-dichloro-phenoxy)phenoxy]propionate) and sethoxydim (2-[1-(ethoxyimino)-butyl]-5-[2-(ethylthio)-propy]-3-hydroxy-2-cyclohexene-1-one). Similar measurements involving maize seedlings (Zea Mays L. var. Sunny Vee) were also used to determine residues of the herbicide chlorsulfuron (2-chloro-N-[(4-methoxy-6-methyl-1,3,5-triazin-2-yl)aminocarbony]benzenesulfonamide) in soils. The procedure appeared to be reproducible with residues of chlorsulfuron, diclofop acid and sethoxydim being detectable at amounts of 0.001, 0.2 and 0.05 μg g^?1 respectively.     

Soil persistence experiments with [14C]2,4-D in herbicidal mixtures, and field persistence studies with tri-allate and trifluralin both singly and combined

The persistence of [¹⁴C]2,4-D at a rate equivalent to 1 kg/ha was studied in the laboratory on a heavy clay and a sandy loam at 85%of field capacity and 20°C both alone and in the presence of 1 kg/ha dicamba, dichlorprop, difenzoquat, TCA, and 2,4,5-T. The persistence of 2,4,5-T was also monitored in both soils under the same conditions in the presence and absence of [¹⁴C]2,4-D. All soils were extracted at weekly intervals using aqueous acidic acetonitrile and analysed for [¹⁴C]2,4-D remainining radiochemical techniques. The extracts containing 2,4.5-T were additionally analysed gas chromatographically for that herbicide. In each soil type the half-life of the 2,4-D was similar regardless of whether applied singly or in combination with the five herbicides tested. Similarly, [¹⁴C]2,4-D did not affect the breakdown of 2,4,5-T in either soil type. The persistence of tri-allate (1·5 kg/ha) and trifluralin (0·75 kg/ha) both singly and in combination were compared using small field plots at two locations in Saskatchewan. Applications were made during May of 1977 and 1978 and the plots were sampled and analysed for herbicide(s) remaining after 10 and 20 weeks, respectively. The results indicate that within experimental error the loss of both tri-allate and trifluralin from the plots treated with the mixture was the same as from plots treated with the individual compounds.     

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.     

The degradation of methazole in soil. I. Effects of soil type,soil temperature and soil moisture content

[¹⁴C]-Labelled methazole was incubated in six soils at 25°C and with soil moisture at field capacity. Under these conditions, methazole was unstable, the concentration declined following first-order kinetics with half-life values in the soils ranging from 2.3 to 5.0 days. The main degradation product was 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU) which was more stable than the parent compound. After about 160 days, DCPMU accounted for 30 to 45% of the initial methazole concentration. Degradation of methazole and DCPMU was affected by soil temperature and moisture content. With methazole, half-lives in one soil at field capacity moisture content and temperatures of 25, 15 and 5°C were 3.5, 8.7 and 31.1 days respectively. The half-life at 25°C was increased to 5.0 days at 50% of field capacity and 9.6 days at 25% of field capacity. A proportion of the initial radioactivity added to the soil could not be extracted and this proportion increased with time. After 160 days this unextractable radioactivity accounted for up to 70% of the amount applied.     

Degradation of [14C]Terbuthylazine and [14C]Atrazine in Laboratory Soil Microcosms

The degradation and formation of major chlorinated metabolites of terbuthylazine and atrazine in three soils (loamy clay, calcareous clay and high clay) were studied in laboratory experiments using molecules labelled with ¹⁴C on the s-triazine ring. Soil microcosms were treated with the equivalent of 1 kg ha^-1 of herbicide and incubated in the dark for 45 days at 20(±1)°C. The quantity of [¹⁴C]carbon dioxide evolved in the soils treated with atrazine was negligible and could not be attributed to mineralization of the parent molecule. The mineralization of terbuthylazine accounted for 0·9–1·2% of the initial radioactivity. In the soils studied, the extrapolated half-lives varied from 88 to 116 days for terbuthylazine and 66 to 105 days for atrazine, with no significant differences for the three soils and the two molecules. The deethyl metabolites of the two s-triazines and the deisopropyl-atrazine metabolite appeared during the incubation in the three soils. The completely dealkylated metabolite was not detected in any of the soils. After 45 days of incubation, the non-extractable soil residues for the high clay, loamy clay and calcareous clay soils represented for terbuthylazine, 33·5, 38·3 and 43·1% and for atrazine, 19·8, 20·8 and 22·3% of the initial radioactivity. © 1997 SCI.     

Influence of soil pH and contents of organic carbon and clay on the volatilization of [14C]fenpropimorph after application to bare soil

The behaviour of the morpholine fungicide fenpropimorph applied to soil was investigated in a laboratory chamber. The volatility and metabolism of a ¹⁴C-labelled fenpropimorph formulation (Corbel^®) was studied after application to three soils (sandy loam, loamy clay and loamy sand), simulating a four-day weather scenario in the volatilization chamber. Additional experiments were conducted under standard climatic conditions over a period of 24 h using sandy soils with different pH values. The results of the first experiments showed that most of the radioactivity applied remained in the soils as unchanged fenpropimorph four days after application. In the experiments with the sandy loam and loamy clay, less than 5% of the applied radioactivity was removed by volatilization whereas 11·4% volatilized from the surface of the loamy sand. The comparatively higher volatilization of the fungicide from the loamy sand was confirmed by the later experiments indicating that higher soil pH favoured volatilization of [¹⁴C]fenpropimorph from sandy soils. Thus 5·6% (pH 5·0), 18·9% (pH 5·8) and 28·3% (pH 6·6) of the radioactivity applied volatilized within one day after application. The overall recoveries were between 93·8% and 111·3% in these experiments. © 1998 SCI     

Degradation of the herbicide flamprop-methyl in soil

The degradation of the wild oat herbicide flamprop-methyl [methyl DL -N-benzoyl-N-(3-chloro-4-fluorophenyl)alaninate] in four soils has been studied under laboratory conditions using ¹⁴C-1abelled samples. The flamprop-methyl underwent degradation more rapidly than its analogue flamprop-isopropyl. However, similar degradation products were formed, namely the corresponding carboxylic acid and 3-chloro-4-fluoroaniline. The latter compound occurred mainly as ‘bound’ forms although evidence was obtained of limited ring-opening to give [¹⁴C]carbon dioxide. The time for depletion of 50% of the applied herbicide was approximately 1-2 weeks in sandy loam, clay and medium loam soils and 2-3 weeks in a peat soil.     

Chlorpyrifos degradation in soil at termiticidal application rates

Chlorpyrifos [O,O-diethyl O-(3,5,6-trichloro-2-pyridyl) phosphorothioate] is an organophosphorus insecticide applied to soil to control pests both in agricultural and in urban developments. Typical agricultural soil applications (0.56 to 5.6 kg ha^?1) result in initial soil surface residues of 0.3 to 32 μg g^?1. In contrast, termiticidal soil barrier treatments, a common urban use pattern, often result in initial soil residues of 1000 μg g^?1 or greater. The purpose of the present investigation was to understand better the degradation of chlorpyrifos in soil at termiticidal application rates and factors affecting its behaviour. Therefore, studies with [¹⁴C]chlorpyrifos were conducted under a variety of conditions in the laboratory. Initially, the degradation of chlorpyrifos at 1000 μg g^?1 initial concentration was examined in five different soils from termite-infested regions (Arizona, Florida, Hawaii, Texas) under standard conditions (25°C, field moisture capacity, darkness). Degradation half-lives in these soils ranged from 175 to 1576 days. The major metabolite formed in chlorpyrifos-treated soils was 3,5,6-trichloro-2-pyrid-inol, which represented up to 61% of applied radiocarbon after 13 months of incubation. Minor quantities of [¹⁴C]carbon dioxide (< 5%) and soil-bound residues (? 12%) were also present at that time. Subsequently, a factorial experiment examining chlorpyrifos degradation as affected by initial concentration (10, 100, 1000 μg g^?1), soil moisture (field moisture capacity, 1.5 MPa, air dry), and temperature 15, 25, 35°C) was conducted in the two soils which had displayed the most (Texas) and least (Florida) rapid rates of degradation. Chlorpyrifos degradation was significantly retarded at the 1000 μg g^?1 rate as compared to the 10 μg g^?1 rate. Temperature also had a dramatic effect on degradation rate, which approximately doubled with each 10°C increase in temperature. Results suggest that the extended (3–24 + years) termiticidal efficacy of chlorpyrifos observed in the field may be due both to the high initial concentrations employed (termite LC ₅₀ = 0.2– 2 μg g^?1) and the extended persistence which results from employment of these rates. The study also highlights the importance of investigating the behaviour of a pesticide under the diversity of agricultural and urban use scenarios in which it is employed.     

Degradation of the herbicide benzoylprop-ethyl in soil

The degradation of [¹⁴C] benzoyl prop ethyl (SUFFIX,^a ethyl N-benzoyl-N-(3,4-dichlorophenyl)-2-aminopropionate) in four soils has been studied under laboratory conditions. The major degradation product of benzoylprop ethyl at up to 4 months after treatment was its corresponding carboxylic acid (II). On further storage this compound became firmly bound to soil before it underwent a slow debenzoylation process which led to the formation of a number of products including N-3,4-dichlorophenylalanine (IV), benzoic acid, 3,4-dichloroaniline (DCA), which was mainly present complexed with humic acids, and other polar products. Although these polar products were not identified, they were probably degradation products of DCA, since they were also formed when DCA was added to soil. No 3,3′,4,4′-tetrachloroazobenzene (TCAB) was detected in any of the soils at limits of detectability ranging from 0.01-0.001 parts/million. Since N-3,4-dichlorophenylalanine (IV) and 3,4-dichloroaniline were transient degradation products of benzoylprop ethyl, the metabolism in soil of radiolabelled samples of these compounds was also studied. In these laboratory experiments the persistence of the herbicide increased as the organic matter content of the soil increased and the time for depletion of half of the applied benzoylprop ethyl varied from 1 week in sandy loam and clay loam soils to 12 weeks in a peat soil.     

Sulcotrione soil persistence and mobility in summer maize and winter wheat crops

Sulcotrione soil persistence in spring maize ( Zea mays L.) crops grown on a sandy loam soil was greater at pH 5·5 and 6·0 (soil half-life T _1/2≈58 days) than at pH 7·1 ( T _1/2 = 44 days). Sulcotrione was also applied as recommended on a summer maize crop at the five- to six-leaf growth stage, grown on a sandy loam soil. Sulcotrione soil half-life was 44 days, and the herbicide remained mainly in the 0- to 5-cm surface soil layer during the cropping period, in spite of the high water solubility and the heavy rains at the end of August; lower sulcotrione concentrations (10–18% of the total during the 2-month period after sulcotrione application) were detected in the 5- to 10-cm surface soil layer. The herbicide was applied pre-emergence to winter wheat ( Triticum aestivum L.) at four sites that differed in their soil texture and composition: loamy sand, sandy loam, loam and clay loam. Persistence was greater in the soils containing more organic matter. In soils having similar organic matter contents, persistence was lower in the soil containing more sand relative to loam and clay. During the winter crops, sulcotrione moved down to the 10- to 15-cm soil layer, in spite of the fact that the rains were lower in winter than in summer. Sulcotrione most generally was not detected in the 15–20 cm soil layer of the maize and winter wheat crops.     

Factors affecting the distribution of pesticide drenches in soil using the fungicide metazoxolon as a model

The distribution resulting from the drenching of soil with a suspension concentrate of [¹⁴C]metazoxolon was studied in the laboratory and the field. Penetration of soil columns was increased by (a) increasing the drench volume from 1 to 7.8 litres m^?2, (b) changing the original soil moisture content from air-dry to field-capacity, and (c) including 1 % of ‘Renex 30’ surfactant in the drench. Penetration was greatest in soils containing large pores and was reduced when aggregates were broken down by sieving. Leaching the column with 1.56 cm of ‘rain’, 15 h after treatment, did not increase the penetration by metazoxolon. In all experiments, the maximum concentration of metazoxolon occurred in the top 2 cm of soil. Equivalent effects were found when metazoxolon was applied to a poorly-structured sandy clay loam in the field.     

A new extraction method for benomyl residues in soil and its application in movement and persistence studies

A simple and rapid method for extracting benomyl residues from soils was compared with previous methods. Soil was extracted by shaking for 2 h at room temperature with (1:1) acetone/M aqueous ammonium chloride followed by clean-up by solvent partition and ultraviolet absorption estimation of carbendazim. Recoveries were comparable to those obtained by refluxing with methanolic hydrochloric acid for 4 h, hitherto the most efficient method reported, and were much greater than those obtained by extraction with ethyl acetate or chloroform. The new method gave more tractable extracts than those obtained by refluxing with methanolic hydrochloric acid, which form troublesome metal hydroxide precipitates during clean-up. In field experiments with 2-[¹⁴C]-benomyl and 2-[¹⁴C]-carbendazim, no radioactivity was found more than 25 mm from the soil surface during 10 months after surface application of 1 kg/ha. Carbendazim residues in soils from three field experiments indicated that its persistence is very sensitive to soil pH. The time for 50% loss of initial dose ranged from 26 months at pH 5.5 to less than 3 months at pH 7.2. Biological effectiveness in a crop may therefore depend markedly on differences in soil pH.     

An evaluation of the persistence in soil of two non-chlorinated insecticides analogous to DDT

Two insecticides 1,1-bis(4-ethoxyphenyl)-2-nitrobutane (ENB) and 1,1-bis(4-ethoxyphenyl)-2-nitropropane (ENP), at concentrations varying from 0.4-40 mg kg^?1 have been examined for persistence in three soils. The proportions of the original compounds recovered decreased rapidly in a clay loam (falling to 50% in 2-6 weeks), decreased slowly in a sand (falling to 50% in 16-64 weeks) and decreased variably in a heavy clay (falling to 50% in 2-10 weeks). After the initial phase, the proportion recovered generally decreased at a slower rate. Breakdown of ENP appeared to proceed via a stepwise oxidation to give 1,1-bis(4-ethoxyphenyl)propan-2-one and 4,4′-diethoxybenzophenone. These products were readily broken down under aerobic conditions and were isolated only from waterlogged or sterile soil. Experiments under sterile conditions indicated that biological breakdown was a major factor and that chemical breakdown was specifically surface-activated.     

Degradation of fluridone in submersed soils under controlled laboratory conditions

The experimental, aquatic herbicide fluridone (1-methyl-3-phenyl-5-[3-(trifluoromethyl)phenyl]-4(1H)-pyridinone) was degraded in two submersed soils and in the water above those soils to one acidic metabolite (identified as 1,4-dihydro-1-methyl-4-oxo-5-[3-(trifluoromethyl)phenyl]-3-pyridinecarboxylic acid by mass spectrometry). A sandy and a silt loam soil were treated with [¹⁴C]fluridone, immersed in water, and analyzed after 1, 3, 5, 7, 9, and 12 months. Seven to fifteen percent of the ¹⁴C applied to the soils was recovered in the water on each of the various collection dates. The acidic metabolite accounted for 86 to 93% of the radioactivity in the water fraction 7 months after treatment. The metabolite was absorbed strongly by both soils and comprised about 60% of the total ¹⁴C in each soil after 12 months. The remainder of the ¹⁴C in the soils after 12 months was either the parent compound (～30%) or an undefined insoluble residue (～10%).     

Degradation of the herbicide flamprop-isopropyl in soil under laboratory conditions

The degradation of the wild-oat herbicide flamprop-isopropyl, [isopropyl (±)-N-benzoyl-N-(3-chloro-4-fluorophenyl)alaninate], in four soils has been examined under laboratory conditions with sampling times of up to 45 weeks after treatment. The major degradation product of [¹⁴C]flamprop-isopropyl in all soils at up to 10 weeks after treatment was the carboxylic acid (±)-N-benzoyl-N-(3-chloro-4-fluorophenyl)alanine. This compound in turn underwent degradation by loss of the benzoyl group and the propionic acid moiety, with evolution of [¹⁴C]carbon dioxide to form 3-chloro-4-fluoroaniline (CFA). The CFA was formed slowly in soil and occurred mainly as a bound form. There was evidence to show that the CFA was subsequently converted into other polar products. The time for depletion of 50% of the applied herbicide was approximately 10 weeks in sandy loam and medium loam soils, 11 weeks in a clay loam soil and 23 weeks in a peat soil.