The degradation of methazole in soil. II. Studies with methazole,methazole degradation products and diuron

[¹⁴C]-Labelled methazole, 1-(3,4-dichlorophenyl)-3-methylurea (DCPMU), 1-(3,4-dichlorophenyl)urea (DCPU), and diuron were incubated in soil at 20°C and field capacity soil moisture content. Decomposition followed first-order kinetics; half-lives for degradation of these four compounds were 2.4, 144, 30 and 108 days respectively. The amount of DCPMU and DCPU that could be extracted decreased with time and the decrease was accompanied by the generation of an equivalent amount of ¹⁴CO₂. This was not so in the studies with diuron and methazole, however, and the decrease in the concentrations of radioactivity extracted from soil treated with these compounds could not be entirely accounted for as carbon dioxide. It is concluded that the unextractable radiochemical that was present was DCPMU. Methazole appeared to be degraded through DCPMU to 3,4-dichloroaniline (DCA) with the production of only traces of DCPU.     

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.     

Persistence of the herbicide AC 92,553, N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine,in soils

The effects of soil temperature and soil moisture content on the rate of loss of N-(1-ethylpropyl)-2,6-dinitro-3,4-xylidine (I, AC 92,553) were measured under controlled conditions. The time for 50% disappearance in a sandy loam soil at 75% of field capacity was inversely related to temperature (98 days at 30°; 409 days at 10°). At 25°, the half-life increased with decreasing soil moisture content (122 days at 75% of field capacity; 563 days at 12.5%). In seven soils with different properties there was a trend towards a slower rate of loss as the organic matter content of the soils increased and the half-life varied from 72 to 172 days, first-order kinetics being obeyed. The herbicide was lost rapidly from an inert surface and 97% loss was recorded after 28 days at 25°. Losses from soil surfaces occurred more slowly and were greater from wet compared with dry soil. In the field, it was more persistent when incorporated than when applied to the soil surface. More than 60% of I incorporated in April 1975 could be detected the following September, but when applied to the soil surface, only about 20% of the applied dose remained by this time. Residues measured by gasliquid chromatography using a thermionic nitrogen detector closely paralleled those measured by a bioassay based on the root growth of buckwheat.     

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     

The degradation of simazine, linuron and propyzamide in different soils

Simazine, linuron and propyzamide were incubated in 18 different soils at 25°C and field capacity soil moisture content. The degradation of each herbicide followed first-order kinetics. The half-life of simazine varied from 20 to 44 days, that of linuron from 22 to 86 days and that of propyzamide from 10 to 32 days. The rate of linuron degradation was highly significantly correlated with soil organic matter content, clay content, soil respiration and the extent of herbicide adsorption by the soil. The rate of simazine degradation was significantly and negatively correlated with soil pH, but the rate of propyzamide degradation was not related with any of the soil factors examined.     

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.     

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.     

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.     

Simulation of herbicide persistence in soil .II. Simazine and linuron in long-term experiments

The rates of degradation of simazine and linuron were measured in soil from plots not treated previously with these herbicides. Degradation of both compounds followed first-order kinetics and soil temperature and soil moisture content had a marked effect on the rate of loss. With linuron, half-lives increased from 36 to 106 days with a reduction in temperature from 30° to 5°C at 4% soil moisture, and from 29 to 83 days at 12% soil moisture. Similar temperature changes increased the half-life of simazine from 29 to 209 days and from 16 to 125 days at soil moisture contents of 4 and 12% respectively. A computer program which has been developed for simulation of herbicide persistence was used in conjunction with the laboratory data and the relevant meteorological records for the years 1964 to 1968 in order to test the model against previously published field persistence data for the two herbicides. The results with simazine showed a close correspondence between observed and predicted residue levels but those for linuron, particularly in uncropped plots, were satisfactory for limited periods only.     

Aspects of the selective phytotoxicity of methazole. HI. Behaviour in plants following foliar treatment

Absorption of methazole by leaves of onion (Allium cepa), Stellaria media, Matricaria matricarioides and Veronica persica was rapid for the first 24 h after treatment and continued at a slower rate for up to 6 days to reach a maximum of between 35 and 60% of the amount applied. Differences in absorption between species were generally small. Absorption by the cotyledon of onion was greater than absorption into true leaves. Methazole on the leaf surface degraded to 3-(3,4-dichlorophenyl)-1-methylurea (DCPMU) and small amounts of this degraded to 3-(3,4-dichlorophenyl) urea (DCPU). Methazole absorbed into leaves was relatively stable in M. matricarioides and DCPMU accumulated slowly. The rate of degradation was more rapid in the cotyledons than in the true leaves. Both in leaves and in cotyledons of onion and S. media, methazole degraded rapidly to DCPMU and this accumulated; in those of V. persica, DCPMU was degraded quickly to DCPU and unidentified products. The amount of DCPMU accumulated in the shoots was broadly correlated with the relative phytotoxicity of methazole to the different species, except for young seedlings of V. persica which contained no DCPMU but were susceptible to methazole.     

Aerobic Metabolism of Flupropacil in Sandy Loam Soil

The aerobic soil metabolism of [¹⁴C]flupropacil (isopropyl 2-chloro-5-(1,2,3,6-tetrahydro-3-methyl-2,6-dioxo-4-trifluoromethylpyrimidin-1-yl)benzoate) was determined in microbially active, sieved (2-mm) sandy loam soil with a soil moisture content of 75% at 1/3 bar. The soil was treated with [¹⁴C]flupropacil at 0·5 mg kg⁻¹ (twice the field use rate) and placed in incubation flasks connected to a series of traps (50 g litre⁻¹ NaOH, 0·5M H₂SO₄, ethylene glycol) and incubated at 25(±1)°C. Soil was sampled at 0, 3, 9, 20, 30, 48, 76, 120, 181 and 238 days of aerobic incubation. Volatiles were collected once every two weeks and on the day of soil sampling. Flupropacil metabolized with a half-life of 79 days under aerobic conditions. The major metabolite was flupropacil acid which accounted for up to 69·1% of the initially applied radioactivity at Day 238. Each of the two minor metabolites detected at the end of the study accounted for less than 0·5%. One of the minor metabolites was identified as C4242 acid (2-chloro-5-(1,2,3,6-tetrahydro-2,6-dioxo-4-trifluoromethylpyrimidin-1-yl)benzoic acid). Only a negligible portion (less than 0·3%) of the applied flupropacil was mineralized to [¹⁴C]carbon dioxide. Extractable radioactivity ranged from 78·9% to 95·5%, with bound residues accounting for 3·2%–23·4%. The material balance ranged from 91·6% to 104·4%.     

Persistence studies with the herbicide sethoxydim in prairie soils

The persistence of [¹⁴C]sethoxydim (2-[1-(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexene-1-one) at the 2 μg g^?1 level was studied under laboratory conditions in three soils at 20°C and 85% of their field capacity moistures. Following extraction of the soils with methanol, the herbicide remaining was determined using radiochemical techniques. Loss of radioactivity was more rapid on moist clay loam and sandy loam, where the half-lives were 12 days, than on heavy clay in which the half-life was 26 days. Loss of radioactivity from air-dried soils (15% of field capacity) was negligible with over 94% of the applied activity being recovered after 28 days. The persistence of sethoxydim at a rate of 1 kg ha^?1 was investigated under field conditions using small plots at three prairie locations for 3 successive years. Using an oat-root bioassay procedure, no residues were detected in the 0–10 cm depths of any soils, any year, in September following May treatments.     

Metabolic fate of methazole in purple and yellow nutsedge

The mechanisms for the tolerance of purple nutsedge (Cyperus rotundus L.) and susceptibility of yellow nutsedge (Cyperus esculentus L.) to methazole [2-(3,4-dichlorophenyl)-4-methyl-1,2,4-oxadiazolidine-3,5-dione] were studied. Both species absorbed and translocated[¹⁴C]methazole and metabolites from nutrient solution; however, greater amounts of ¹⁴C per unit weight were detected in yellow than in purple nutsedge. Although intact plants and excised leaves of both species rapidly metabolized methazole to DCPMU [1-(3,4-dichlorophenyl)-3-methylurea], detoxification of DCPMU to DCPU [1-(3,4-dichlorophenyl) urea] occurred more slowly in yellow than in purple nutsedge. Compared to yellow nutsedge, a greater percentage of the radioactivity in purple nutsedge was recovered as polar products. Polar products were converted to the free forms of the parent herbicide and to phytotoxic DCPMU by proteolytic enzyme digestion. Based on the findings of this study, at least three mechanisms (differential absorption, metabolism, and formation of polar products) account for the differential tolerance of these two species to methazole.     

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.     

A quantitative study of asulam persistence in soil

The persistence of the herbicide asulam was studied at different controlled temperature and moisture levels in Regina heavy clay. Degradation was rapid, approximating to first-order kinetics with a half-life of about 7 days, at temperatures in the range 20–35° and at moistures of above 50% of field capacity. At lower soil temperature and/or moisture regimes, breakdown was slower. The laboratory data were used in conjunction with the appropriate meteorological records in a computer program to simulate the degradation pattern for asulam in six separate microplot field studies carried out during May to November 1976. In three of the six experiments there was close correspondence between observed and predicted residue levels, but in the other three experiments, the model underestimated rates of loss.     

Limited survival of Ralstonia solanacearum Race 3 in bulk soils and composts from Egypt

Survival of Ralstonia solanacearum race 3 biovar 2 (phylotype II sequevar 1) in Egyptian soils and compost was studied under laboratory and field conditions. Survival of the pathogen under laboratory conditions varied with temperature, water potential and soil type, with temperature being the major determinant of survival of the pathogen. The effects of temperature and moisture content were variable between different experiments, but survival was generally longer at 15°C than at 4, 28 and 35°C respectively. Survival was also longer when moisture levels were constant compared with varying moisture levels at all temperatures. In experiments to compare the effects of progressive drying in sandy and clay soils there was a difference in survival times between the two soil types. In sandy soils, the pathogen died out more rapidly when soil was allowed to dry out than in controls where the soil was kept at constant water potential. In clay soils there was little difference between the two treatments, possibly due to the formation of a hard impermeable outer layer during the drying process, which retarded water loss from within. Survival in mature composts at 15°C was of the same order of magnitude as in soils but shorter at 28°C, possibly owing to increased biological activity at this temperature, or a resumption of the composting process, with concomitant higher temperatures within the compost itself. The maximum survival time recorded over all soil types and conditions during in vitro studies was around 200 days. In field studies, the maximum survival time in both bare sand and clay was around 85 days at depths up to 50 cm. The survival time was reduced in field experiments carried out in summer to less than 40 days and in one study when the ground was flooded for rice cultivation, the bacterium could not be detected 14 days after flooding. The maximum survival time of R. solanacearum in infected plant material or in infested soil samples incorporated into compost heaps was less than 2 weeks. At the culmination of field soil and compost experiments, no infection was detected in tomato seedlings up to 10 weeks after transplanting into the same soils or composts under glasshouse conditions at a temperature of 25°C.     

Dissipation rates in soil of 1,2-dichloropropane and 1,3- and 2,3-dichloropropenes

The rates of dissipation in soil and chloride-ion release, of the main components of dichloropropane-dichloropropene mixtures used as nematicides, were studied in sealed glass containers at different temperatures and moisture conditions. Half-lives of (Z)- and (E)-1,3-dichloropropenes at 20°C in soils were found to vary from 3 to 25 days; those of 1,2-dichloropropane and 2,3-dichloropropene were about four times and twice as long, respectively. The dissipation rates changed by a factor of about 2 per 10°C change in temperature. Judging from the release rates of waterextractable inorganic chloride in the soil (0-4% per week), the total degradation of all components applied at normal field rates was extremely slow. This indicated the formation of residues containing covalently bound chlorine. Only in ‘enrichment cultures’ was complete degradation indicated.     

Comparative effects of soil solarization with single and double layers of polyethylene film on survival ofFusarium oxysporum F. SP.Vasinfectum

Soil solarization (SoSol) with a single layer of transparent polyethylene (PE) film, traps considerable heat and moisture in soil. Solarization of field soil with two layers of 1 mil (25 μm thick) PE film, separated by a 6-cm air layer, caused soil temperatures at 15 cm depth to rise by 12.7°C and 3.6°C over those in noncovered soil or soil covered by one layer of film, respectively; at 30 cm depth the respective differences in temperature were 11.2°C and 2.7°C. Viability of propagules (mainly chlamydospores) ofFusarium oxysporum f. sp.vasinfectum that had been buried at 30 cm depth, was reduced after 31 days of solarization by 97.5%, 58%, and 0% under a double film layer, a single layer, and in non-covered soil, respectively. The insulating effect of a double layer of PE film improved heat retention in soil and the solarization effect.     

Aspects of the selective phytotoxicity of methazole. II. Behaviour in plants following root exposure

The absorption, translocation and degradation of methazole were examined in onion, Stellaria media, Matricaria matricarioides and Veronica persica grown in culture solution. After a short period of initial rapid uptake, all four species absorbed herbicide and water in the same proportions. Translocation of herbicide to the shoots was directly proportional to transpiration, but the apparent solute concentration in the xylem was less than that in the external solution and varied between the species. A smaller percentage of the total absorbed herbicide was translocated to the shoot in V. persica, the most tolerant species. Methazole was relatively stable in M. matricariodes and was degraded slowly to 3-(3,4-dicnlorophenyl)-1-methylurea (DCPMU). It was degraded rapidly to DCPMU in the other three species and this accumulated in onion and S. media. In V. persica DCPMU was degraded further to 3-(3,4-dichlorophenyl) urea (DCPU). Methazole was not an active inhibitor of photosynthesis by isolated spinach chloroplasts. Both DCPMU and DCPU inhibited photosynthesis but DCPMU was 200-times more active than DCPU. Variations in the concentrations of DCPMU in the shoots of the different species largely accounted for the variations in their response to methazole applied pre-emergence.