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.     

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.     

Effect of pH on the degradation of atrazine,dichlorprop, linuron and propyzamide in soil

The rates of disappearance of atrazine, dichlorprop, linuron and propyzamide were measured in two soils incubated at 22°C and 80% water holding capacity. Observations were made at four pH levels in each soil. Atrazine degradation was relatively insensitive to pH; it increased slightly with increasing pH in one soil and decreased in the other. The other compounds all degraded more slowly at low pH in both soils although dichlorprop had essentially disappeared in 14 days under all conditions, so that the effect of pH is not unlikely to be of practical interest. The ratios of the degradation rates of atrazine, linuron, and propyzamide varied with the soil and the pH.     

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.     

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     

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.     

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.     

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.     

Adsorption and degradation of thiazopyr in compost‐amended and non‐amended soils

Adsorption and degradation of thiazopyr on two unamended soils and a soil amended annually during 8 years with compost were studied under laboratory conditions and compared with the results obtained on soils amended with fresh sewage sludge compost. The adsorption isotherms fitted the Freundlich equation well and a marked sorption increase was found in amended soils. Degradation data followed first‐order kinetics and thiazopyr had a half‐life of about 75 days at 25 °C and 60% water‐holding capacity of soil. The addition of fresh compost markedly decreased the rate of thiazopyr degradation, whereas the compost mineralised in the field after annual additions had only a small influence. Incubation studies with sterile soils showed a very significant decrease of the degradation rate, indicating that degradation by micro‐organisms was the main pathway of thiazopyr degradation in the soils studied. © 2001 Society of Chemical Industry     

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.     

EWRS Herbicide-Soil Working Group: Collaborative experiment on simazine persistence in soil

The effects of soil temperature and soil moisture content on the rate of simazine degradation were measured in the laboratory in soils from sixteen sites located in several different countries. First-order half-lives under standard incubation conditions were significantly correlated with clay content, organic carbon content and soil pH in a multiple linear regression. The temperature dependence of degradation was similar in the different soils whereas the moisture dependence showed considerable variation between soils. Persistence of simazine was also measured in the same soils in the field and at live additional sites. Weather records from the different sites for the periods of the Held experiments were used in conjunction with constants derived from the laboratory data in a computer program to simulate persistence in the field. In general, the model overestimated residues in the field. About half of the calculated residues were within 25% of those observed, an accuracy sufficient for practical purposes, but on several occasions the discrepancies between calculated and observed residues were greater than 50%. Possible reasons for the discrepancies and requirements for further experiments are discussed.     

Induction and Transfer of Enhanced Biodegradation of the Herbicide Napropamide in Soils

In laboratory incubations, the times to 50% loss (DT₅₀) of a first application of napropamide were approximately 25, 45 and 75 days in soil incubated at 25, 15 and 5°C respectively. When treated for a second time, the DT₅₀ values were 4, 7 and 15 days at the same temperatures, irrespective of the temperature of the first incubation. This indicates that enhanced degradation of napropamide in soil can be both induced and expressed at low temperature. A mixed microbial culture able to degrade the herbicide to a single degradation product, identified by HPLC retention time as naphthoxypropionic acid, was obtained from a soil capable of rapid degradation. Addition of a sub-sample of this mixed culture to a previously untreated soil introduced rapid degrading ability. When small amounts of soil capable of rapid degradation were added to previously untreated soil, in both the laboratory and the field, the degradation rate of napropamide increased compared with that in unamended soils.     

Degradation of chlorsulfuron and triasulfuron in alkaline soils under laboratory conditions

The degradation of chlorsulfuron and triasulfuron was investigated in alkaline soils (pH 7.1–9.4) spiked at 40 μg a.i. kg^–1 under laboratory conditions at 25 °C and a moisture content corresponding to 70% field capacity (–33 kPa), using high-performance liquid chromatography. Degradation data for the two herbicides did not follow first-order kinetics, and observed DT₅₀ values in surface soils ranged from 19 to 42 days and from 3 to 24 days for chlorsulfuron and triasulfuron respectively. Disappearance of both chlorsulfuron and triasulfuron was faster in non-sterile than in sterile soil, demonstrating the importance of microbes in the breakdown process. The persistence of chlorsulfuron increased with increasing depth, which can be attributed to the decline in the microbial populations down the profile. The DT₅₀ value for chlorsulfuron at 30–40 cm depth was nearly four times higher than that in the top-soil. The results obtained show that persistence of these herbicides in alkaline surface soils at 25 °C and at a moisture content of 70% field capacity is similar to those reported in other European and North American soils. The study shows that if these herbicides are contained in surface soil layers, the risk of residue carry-over under southern Australian conditions is small. However, the rate of their degradation in alkaline subsoils is very slow, and under conditions conducive to leaching their prolonged persistence in the soil profile is possible.     

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.     

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.     

Relations entre la vitesse de dégradation du propyzamide et les propriétés physicochimiques des sols

Relationship between rate of propyzamide degradation and physico-chemical properties Propyzamide was incubated in 29 different soils at a soil moisture content of 25%. The degradation of the herbicide followed first-order kinetics. The rate of propyzamide degradation was correlated with soil resistivity. Addition of mineral fertilizers inhibited propyzamide degradation.     

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     

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.     

Structure–degradability relationships for propyzamide analogues in soils

The kinetics of the degradation in soil of propyzamide and nine analogues have been measured at two temperatures (28 and 60°C). The degradation rates of propyzamide were determined in sterilised soil and in soil perfusates. The results obtained are in agreement with a chemical degradation of propyzamide and its analogues. Relationships were established between degradation rate constants and physicochemical parameters of the compounds: log k = ?aσ+bπ + c where σ = Hammett constant, π = hydrophobic constant, a, b, c are constants.