The relative movement and persistence in soil of chlorsulfuron, metsulfuron-methyl and triasulfuron

Summary. Adsorption and degradation rates of triasulfuron in 8 different soils were negatively correlated with soil pH and were generally lower in subsoils than in soils from the plough layer. The half-life at 20°C varied from 33 days in a top soil at pH 5·8 to 120 days in a subsoil at pH 7·4. Adsorption distribution coefficients in these two soils were 0·55 and 0·19, respectively. Movement and persistence of residues of chlorsulfuron, triasulfuron and metsulfuron-methyl were compared in a field experiment prepared in spring 1987. Triasulfuron was less mobile in the soil than the other two compounds. Residues of all three herbicides were largely confined to the upper 40–50 cm soil 148 days after application. With an initial dose of 32 g ha⁻¹, residues in the surface soil layers were sufficient to affect growth of lettuce and sugar-beet sown approximately one year after application. Laboratory adsorption and degradation data were used with appropriate weather data in a computer model of herbicide transport in soil. The model gave good predictions of total soil residues during the first five months following application, and also predicted successfully the maximum depth of penetration of the herbicides into the soil during this period. However, more herbicide was retained close to the soil surface than was predicted by the model. The model predicted extensive movement of the herbicides in the soil during winter but did not predict that residues sufficient to affect crop growth could be present in the upper 15–20 cm soil after one year.     

Evaluation of a simulation model for prediction of herbicide movement and persistence in soil

The movement and persistence of residues of propyzamide, linuron, isoxaben and R-40244 were measured in a sandy loam soil in field experiments prepared in spring and autumn. None of the herbicides moved to depths greater than 12 cm in the soil during the winter period, following application in autumn, and none moved more than 6 cm in the soil, following application in spring. The general order of persistence of total soil residues was isoxaben > linuron = R-40244 > propyzamide. Appropriate constants to describe the moisture and temperature dependence of degradation were derived from laboratory incubation experiments and used with measurements of the strengths of adsorption of the different herbicides by the soil, in a computer model of herbicide movement. The model, in general, gave good predictions of total soil residues, but overestimated herbicide movement, particularly in winter. Measurements of herbicide desorption from the soil at intervals, during a laboratory incubation experiment, demonstrated an apparent increase in the strength of adsorption with time. When appropriate allowance was made for these changes in adsorption in the computer model, improved predictions of the vertical distribution of the herbicide residues were obtained.     

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.     

Measurement and simulation of the movement and degradation of atrazine and metribuzin in a fallow soil

The movement and persistence of atrazine and metribuzin, in a sandy loam soil following application in spring, was simulated using two models. The first model, based on the physical laws describing water and solute movement and using measured values of soil hydraulic properties, underestimated herbicide mobility in the soil and predicted too rapid drying of the deeper soil layers. The accuracy of the simulations was improved by empirically reducing the measured hydraulic conductivities by a factor of 4. This probably reflects the difficulties of obtaining reliable measurements of soil hydraulic properties. A second and simpler model, which simulated water and herbicide movement using mobile and immobile water categories, accurately predicted soil water contents. It tended to underestimate herbicide movement at short times after application, and to overestimate movement later in the experiments. A comparison of different methods of simulating herbicide degradation showed that prediction of degradation rates in the field from laboratory data can be unsatisfactory with some compounds.     

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.     

Evaluation of three pesticide leaching models with experimental data for alachlor, atrazine and metribuzin

The persistence and movement of residues of alachlor, alrazine and metribuzin were measured in a mini-lysimeter system in the field. This comprised a number of soil columns (11 cm diametert; 30 cm long), and permitted the vertical distribution of residues to be determined at. intervals alter application and the collection and analysis of leaehale water. Laboratory experiments were also performed to determine the degradation rates of the three herbicides and their strengths of adsorption by the test soil. The results showed an order of degradation rate of metribuzin> alachlor>atrazine and an order of adsorption of alacblor>atrazine>melribuzin. Movement of residues in the soil columns and concentrations in the leachate were inversely related to the strength of adsorption. Parameters derived from the laboratory data were used in conjunction with weather data for the period of the field experiment in three mathematical models of pesticide leaching: VARLEACH, LEACHP and PRZM2. In most instances, the models gave acceptable predictions of the distribution of residues in soil. This was particularly so for the less mobile compound alachlor. With the most mobile compound, metribuzin, residues were not well predicted at the later sampling dates. All three models gave accurate predictions of the volumes of drainage water, but none of them predicted the concentrations of herbicide in the leachate, presumably because they do not take account of preferential flow pathways of water and solute in the soil.     

Persistence of pyrazosulfuron in rice-field and laboratory soil under Indian tropical conditions

BACKGROUND: Pyrazosulfuron ethyl, a new rice herbicide belonging to the sulfonylurea group, has recently been registered in India for weed control in rice crops. Many field experiments revealed the bioefficacy of this herbicide; however, no information is available on the persistence of this herbicide in paddy soil under Indian tropical conditions. Therefore, a field experiment was undertaken to investigate the fate of pyrazosulfuron ethyl in soil and water of rice fields. Persistence studies were also carried out under laboratory conditions in sterile and non‐sterile soil to evaluate the microbial contribution to degradation. RESULTS: High‐performance liquid chromatography (HPLC) of pyrazosulfuron ethyl gave a single sharp peak at 3.41 min. The instrument detection limit (IDL) for pyrazosulfuron ethyl by HPLC was 0.1 µg mL^?1, with a sensitivity of 2 ng. The estimated method detection limit (EMDL) was 0.001 µg mL^?1 and 0.002 µg g^?1 for water and soil respectively. Two applications at an interval of 10 days gave good weed control. The herbicide residues dissipated faster in water than in soil. In the present study, with a field‐soil pH of 8.2 and an organic matter content of 0.5%, the pyrazosulfuron ethyl residues dissipated with a half‐life of 5.4 and 0.9 days in soil and water respectively. Dissipation followed first‐order kinetics. Under laboratory conditions, degradation of pyrazosulfuron ethyl was faster in non‐sterile soil (t_1/2 = 9.7 days) than in sterile soil (t_1/2 = 16.9 days). CONCLUSION: Pyrazosulfuron ethyl is a short‐lived molecule, and it dissipated rapidly in field soil and water. The faster degradation of pyrazosulfuron in non‐sterile soil than in sterile soil indicated microbial degradation of this herbicide. Copyright © 2012 Society of Chemical Industry     

Effect of long‐term field application of pendimethalin: enhanced degradation in soil

The effect of long‐term application of pendimethalin in a maize–wheat rotation on herbicide persistence was investigated. Pendimethalin was applied at 1.5 kg AI ha⁻¹ separately as one or two annual applications for five consecutive years in the same plots. Residues of pendimethalin were determined by gas chromatography. Harvest‐time residues of the herbicide decreased gradually over the years and at the end of five years less than 3% of applied pendimethalin was recovered from soil as against 18% in the first year. Residues were found distributed in the soil profile up to 90 cm depth at the end of the experiment with peak distribution of 0.03 µg g⁻¹ in the surface layer of soil treated with 10 herbicide applications. The minimum distribution was, however, in the deepest soil (75–90 cm) profile. Some of the metabolites of pendimethalin ie dealkylated pendimethalin derivative, partially reduced derivative and cyclized product were also traced in surface and sub‐surface soils up to 90 cm. A study of the rate of degradation of pendimethalin in field‐treated soils under laboratory conditions revealed faster degradation compared to control soils. Only the surface soil (0–15 cm) showed this enhanced degradation of the herbicide, which could be due to the adaptability of the aerobic micro‐organisms to degrade pendimethalin. Microbes capable of degrading herbicide were isolated, identified and pendimethalin degradation was confirmed in nutrient broth. © 2000 Society of Chemical Industry     

Dissipation of imazapyr, flumetsulam and thifensulfuron in soil

The degradation of imazapyr, flumetsulam and thifensulfuron applied at 500.40 and 30 g active ingredient (a.i.) ha^-1, respectively, to silt loam soil was studied under laboratory and field conditions. Herbicide residues were analysed by a lentil ( Lens culinarits L.) bioassay. Results showed that temperature had a significant effect on herbicide degradation, whereas the impact of soil organic matter ami pH were less well defined. Half-lives for imazapyr, flumetsulam and thifensulfuron in soil samples from the 0-5 cm layer (6.4% organic carbon) at 15 °C were 125, 88 and 5.4 days, respectively, and 69, 30 and 3.9 days at 30°C. In soil sampled from the 15-20 cm layer (3.5% organic carbon) half-lives were 155. 70 and 6.4 days, respectively, at 15 °C and 77, 24 and 4.8 days at 30 °C, A field experiment investigated the degradation and teaching of each herbicide under two precipitation regimes [natural precipitation (208 mm), and natural precipitation plus 75 mm irrigation (283 mm) over 4 months to a soil depth of 25 cm. Thifensulfuron degraded rapidly, whereas residues of flumetsulam and imazapyr leached below 25 cm in both the low-and high-precipitasion treatments after 4 months. Significant imazapyr residues were still present in the soil to 25 cm depth after 3 months, A multi-component model for herbicide dissipation was developed and evaluated using data from the laboratory and field experiments.     

Effect of recent cropping history and herbicide use on the degradation rates of isoproturon in soils

Five soil samples were taken from each of five fields with different crop management histories. Three of the fields were in an arable rotation, the fourth field was temporary grassland, and the final field was under permanent grass. Of the three arable fields, two had been cropped with winter wheat in three of the preceding 6 years, and the third had last been cropped with winter wheat once only, 6 years previously. With one exception, the winter wheat had been sprayed with the herbicide isoproturon. The rate of isoproturon degradation in laboratory incubations was strongly related to the previous management practices. In the five soils from the field that had been treated most regularly with isoproturon in recent years, <2.5% of the initial dose remained after 14 days, indicating considerable enhancement of degradation. In the soils from the field with two applications of the herbicide in the past 6 years, residues after 27 days varied from 5% to 37% of the amount applied. In soils from the other three sites, residue levels were less variable, and were inversely related to microbial biomass. In studies with selected soils from the field that had received three applications of isoproturon in the previous 6 years, kinetics of degradation were not first‐order but were indicative of microbial adaptation, and the average time to 50% loss of the herbicide (DT₅₀) was 7.5 days. In selected soils from the field that had received just one application of isoproturon, degradation followed first‐order kinetics, indicative of cometabolism. Pre‐incubation of isoproturon in soil from the five fields led to significant enhancement of degradation only in the samples from the two fields that had a recent history of isoproturon application.     

Simulation of the persistence of atrazine, linuron and metolachlor in soil at different sites in the USA

The effects of soil temperature and soil moisture content on the rates of degradation of atrazine, linuron and metolachlor were measured in the laboratory in soil from different sites in the USA. Persistence of the herbicides was measured in the same soils in the field during the summers of 1978 and 1979. Weather records from the different sites for the periods of the field experiments were used in conjunction with appropriate constants derived from the laboratory data in a computer program to simulate persistence in the field. There was a general tendency for the model to overestimate the observed soil residues. For example, with atrazine, 40 of the 48 measured residues were lower than those predicted by the model; seven were more than 30% below and two were more than 50% below. With metolachlor, 16 of the 48 measured residues were more than 30% below those predicted and six were more than 50% below; almost identical results were obtained with linuron. When the model overestimated late-season residues by a large amount, the discrepancies between predicted and observed data were usually apparent from early in the experiment. Possible reasons for the discrepancies are discussed.     

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.     

Residualwirkung von Chlortriazin-Herbiziden im Boden an drei rumänischen Standorten. I. Prognose der Persistenz von Simazin und Atrazin im Boden

Residual effects of chlorotriazine herbicides in soil at three Rumanian sites. I. Prediction of the persistence of simazine and atrazine Persistence of simazine and atrazine in the top 10 cm soil was measured at three sites in Rumania with variations in climate and soil conditions. Both herbicides were applied at 1 and 3 kg ai ha^?1 to uncropped plots and to plots cropped with maize (Zea mays L.). Rates of residue decline were independent of application rate and crop cover but varied between sites. The time for 50% loss of atrazine varied from 36 to 68 days and that of simazine from 48 to 70 days. Laboratory studies were made with atrazine to characterize degradation rates under standard conditions and to measure adsorption and leaching behaviour in the different soils. Weather records for the periods of the field experiments were used in conjunction with appropriate constants derived from the laboratory results, or from data in the literature, in a computer program to simulate persistence in the field. Results from the model were in reasonable agreement with the observed soil residues although there was a tendency to overestimate rates of loss on some occasions. The results suggest that the model of persistence was sufficiently accurate for practical purposes, and that its use could preclude the need for extensive analytical measurements of residues.     

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 the persistence of atrazine in soil at different sites in Portugal

The effects of soil temperature and soil moisture content on the rates of degradation of atrazine, were measured in the laboratory in soils from different sites in Portugal. Persistence of atrazine was measured in the same soils in the field during the spring and summer of 1984, 1985, 1986 and 1987. Weather records from the different sites, measured during the periods of the field experi ments, were used in conjunction with appropriate constants derived from the laboratory data in a computer program to simulate persistence in the field. The model generally overestimated the ob served soil residues, particularly during the first 7–14 days after application. The fit from the model was good from day 14 to the end of the experiments.     

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.     

Enhanced degradation of atrazine under field conditions correlates with a loss of weed control in the glasshouse

Enhanced degradation of atrazine has been reported in the literature, indicating the potential for reduced residual weed control with this herbicide. Experiments were conducted to determine the field dissipation of atrazine in three cropping systems: continuous Zea mays L. (CC) receiving atrazine applications each year, Gossypium hirsutum L.-Z. mays rotation (CCR) receiving applications of atrazine once every 2 years and a no atrazine history soil (NAH). Subsequent laboratory and greenhouse experiments were conducted with soil collected from these cropping systems to determine atrazine degradation, mineralization and residual weed control. Field dissipation of atrazine followed first-order kinetics, and calculated half-life values for atrazine combined over 2003 and 2005 increased in the order of CC (9 d) = CCR (10 d) < NAH (17 d). Greenhouse studies confirmed that the persistence of atrazine was approximately twofold greater in NAH soil than in CC or CCR soil. Biometer flask mineralization studies suggested that enhanced degradation of atrazine was due to rapid catabolism of the s-triazine ring. Glasshouse efficacy studies revealed a loss of residual weed control in CC and CCR soil compared with NAH soil. These data indicate that, under typical Mississippi Delta field conditions and agronomic practices, the persistence of atrazine may be reduced by at least 50% if the herbicide is applied more than once every 24 months. Glasshouse studies suggest that under these conditions a loss of residual weed control is possible.     

Spatial variability in 14C-herbicide degradation in surface and subsurface soils

The spatial variability in mineralization of atrazine, isoproturon and metamitron in soil and subsoil samples taken from a 135-ha catchment in north France was studied. Fifty-one samples from the top layer were taken to represent exhaustively the 31 agricultural fields and 21 soil types of the catchment. Sixteen additional samples were collected between depths of 0.7 and 10 m to represent the major geological materials encountered in the vadose zone of the catchment. All these samples were incubated with 14C-labelled atrazine under laboratory conditions at 28 degrees C. Fourteen selected surface samples which exhibited distinctly different behaviour for atrazine dissipation (including sorption and mineralization) were incubated with 14C-isoproturon and 14C-metamitron. Overall soil microbial activity and specific herbicide degradation activities were monitored during the incubations through measurements of total carbon dioxide and 14C-carbon dioxide respectively. At the end of the incubations, extractable and non-extractable (bound) residues remaining in soils were measured. Variability of herbicide dissipation half-life in soil surface samples was lower for atrazine and metamitron (CV < 12%) than for isoproturon (CV = 46%). The main contributor to the isoproturon dissipation variability was the variability of the extractable residues. For the other herbicides, spatial variability was mainly related to the variability of their mineralization. In all cases, herbicide mineralization half-lives showed higher variability than those of dissipation. Sorption or physicochemical soil properties could not explain atrazine and isoproturon degradation, whose main factors were probably directly related to the dynamics of the specific microbial degradation activity. In contrast, variability of metamitron degradation was significantly correlated to sorption coefficient (K(d)) through correlation with the sorptive soil components, organic matter and clay. Herbicide degradation decreased with depth as did the overall microbial activity. Atrazine mineralization activity was found down to a depth of 2.5 m; beyond that, it was negligible.     

Enhanced degradation of some soil-applied herbicides

In a field experiment involving repeated herbicide application, persistence of simazine was not affected by up to three previous doses of the herbicide. With propyzamide, there was a trend to more rapid rates of degradation with increasing number of previous treatments. Persistence of linuron and alachlor was affected only slightly by prior applications. In a laboratory incubation with soil from the field that had received four doses of the appropriate herbicide over a 12–month period, there was again no effect from simazine pretreatments on rates of loss. However, propyzamide, linuron and alachlor all degraded more rapidly in the previously treated than in similar untreated soil samples. Propyzamide, linuron, alachlor and napropamide degradation rates were all enhanced by a single pretreatment of soil in laboratory incubations, whereas degradation rates of isoproturon, metazachlor, atrazine and simazine were the same in pretreated and control soil samples.