Influence of topsoil tilth and soil moisture status on losses of pesticide to drains from a heavy clay soil

Twelve lysimeters with a surface area of 0.5 m² and a length of 60 cm were taken over mole drains from a Denchworth heavy clay soil and divided into two groups with either a standard agricultural tilth or a finer topsoil tilth. The influence of topsoil tilth on leaching of the herbicide isoproturon and a bromide tracer was evaluated over a winter season. The effect of variations in soil moisture status in the immediate topsoil on leaching of isoproturon, chlorotoluron and linuron was investigated in the following winter season. Here, water inputs were controlled such that lysimeters received 50 mm at a maximum intensity of 2 mm h^?1 over a 4‐week period with herbicides applied on day 15. Three treatments received the water either all prior to application, all after application, or evenly spread over the 4‐week period. Leaching losses of the three herbicides were monitored for a subsequent drainage event. Analysis of covariance showed a significant effect of topsoil tilth and total flow on both the maximum concentrations (P = 0.034) and total losses (P = 0.012) of isoproturon in drainflow. Both concentrations and losses were c 35% smaller from lysimeters with the finer tilth. However, generation of the fine tilth in the field was restricted by a wet autumn and this is not considered a reliable management option for reducing pesticide losses from heavy clay soils. In the second experiment, variation in soil moisture content prior to and after application did not have any significant effect (P < 0.05) upon subsequent losses of the three herbicides to drains. © 2001 Society of Chemical Industry     

Measurement and simulation of herbicide transport in macroporous soils

Lysimeter experiments were carried out to study pesticide transport through macroporous soils. In order to differentiate between the effects of soil structure and chemical behaviour, the leaching experiments were conducted using disturbed and undisturbed soil samples. Two herbicides with different sorption behaviours, and bromide as tracer were applied. The results were used to validate a dynamic simulation model which considers bypass flow in macropores. The simulation results show that the model is able to reproduce the soil suction within the soil as well as the spatial distribution of bromide and the herbicides. The continuity of the macropores is most important for the efficiency of bypass flow. The results indicate that cultivation practices like ploughing significantly influence the temporal and spatial distribution of the macropores. © 1998 SCI.     

Leaching of pesticides and a bromide tracer through lysimeters from five contrasting soils

In each of two seasons, undisturbed lysimeters 0.8 m in diameter and 1.05 m in length taken from five soil types were cropped with winter wheat. They received autumn applications of the pesticides isoproturon and linuron as well as a bromide tracer and spring applications of dimethoate and MCPA. Leachate was collected at regular intervals and concentrations of the various solutes determined. Rainfall from December to March was 290 and 191 mm in the first and second seasons, respectively. Both springs were exceptionally dry with less than 50% of the mean April‐to‐June rainfall of 138 mm. Total flow from the lysimeters ranged from 335 to 477 mm (and from 0.78 to 3.95 pore volumes) over the two seasons. Leaching to drainage of bromide highlighted soils where preferential flow was influential with total losses ranging from 24% of applied for a strongly structured, alluvial clay loam to 79% for an unstructured sand. Leaching to drainage of isoproturon (K_oc ≈ 100 ml g⁻¹) was observed from all but a peat soil with losses greater (0.31–1.01% of applied) from the clay loam and a deep medium loam, where patterns of leaching clearly indicated preferential flow mechanisms, than from the sand and a light loam over gravel (0.04–0.18% of applied) where a broad breakthrough curve indicated that matrix flow was more important. Linuron (K_oc ≈ 500 ml g⁻¹) was detected in occasional samples of leachate from the clay loam, the light loam over gravel and the medium loam during the first season only (maximum loss 0.12% of applied). The sandy soil, often considered most vulnerable to leaching, gave the smallest total losses of pesticide of the four mineral soils, whilst significant preferential flow in the deep, medium loam was believed to result from a compacted topsoil. Neither of the spring‐applied pesticides was detected in the leachate, as flow following application was very small and relatively slow. © 2000 Society of Chemical Industry     

Modelling approaches to compare sorption and degradation of metsulfuron-methyl in laboratory micro-lysimeter and batch experiments

Results of laboratory batch studies often differ from those of outdoor lysimeter or field plot experiments--with respect to degradation as well as sorption. Laboratory micro-lysimeters are a useful device for closing the gap between laboratory and field by both including relevant transport processes in undisturbed soil columns and allowing controlled boundary conditions. In this study, sorption and degradation of the herbicide metsulfuron-methyl in a loamy silt soil were investigated by applying inverse modelling techniques to data sets from different experimental approaches under laboratory conditions at a temperature of 10 degrees C: first, batch-degradation studies and, second, column experiments with undisturbed soil cores (28 cm length x 21 cm diameter). The column experiments included leachate and soil profile analysis at two different run times. A sequential extraction method was applied in both study parts in order to determine different binding states of the test item within the soil. Data were modelled using ModelMaker and Hydrus-1D/2D. Metsulfuron-methyl half-life in the batch-experiments (t1/2 = 66 days) was shown to be about four times higher than in the micro-lysimeter studies (t1/2 about 17 days). Kinetic sorption was found to be a significant process both in batch and column experiments. Applying the one-rate-two-site kinetic sorption model to the sequential extraction data, it was possible to associate the stronger bonded fraction of metsulfuron-methyl with its kinetically sorbed fraction in the model. Although the columns exhibited strong significance of multi-domain flow (soil heterogeneity), the comparison between bromide and metsulfuron-methyl leaching and profile data showed clear evidence for kinetic sorption effects. The use of soil profile data had significant impact on parameter estimates concerning sorption and degradation. The simulated leaching of metsulfuron-methyl as it resulted from parameter estimation was shown to decrease when soil profile data were considered in the parameter estimation procedure. Moreover, it was shown that the significance of kinetic sorption can only be demonstrated by the additional use of soil profile data in parameter estimation. Thus, the exclusive use of efflux data from leaching experiments at any scale can lead to fundamental misunderstandings of the underlying processes.     

Lysimeter study to investigate the effect of rainfall patterns on leaching of isoproturon

The influence of five rainfall treatments on water and solute leaching through two contrasting soil types was investigated. Undisturbed lysimeters (diameter 0.25 m, length 0.5 m) from a sandy loam (Wick series) and a moderately structured clay loam (Hodnet series) received autumn applications of the radio-labelled pesticide isoproturon and bromide tracer. Target rainfall plus irrigation from the end of November 1997 to May 1998 ranged from drier to wetter than average (235 to 414 mm); monthly rainfall was varied according to a pre-selected pattern or kept constant (triplicate lysimeters per regime). Leachate was collected at intervals and concentrations of the solutes were determined. Total flow (0.27-0.94 pore volumes) and losses of bromide (3-80% of applied) increased with increasing inputs of water and were larger from the Wick sandy loam than from the Hodnet clay loam soil. Matrix flow appeared to be the main mechanism for transport of isoproturon through the Wick soil whereas there was a greater influence of preferential flow for the Hodnet lysimeters. The total leached load of isoproturon from the Wick lysimeters was 0.02-0.26% of that applied. There was no clear variation in transport processes between the rainfall treatments investigated for this soil and there was an approximately linear relationship (r2 = 0.81) between leached load and total flow. Losses of isoproturon from the Hodnet soil were 0.03-0.39% of applied and there was evidence of enhanced preferential flow in the driest and wettest treatments. Leaching of isoproturon was best described by an exponential relationship between load and total flow (r2 = 0.62). A 45% increase in flow between the two wettest treatments gave a 100% increase in leaching of isoproturon from the Wick soil. For the Hodnet lysimeters, a 35% increase in flow between the same treatments increased herbicide loss by 325%.     

Leaching potential and decomposition of fluroxypyr in Swedish soils under field conditions

The mobility and decomposition of the herbicide fluroxypyr (4-amino-3,5-dichloro-6-fluoro-2-pyridyloxyacetic acid) was studied under field conditions in a sandy soil and a clay soil. Leachate was collected in lysimeters with undisturbed soil (sand) and in tile-drained plots (clay). Soil samples to a depth of one metre were also collected in both soils to characterize the temporal depth distribution of fluroxypyr in the profiles. The herbicide was applied as the I-methylheptyl ester of fluroxypyr at two rates, 187.5 and 375.0 g a.e. ha^?1, representing the normal and double the dose of the compound used for spring cereals. Some lysimeters received supplementary watering. Only two leachate samples (one from each soil) had concentrations of fluroxypyr above the detection limit (1 μg litre^?1), i.e. 2 and 5 μg litre^?1. Both samples were collected within two months after application, when less than 2 mm of drainage had been collected. The methylheptyl ester of fluroxypyr was not found in any of the samples. Fluroxypyr levels above the detection limit in soil (5 μg kg^?1 dry soil), were never found below the topsoil (0.2 m) in the clay profile, while, in the sandy profile, levels just above the detection limit were found occasionally in deeper soil layers. Concentrations were reduced to undetectable or very low levels within three months after spraying.     

Mobility and persistence of isazofos in granular and microencapsulated formulations in two soils,using field lysimeters

Field lysimeters were used to assess the mobility and persistence of microencapsulated and granular formulations of the soil insecticide, isazofos, in Plainfield sand, and the microencapsulated formulation in Vittoria loam soil, using two moisture regimes, rainfall and supplementary watering. Mobility and persistence comparisons were made with an earlier lysimeter study which used emulsifiable concentrate (EC) and granular formulations of isazofos in Plainfield sand. Isazofos mobility in Plainfield sand increased in the following order for the tested formulations: microencapsulated < granular ≦ EC. Atrazine, which was applied as a suspension concentrate to all lysimeters as an internal reference, appeared to exhibit retarded disappearance rates during initial stages of the study when in the presence of granular isazofos in the rainfall treatment. The degradation of isazofos was faster in Vittoria loam than in Plainfield sand for the microencapsulated formulation in the field lysimeters (only formulation tested), and for all three formulations in a laboratory study.     

Improved control of Stellaria media (L.) Vill in leys with Trifolium repens, using bentazone and benazolin with additives

In pot experiments, mixtures of ammonium sulphate with surfactants or oil additives increased the phytotoxicity of commercially formulated bentazone (‘Basagran’) to Stellaria media (chickweed). Ammonium sulphate with a proprietary oil adjuvant Actipron had similar effects with benazolin potassium salt, but not with an ethyl ester formulation of benazolin. The phytotoxicity of bentazone and benazolin salts to Trifolium repens (white clover) and Lolium perenne (perennial ryegrass) was almost unaffected by these additives. In a field experiment, a mixture of ammonium sulphate with Actipron improved the control of S. media by bentazone and benazolin salts. Oils and surfactants markedly increased rates of entry of ¹⁴C bentazone into leaves of S. media, white clover and Chrysanthemum segetum. Ammonium sulphate sometimes had similar effects but on other occasions reduced uptake. In some circumstances the additives apparently interacted synergistically, to increase uptake of labelled herbicide into the leaf or to enhance its transport within the plant. In these test species, differential absorption of bentazone could not explain differences in species susceptibility, suggesting that the main cause of resistance was the ability of plants to degrade the herbicide.     

Model Predictions and Field Measurements of Chlorsulfuron Leaching under Non-steady-state Flow Conditions

Model simulations of chlorsulfuron (1-(2-chlorophenylsulfonyl)-3-(4-methoxy-6-methyl-1,3,5-triazin-2-yl)urea) leaching in a loamy soil were made with the mechanistic dual-porosity model MACRO. Comparisons were made with a data set obtained in a lysimeter experiment in which leaching was measured during an 11-month period after applying chlorsulfuron at two rates (4 and 8 g ha⁻¹). In this experiment, peak concentrations appeared c.6 months after pesticide application, reaching levels of 14 and 21 ng litre⁻¹ in the low- and high-dose treatments, respectively. These peak concentrations appeared after c.70 mm of accumulated leachate, implying that some of the herbicide was displaced through the soil columns by non-equilibrium flow processes. Model calibration was limited to parameters related to evapotranspiration, water uptake by roots and degradation rates in the subsoil. With this minimum amount of calibration, the model successfully described the leaching pattern of chlorsulfuron, provided that the two-flow domain option in the model was used. Running the model in one-flow domain resulted in considerable underestimates of leaching of chlorsulfuron over the short-term (<1 year). The degradation rate in the subsoil was also found to be critical. It had to be increased about fivefold to match measured chlorsulfuron concentrations in leachate. At such concentrations, 0·012 g ha⁻¹ of chlorsulfuron (0·3% of that applied) was predicted to leach through the soil profile during the 11-month simulation period when the lower dose of the compound was applied.     

Evaluation of the bioefficacy and leaching of a controlled–release formulation of chlorsulfuron

A controlled–release formulation of chlorsulfuron (DT26B) and a conventional granular formulation were compared for their initial bioefficacy and leaching in laboratory and field experiments. Three alkaline soil types, representative of farm soils in SE Australia, were used for these experiments. Laboratory tests of initial bioefficacy using a Mallee sand (pH 7.2) were able to detect, within ED_05–95 limits, that approximately 50% of active ingredient remained as a non–available reserve in DT26B immediately after spraying, when compared with the granular formation. There was also an indication that the bioavailable component of active ingredient in DT26B would be sufficient for weed control. This was confirmed at field sites on a Kattyoong sand (pH 7.9) and a Wimmera Grey clay (pH 8.4), after using recommended application rates of chlorsulfuron during the 1994 winter growing season. The formulations produced only marginal differences in control of Lolium rigidum Gaud. The field leaching trials at these sites showed that there can be rapid leaching of chlorsulfuron with only small amounts of rainfall. Under these conditions, there was an overall trend of reduced leaching by DT26B at both trial sites, although these reductions were not significant when compared with the granular formation, A laboratory system designed to measure the mobility of herbicides is described. It was used to impose upon each formulation a greater level of leaching than in the field trials, using Mallee sand columns irrigated with 50 mm day^–1 under –50 kPa suction. This test revealed a negligible reduction in leaching of chlorsulfuron by DT26B as compared with the granular formulation. Therefore, although the controlled–release formulation DT26B demonstrated that it could provide a viable alternative for weed control under conventional spraying conditions in arable farming, the agricultural usage of DT26B for the purpose of reducing leaching could not be warranted at its current stage of development.     

鲁北平原咸水滴灌对土壤水盐分布和棉花产量的影响

鲁北平原是山东省重要的粮棉油生产基地,合理利用微咸水和咸水资源是亟待解决的问题。通过田间小区试验,以淡水滴灌处理为对照,设置不同矿化度咸水滴灌处理,研究全地膜覆盖条件下,咸水滴灌对棉花农田土壤水盐分布和产量的影响。结果表明,灌出苗水可以明显降低棉田主要根层土壤EC值,降低率在26．8％～29．0％之间。咸水滴灌减少了棉花对土壤水分的吸收,主要影响土层在40～100 cm,灌溉水矿化度越高,影响越大。与淡水滴灌相比,滴灌补灌矿化度6g·L-1以下的咸水对棉花产量没有明显的影响,而滴灌8g·L-1的咸水在降水偏少的年份能明显降低棉花产量。从土壤盐分的积累来看,利用滴灌补灌一次6g·L-1以下的咸水,通过黄河水和夏季降水淋洗土壤盐分,不会造成棉花根系分布层土壤盐分的积累。该研究结果可为鲁北平原区咸水利用提供科学依据。     

[14C]Glyphosate transport in undisturbed topsoil columns

Although glyphosate (N‐(phosphonomethyl)glycine) is one of the most frequently used herbicides, few controlled transport experiments in undisturbed soils have been carried out to date. The aim of this work was to study the influence of the sorption coefficient, soil‐glyphosate contact time, pH, phosphorus concentration and colloid‐facilitated transport on the transport of [¹⁴C]glyphosate in undisturbed top‐soil columns (20 cm height × 20 cm diameter) of a sandy loam soil and a sandy soil. Batch sorption experiments showed strong Freundlich‐type sorption to both soil materials. The mobility of glyphosate in the soil columns was strongly governed by macropore flow. Consequently, amounts of glyphosate leached from the macroporous sandy loam soil were 50–150 times larger than from the sandy soil. Leaching rates from the sandy soil were not affected by soil‐glyphosate contact time, whereas a contact time of 96 h strongly reduced the leaching rates from the sandy loam soil. The role of pH and phosphorus concentration in solution was relatively unimportant with respect to total glyphosate leaching. The contribution of colloid‐facilitated transport was <1 to 27% for the sandy loam and <1 to 52% for the sandy soil, depending on soil treatment. The risk for glyphosate leaching from the top‐soils seems to be limited to conditions where pronounced macropore flow occurs shortly after application. © 2000 Society of Chemical Industry     

Lysimeter studies

Traditionally, the potential mobility of agricultural xenobiotics in soil has been assessed under controlled laboratory conditions. This continues to have a role in determining relative movement, but there is an increasing need to be confident that xenobiotics will not reach the groundwater under field conditions. Outdoor lysimeters using intact soil cores provide a more realistic means of assessment than laboratory techniques without the complications and uncertainties of field studies. Two methods of collecting intact lysimeter cores are described, together with a series of experiments involving the degradation and mobility of the radiolabelled herbicide benazolin-ethvl (ethyl 4-chloro-2,3-dihydro-2-oxo-1,3-henzothiazol-3-ylacetate).     

From laboratory to field: uses and limitations of pesticide behaviour models for the soil/plant system

Modelling is an economic way of assessing pesticide behaviour under field conditions; it is cheaper and faster than field experiments. Modelling attempts to generalize knowledge of pesticide field behaviour through identification of the most important pesticide/soil properties that can be measured in the laboratory. The technology to simulate volatilization of volatile pesticides that are incorporated or injected into the soil is well developed. However, modelling of volatilization rates from plant and soil surfaces before the first significant rainfall event after application is barely possible with current knowledge. The technology to simulate pesticide persistence in the plough layer is well developed; the PERSIST model has been tested at least 178 times, usually resulting in a slightly faster decline in the field than was simulated. In general, available pesticide leaching models are reliable enough to assess the leaching of the bulk of the dose (leaching levels above 1%). The EU drinking water limit of 0.1 μg L^?1 implies leaching of less than 0.1% of a dose of 1 kg ha^?1. At such a low leaching level, the validation status of the models is still low, mainly because preferential flow processes in both structured and unstructured soils and the factors controlling the transformation rate in subsoil are not well enough understood.     

莠去津在土壤中的残留动态和淋溶动态

利用HPLC法对土壤中莠去津的残留动态、淋溶动态进行了研究。结果显示,莠去津以有效成分2.25 kg/hm2和4.50 kg/hm2的剂量施用时,在土壤中的半衰期分别为19.1 d和18.1 d,即其半衰期与莠去津的施用浓度无关,属于典型的一级动力学反应。在120 d的玉米生长期中,土壤中莠去津在不断降解代谢的同时,逐渐向深层土壤中淋溶,多数莠去津持留在表层土壤中。施用莠去津27 d后,高浓度处理小区莠去津的淋溶深度超过30 cm,深度为10~15 cm处的土壤在施用后27 d莠去津的浓度最大。同一土壤深度,莠去津在高浓度处理小区的残留量要远高于低浓度处理小区。这些结果显示,减小莠去津的用量可以减少莠去津在土壤中的移动,表明低剂量施用莠去津是保护地下水免受污染的一种有效措施。影响莠去津的淋溶作用的主要因素包括使用量和土壤的理化特性。     

Influence of methyl bromide soil fumigation method on fungicidal efficacy and bromide residues

Highest inorganic bromine residues (30 ppm) were found when soil was fumigated with liquid methyl bromide (MB) introduced by conventional means into evaporating dishes. With preheated (vaporized) MB or injection of MB/chloropicrin (CP) mixtures, bromide concentrations were reduced by 50%. They were uniform throughout the soil (0 to 60 cm) except after MB/CP injection, when larger residues were found in the 30-60 cm layer. Leaching with 2000 m^p3/ha (20 cm) of water always reduced bromide content to 7.5 and 10 ppm at the 0-30 and 30-60 cm depth, respectively. Organic amendments to soils substantially increased bromide levels up to 118 ppm, most of which was found in the upper soil layers; two teachings with 2000 m^p3 /ha water were required to return the soils to their normal state.Sclerotium rolfsii andFusarium oxysporum f. sp.dianthi cultures buried in soil were eliminated from the upper 30 cm with MB applied either conventionally or preheated. At 50 cm, 500 kg/ha of the preheated gas was superior to 1000 kg/ha of the cold gas. All MB fumigations suppressed carnation flower yields compared with CP alone but were superior to no treatment. After leaching, MB-fumigated soils yielded the highest number of flowers.     

Application and validation of pesticide leaching models

Two lysimeter studies with the pesticides cloethocarb and bentazone were used to validate a modified version of the computer model SESOIL (Seasonal Soil Compartment Model), developed by Bonazountas and Wagner and to transfer the results of the lysimeter study to another climate scenario. For cloethocarb, an experimental insecticide without marked mobility in the soil, as indicated by the lysimeter study, the simulations were in excellent agreement with the lysimeter study. Calculations for bentazone revealed higher leaching than found in the lysimeter study. This discrepancy was resolved by a more careful and realistic inclusion of sorption in the lysimeter soil into the simulation model.     

Leaching of Methyl Isothiocyanate in Plainfield Sand Chemigated with Metam-Sodium

Soil column studies were undertaken to investigate the influence of soil water content and irrigation on leaching, distribution and persistence of methyl isothiocyanate (MITC) in a sandy soil chemigated with the soil fumigant metam-sodium. No leaching was obtained from columns at low water content (0·042 or 0·074 cm³ cm⁻³). However, 8·4 (±2·8), 34·2 (±7·4) and 119·4 (±8·3) μg of MITC leached from columns at 0·105, 0·137 and 0·168 cm³ cm⁻³ water content, respectively. Increased leaching resulted from sprinkler application of 25-mm of water to columns at 0·137 cm³ cm⁻³ water content. Leaching of MITC constituted only a small fraction of the amount applied even in the worst case. Methyl isothiocyanate persisted in soil for 15 days at 2°C in varying amounts under the different water regimes. Relatively high amounts of MITC residues (8–12 mg kg⁻¹ soil) were detected in the top 25-cm layer of all the soil columns. Degradation was the major pathway of dissipation for the chemical despite the soil water regime.     

膜下滴灌滴水流量对棉花根系分布的影响

为探明膜下滴灌条件下滴水流量对棉花根系分布的影响,采用田间定位试验,设置3组滴水流量,分别为1.69 L·h^-1（W169）、3.46 L·h^-1（W346）和6.33 L·h^-1（W633）,分析膜下滴灌土壤水吸力对棉花根系空间分布特征的影响。结果表明：滴水流量越大,膜外裸地和膜内土壤的基质吸力越小,边行棉花根系吸水受到的胁迫程度也越小,W633处理膜下根区土壤水吸力分布均匀且适合棉花生长,棉花根长密度水平分布不论是初期还是后期均呈双峰抛物线分布;棉花根重密度呈现出相同的变化规律,内、边行棉花根重密度在花期、铃期、吐絮期的差值分别为42.26、-0.22、97.40 g·m^-3。当滴水流量越小时,膜外裸地的基质吸力大于膜内土壤的基质吸力,致使边行棉花的根系吸水受到较大胁迫,W169处理棉花根长密度水平分布由生育初期的双峰抛物线分布变为后期的单峰抛物线分布;内、边行棉花根重密度在花期、铃期、吐絮期的差值分别为299.70、304.86、369.84 g·m^-3。表明随着滴水流量的增加,膜下滴灌行间棉花根系生长更加均匀,研究结果可为棉花膜下滴灌系统设计提供技术参考。