Photolytic versus microbial degradation of clomazone in a flooded California rice field soil

BACKGROUND: Clomazone is a popular herbicide used on California rice fields and exhibits rapid anaerobic microbial degradation (t_1/2 = 7.9 days). To test the potential of direct and indirect photolytic degradation as a cofactor in the overall degradation rate, sacrificial time‐series microcosms were amended with water, non‐sterilized soil + water and sterilized soil + water. Clomazone was added to each microcosm, which was then exposed to natural and artificial sunlight over 35 days. Water and acetonitrile extracts were analyzed for clomazone and metabolites via LC/MS/MS. RESULTS: The calculated pseudo‐first‐order degradation rate constants (k) were k_water = 0–0.005 ± 0.003 day^?1, k_sterile = 0–0.005 ± 0.003 day^?1 and k_non?sterile = 0.010 ± 0.002–0.044 ± 0.007 day^?1, depending on light type. The formation of ring‐open clomazone, a microbial metabolite, correlated with clomazone degradation. Trace amounts of 5‐hydroxyclomazone (m/z = 256 → 125), aromatic hydroxyclomazone (m/z = 256 → 141) and an unknown product (m/z = 268 → 125) were observed. CONCLUSIONS: The photolytic degradation rate depends on both light type and the quality of the chromophores that induce indirect photolysis. Microbial degradation was found to be sensitive to temperature fluctuations. Overall, microbes are shown to be more detrimental to the environmental fate of clomazone than photolysis. Copyright © 2012 Society of Chemical Industry     

Variability of retention process of isoxaflutole and its diketonitrile metabolite in soil under conventional and conservation tillage

BACKGROUND: Sorption largely controls pesticide fate in soils because it influences its availability for biodegradation or transport in the soil water. In this study, variability of sorption and desorption of isoxaflutole (IFT) and its active metabolite diketonitrile (DKN) was investigated under conventional and conservation tillage. RESULTS: According to soil samples, IFT K_D values ranged from 1.4 to 3.2 L kg^?1 and DKN K_D values ranged from 0.02 to 0.17 L kg^?1. Positive correlations were found between organic carbon content and IFT and DKN sorption. IFT and DKN sorption was higher under conservation than under conventional tillage owing to higher organic carbon content. Under conservation tillage, measurements on maize and oat residues collected from the soil surface showed a greater sorption of IFT on plant residues than on soil samples, with the highest sorbed quantities measured on maize residues (K_D ≈ 45 L kg^?1). Desorption of IFT was hysteretic, and, after five consecutive desorptions, between 72 and 89% of the sorbed IFT was desorbed from soil samples. For maize residues, desorption was weak (<50% of the sorbed IFT), but, after two complementary desorptions allowing for IFT hydrolysis, DKN was released from maize residues. CONCLUSION: Owing to an increase in organic carbon in topsoil layers, sorption of IFT and DKN was enhanced under conservation tillage. Greater sorption capacities under conservation tillage could help in decreasing DKN leaching to groundwater. Copyright © 2012 Society of Chemical Industry     

Bromoxynil degradation in a Mississippi silt loam soil

BACKGROUND: The objectives of these laboratory experiments were: (1) to assess bromoxynil sorption, mineralization, bound residue formation and extractable residue persistence in a Dundee silt loam collected from 0–2 cm and 2–10 cm depths under continuous conventional tillage and no‐tillage; (2) to assess the effects of autoclaving on bromoxynil mineralization and bound residue formation; (3) to determine the partitioning of non‐extractable residues; and (4) to ascertain the effects of bromoxynil concentration on extractable and bound residues and metabolite formation. RESULTS: Bromoxynil K_d values ranged from 0.7 to 1.4 L kg^?1 and were positively correlated with soil organic carbon. Cumulative mineralization (38.5% ± 1.5), bound residue formation (46.5% ± 0.5) and persistence of extractable residues (T_1/2 < 1 day) in non‐autoclaved soils were independent of tillage and depth. Autoclaving decreased mineralization and bound residue formation 257‐fold and 6.0‐fold respectively. Bromoxynil persistence in soil was rate independent (T_1/2 < 1 day), and the majority of non‐extractable residues (87%) were associated with the humic acid fraction of soil organic matter. CONCLUSIONS: Irrespective of tillage or depth, bromoxynil half‐life in native soil is less than 1 day owing to rapid incorporation of the herbicide into non‐extractable residues. Bound residue formation is governed principally by biochemical metabolite formation and primarily associated with soil humic acids that are moderately bioavailable for mineralization. These data indicate that the risk of off‐site transport of bromoxynil residues is low owing to rapid incorporation into non‐extractable residues. Published 2009 by John Wiley & Sons, Ltd     

Pedotransfer functions for isoproturon sorption on soils and vadose zone materials

BACKGROUND: Sorption coefficients (the linear K_D or the non‐linear K_F and N_F) are critical parameters in models of pesticide transport to groundwater or surface water. In this work, a dataset of isoproturon sorption coefficients and corresponding soil properties (264 K_D and 55 K_F) was compiled, and pedotransfer functions were built for predicting isoproturon sorption in soils and vadose zone materials. These were benchmarked against various other prediction methods. RESULTS: The results show that the organic carbon content (OC) and pH are the two main soil properties influencing isoproturon K_D. The pedotransfer function is K_D = 1.7822 + 0.0162 OC^1.5 ? 0.1958 pH (K_D in L kg^?1 and OC in g kg^?1). For low‐OC soils (OC < 6.15 g kg^?1), clay and pH are most influential. The pedotransfer function is then K_D = 0.9980 + 0.0002 clay ? 0.0990 pH (clay in g kg^?1). Benchmarking K_D estimations showed that functions calibrated on more specific subsets of the data perform better on these subsets than functions calibrated on larger subsets. CONCLUSION: Predicting isoproturon sorption in soils in unsampled locations should rely, whenever possible, and by order of preference, on (a) site‐ or soil‐specific pedotransfer functions, (b) pedotransfer functions calibrated on a large dataset, (c) K_OC values calculated on a large dataset or (d) K_OC values taken from existing pesticide properties databases. Copyright © 2011 Society of Chemical Industry     

Aminocyclopyrachlor sorption–desorption and leaching in soil amended with organic materials from sugar cane cultivation

The correct application of a new herbicide depends on knowledge concerning its behaviour within the cultivation system. The objective of this study was to evaluate the sorption–desorption process of aminocyclopyrachlor in soils with the addition of three aged organic materials from sugar cane and their transport via leaching. Sugar cane straw (12 t/ha), filter cake (90 t/ha) and vinasse (200 m³/ha) were added to a clayey soil 15, 30 and 60 days before herbicide application. Sorption and desorption were evaluated by the batch equilibrium method. For leaching assessments, the materials were applied to the soil surface. Sorption was relatively low in all treatments (K_d = 0.17–0.41 L/kg), although significantly higher in soil without organic material addition. A negative correlation between herbicide sorption and increased soil base saturation was observed, indicating competition for sorption sites. With the addition of vinasse, 71% of the herbicide reached the leachate, while <50% reached the leachate in the other treatments. Aminocyclopyrachlor availability was not reduced with organic material addition to the soil, which may be favourable for weed control. However, the presence of vinasse leads to the risk of leaching to deeper soil layers than the seed bank.     

Sorption of ionisable organic compounds by field soils. Part 1: Acids

Sorption of different classes of weak organic acids was measured using soils with a range of pH values, taken from long-term field experiments that had received different amounts of lime. Non-ionisable compounds were used to demonstrate that the soils of different pH used in the experiments have similar sorptive properties. Values of the sorption coefficients for chloride ion were negative at all pHs except one. Sorption of moderately polar, monobasic, weak acids was weak in acidic soils and very weak in neutral and alkaline soils where they were predominantly dissociated. A lipophilic weak acid was strongly sorbed even at high pH. A model is presented which estimates soil/water distribution coefficients, at any soil pH, from lipophilicity and pK_a of the acid and organic matter content of the soil. The model was derived using sorption measured for substituted phenoxyacetic acids. Sorption values calculated using the model were compared with values measured for chlorsulfur on and showed useful agreement. Dibasic acids were strongly sorbed, probably by the mechanism of ligand exchange, if they were chelating agents with potential to form 5- or 6-membered rings with an acceptor atom. Phenylphosphonic acid was strongly sorbed, being a strong monodentate ligand.     

Characterization of the sorption of an anthranilate fungicide in soil using thermal analytical and mineralogical techniques

The sorption of pesticides to soil particles has implications for their distribution and fate in the soil environment. A batch equilibrium technique was used to investigate sorption of the fungicide AEC623892 to intact and hydrogen-peroxide-treated whole soils and several particle-size fractions isolated from them. Sorption of AEC623892 to the soil as a whole was low. K(f oc) values measured in the whole soils were 169.2 and 41.9 ml g(-1) for Soil A and Soil B respectively. The highest values of K(f oc) were measured in soil particle-size fractions <53 microm (266.5 ml g(-1) in the 2-20 microm fraction of Soil A; 471.9 ml g(-1) in the 20-53 microm fraction of Soil B). Sorption was most irreversible in the 2-20 microm fractions. Overall, treatment of soil particle-size fractions with hydrogen peroxide resulted in lower values of K(f oc) (112.3 ml g(-1) in Soil A whole soil and 30.9 ml g(-1) in Soil B whole soil). In both soils, the maximum sorption among hydrogen-peroxide-treated samples was observed in the <2 microm fraction (166.6 and 311.0 ml g(-1) for Soil A and Soil B, respectively). Investigation of the mineralogical composition of the soils suggested that the clay mineralogy (dominated by kaolinite and illite) is less likely to account for the differences in sorption observed than differences in the characteristics of the soil organic matter. Thermal analysis of the different soil fractions indicated that hydrogen peroxide treatment preferentially removed aliphatic fractions of organic matter, but had less effect on lignin-like, aromatic fractions.     

Kinetics of sorption-desorption of benfuracarb insecticide in mollisols

BACKGROUND: Sorption‐desorption processes govern the movement of pesticides in soil. These processes determine the potential hazard of the pesticide in a given environment for groundwater contamination and need to be investigated. RESULTS: In the present study, sorption‐desorption processes of benfuracarb were investigated using a batch method in two mollisols. The kinetics of benfuracarb sorption in mollisols conformed to two‐compartment (1 + 1) first‐order kinetics. The fast sorption rate constant was about 3 times higher for silt loam than for loam soil. However, the slow sorption rate constants were statistically similar for both soils. The concentration‐dependent sorption‐desorption isotherms of benfuracarb could not closely conform to the Freundlich isotherm in mollisols of high organic C content. The computed values of both the sorption (log K) and desorption (log K′) capacities were higher for silt loam than for loam soil. The desorption index (n′/n) values in the range 30.0–41.3 indicated poor reversibility of sorbed benfuracarb in mollisols. CONCLUSION: In view of the strong sorption of benfuracarb in mollisols with only partial desorption, the possibility of the leaching of soil‐applied benfuracarb to contaminate groundwaters appears to be low. Copyright © 2010 Society of Chemical Industry     

Behaviour of the insecticides ethoprophos and carbofuran during soil—water transport

The movement of the organophosphate nematicide-insecticide ethoprophos (ethoprop; O-ethyl S,S-dipropyl phosphorodithioate) and the carbamate insecticide-nematicide carbofuran (2,3-dihydro-2,2-dimethylbenzofuran-7-yI methylcarbamate) was studied under steady-state flow in small-scale laboratory soil columns. Miscible displacement column experiments, mass balance calculations, and batch incubation studies furnished information on insecticide sorption and degradation processes that occur during transport through soil. Miscible displacement studies demonstrated that ethoprophos degradation could be described as first-order and that both insecticides exhibited non-equilibrium sorption. Both batch and miscible displacement results showed ethoprophos to be more strongly sorbed by soil than carbofuran. Measured equilibrium sorption coefficients were 1.29 cm³ g^?1 for ethoprophos and 0.29 cm³ g^?1 for carbofuran on a Riverhead soil (0.011 organic carbon fraction); 035 cm³ g^?1 for carbofuran on Valois soil (0.016 organic carbon fraction); and 2.38 cm³ g^?1 for ethoprophos on Rhinebeck soil (0.031 organic carbon fraction). Two solutions to the convection-dispersion equation, one that incorporated equilibrium sorption and another (bicontinuum model) that included a non-equilibrium sorption term, allowed quantitative evaluation of transport processes. The bicontinuum model used in conjunction with experimental batch and mass balance techniques provided estimates of insecticide sorption and degradation parameters.     

Sorption–desorption of 'aged' isoxaflutole and diketonitrile degradate in soil

Isoxaflutole is a relatively new herbicide used for weed control in maize. The objective of this research was to increase the understanding of the behaviour and environmental fate of isoxaflutole and its diketonitrile (DKN) degradate in soil, including determination of the strength of sorption to soil and whether sorption is affected by ageing. In sandy loam (SL) and silty clay (SiCl) soils, ¹⁴C‐isoxaflutole was found to dissipate rapidly after application to soil; recovery ranged from ～42% to 68% at week 0, and recovery had decreased to <10% at week 12. Decreases in ¹⁴C isoxaflutole residues over time in SL and SiCl soils are consistent with hydrolysis of isoxaflutole and formation of bound DKN residues in the soil. DKN recovery from freshly treated SiCl and SL soils was 41% to 52%. After a 12‐week incubation in SL soil at pH 7.1 and 8.0, recoveries were similar, ～40%. However, at week 12 in SL soil pH 5.7, DKN recovery decreased to ～28%. DKN recovery in SiCl soil at week 12 was <10%. Increases in sorption of DKN in SL at pH 5.7 and SiCl soil over time indicate that the DKN degradate is tightly bound to the soil and sorption is affected by soil pH and soil type. Sorption of ¹⁴C‐DKN in the SiCl soil more than doubled with ageing compared with the lower K_d sorption coefficient values of the SL soils. In the SiCl soil at time 0, the K_d was 0.6; at 1 week, K_d increased to 2; and at the end of the 12‐week incubation period, K_d was 4.5. This strong binding of DKN to the soil may be due to chelate formation in the interlayer of the clay.