Metribuzin degradation in soil: I—effects of soybean residue amendment,metribuzin level,and soil depth

Plant residue and soil depth effects on metribuzin degradation were investigated. Dundee silt loam soil collected at depth increments of 0–10 cm (SUR) and 10–35 cm (SUB) was treated with labeled [5^?14 C]metribuzin. Samples were assayed at several time points up to 140 days after treatment. Soybean residue was added to half of the SUR samples (RES), with remaining SUR unamended (NORES). None of the SUB samples were amended with soybean residue. Metribuzin mineralization to ¹⁴CO₂ proceeded more slowly in RES and SUB than in NORES and SUR, respectively. Extractable components in SUR samples included polar metabolites, plus deaminated metribuzin (DA) in the RES, and parent metribuzin in the NORES. Deaminated diketometribuzin (DADK) and metribuzin comprised major ¹⁴C components extracted from SUB, while in SUR, faster degradation of metabolites resulted in metrizubin as the primary identifiable compound. Unextractable ¹⁴C increased until day 35 for both RES and NORES, after which it remained constant for NORES. but declined for RES. A corresponding rise in RES polar ¹⁴C suggested that as soybean residue decomposed, ¹⁴C bound in the residue was released as extractable polar material. Soil with soybean residue accumulation may alter metabolite degradation patterns, but does not impede initial metribuzin degradation. Depth differences in metribuzin degradation were attributed to reductions in microbial activity with increasing soil depth.     

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.     

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.     

Spatial variability in the degradation rates of isoproturon and chlorotoluron in a clay soil

Thirty separate soil samples were taken from different locations at the Brimstone farm experimental site, Oxfordshire, UK. Incubations of isoproturon under standard conditions (15 °C; ?33 kPa soil water potential) indicated considerable variation in degradation rate in the soil, with the time to 50% loss (DT₅₀) varying from 6 to 30 days. These differences were confirmed in a second comparative experiment in which degradation rates were assessed in 11 samples of the same soil in two separate laboratories using an identical protocol. There was a significant negative linear relationship (r²= 0.746) between the DT₅₀ values and soil pH in this group of soils. In a third experiment, degradation rates of the related compound chlorotoluron were compared with those of isoproturon in 12 separate soil samples, six of which had been stored for several months, and six of which were freshly collected from the field. Degradation of both herbicides occurred more slowly in the stored samples than in the fresh samples but, in all of them, chlorotoluron degraded more slowly than isoproturon, and there was a highly significant linear relationship (r²=0.916) between the respective DT₅₀ values.     

Metribuzin degradation in soil: II—effects of tillage

A 140-day laboratory incubation, using surface soil from a long-term soybean tillage study, evaluated tillage influence on [¹⁴C]metribuzin degradation. Higher plant residue conditions in no-tillage (NT) soil inhibited metribuzin mineralization to [¹⁴C]carbon dioxide as compared to metribuzin degradation patterns observed in conventional tillage (CT) soil. At 140 days, relative abundance of extractable ¹⁴C components in NT included polar metabolites > metribuzin = deaminated metribuzin (DA) = deaminated diketometribuzin (DADK), while in CT, components included metribuzin > polar metabolites > DADK?DA. Conditions in NT apparently inhibited polar ¹⁴C degradation, and resulted in its accumulation, while in CT polar ¹⁴C degradation proceeded relatively rapidly. For both NT and CT, more ¹⁴ C was measured in an unextractable fraction than in any other fraction. A greater portion of the unextractable fraction in NT was associated with decomposed plant residue than in CT. Surface accumulation of crop residue, such as occurs under NT, provided a soil environment which altered metribuzin degradation patterns.     

Degradation and adsorption of carbendazim and 2-aminobenzimidazole in soil

Increasing adsorption of [¹⁴C]-labelled carbendazim in soil took place within a few weeks of incubation and was greatest in soil with a high organic matter content. Carbendazim was slowly decomposed in soil, mainly by soil microorganisms. After 250 days of incubation in two unsterilised soils, 13 and 5% respectively of added [¹⁴C]-carbendazim was recovered compared with 70 and 50% respectively from sterile soils; 4–8% of added carbendazim was recovered as 2-aminobenzimidazole (2-AB) from both unsterilised and sterile soil. After 270 days' incubation, 33 and 9% of ¹⁴C was recovered as ¹⁴CO₂ from soil supplied with [¹⁴C]-carbendazim (20 and 100 mg/kg) respectively. Degradation started more rapidly when carbendazim was added to soil preincubated with the fungicide but the degradation rate was very low in all cases, indicating that the compound is a poor microbial energy source and that the degradation is a co-metabolic process. 2-AB was found as a degradation product although it appeared to be unstable in soil, decomposing rapidly after a lag period of about 3 weeks; small amounts remained in the soil for several months, however, presumably adsorbed on soil particles.     

The degradation of dinitramine (N3,N3-diethyl 2,4-dinitro-6-trifluoromethyl-m-phenylenediamine) in soil

Radioactive dinitramine (1) was incorporated at $\text{1}\text{2}$ Ib/acre (0.6 ppm) in Anaheim silty loam soil and its degradation studied over an 8-month period. For both specifically—¹⁴CF₃ and -Ring-UL-[¹⁴C] labeled (1), only ca. 20% of the radioactivity was lost from the incorporated zone. Mehanol- or acetonitrile-extractable radioactivity decreased rapidly over the initial 60 days reaching 20% after 244 days. Two compounds were isolated and characterized as (1), 0.05 ppm, and 6-amino-1-ethyl-2-methyl-7-nitro-5-trifluoromethylbenzimidazole (2), 0.06 ppm. Two other compounds were tentatively identified by TLC as monodealkylated dinitramine (3), 0.01 ppm, and 6-amino-2-methyl-7-nitro-5-trifluoromethylbenzimidazole (4), 0.01 ppm, Sodium hydroxide (10%) and anionic surfactant (10%) were effective in removing up to 50% of the residual bound radioactivity (i.e., nonacetonitrile extractable), while dimethylamine (25%) released 26%; extraction by acid was less effective.     

Changes in the concentrations of isoproturon and its degradation products in soil and soil solution during incubation at two temperatures

Changes in the concentrations of [¹⁴C]carbonyl-isoproturon and its degradation products in a clay-loam soil and in soil solution during incubation at 11°C and 18°C for 6 weeks, were measured following solvent extraction and soil solution sampling with glass microfibre filters. During herbicide degradation, ¹⁴CO₂ was released (up to 20%) and unextractable radioactivity increased (up to 30%). Monomethyl isoproturon was the main metabolite in soil followed by metabolite X₅ (possibly hydroxy di-des-methyl isoproturon). Isoproturon and monomethyl isoproturon were mainly adsorbed by soil whereas metabolite X₅ was found mainly in the soil solution. Isoproturon concentrations declined in both soil and soil solution, but the percentage of the residual herbicide dissolved in the soil solution decreased from 26 to 15%. At low temperature, herbicide degradation occurred more slowly, and the degradation products were generally less abundant. However metabolite X₅ was present at unexpectedly high levels, particularly in the soil solution. Evolution de l'isoproturon et de ses produits dégradation dans le sol et la solution du sol pendant l'incubation de Vherbicide a deux temperatures. L'évolution de l'isoproturon (marqué au ¹⁴C sur le carbonyle) et de ses produits de dégradation dans un sol argilo-limoneux et dans la solution du sol est suivie pendant 6 sêmaines d'incubation de l'herbicide à 11 et 18°C. Pour ce faire, la solution du sol est échantillonnée au moyen de filtres en fibres de verre et les composés sont extraits du sol par des solvants. Au cours de la dégradation, du ¹⁴CO₂ est libéré (jusqu'à 20%) et la radioactivité non extraite s'accroit (jusqu'à 30%). L'isoproturon monométhyle est le principal métabolite dans le sol suivi du metabolite X5 (probablement le dérivé hydroxy didéméthylé). L'isoproturon et son dérivé monométhyle sont surtout adsorbés par le sol alors que le métabolite X5 est surtout en solution. La quantite d'iso-proturon diminue simultanemént dans le sol et la solution du sol mais la fraction dissoute de l'herbicide residuel décroit de 26 à 15%. A basse température, la dégradation de l'herbicide est plus lente et les produits de dégradation sont généralement moins abondants à l'exception notable du métabolite X⁵ qui est présent a un niveau élevé, en particulier dans la solution du sol. Veränderung der Konzentration von Isoproturon und seiner Abbauprodukte im Boden und in der Bodenlösung bei Inkubation Veränderung der Konzentration von [¹⁴C]-Car-bonyl-Isoproturon und seiner Abbauprodukte in einem Lehmboden und in der Bodenlösung wurden nach 6 Wochen Inkubation bei 11 und 18°C und Extraktion bzw. Probennahme durch Glasmikrofaserfilter gemessen. Während des Herbizidabbaus wurden bis zu 20 % der Radioaktivität als ¹⁴CO2 freigesetzt, und die nichtextrahierbare Radioaktivität nahm zu (bis zu 30 %). Monomethyl-Isoproturon war der Hauptmetabolit, gefolgt vom Metabolit X₅ (möglicherweise Hydroxy-didesmethyl-Isoproturon). Isoproturon und Monomethyl-Isoproturon waren weitgehend an Bodenpartikeln adsorbiert, während der Metabolit X₅ vorwiegend in der Bodenlösung gefunden wurde. Die Isoproturon-Konzentrationen nahmen sowohl im Boden als auch in der Bodenlösung ab, aber der Anteil des Herbizidrückstands in der Bodenlösung ging von 26 auf 15 % zurück. Bei der niedrigen Temperatur wurde das Herbizid langsamer abgebaut, und die Menge der Abbauprodukte war allgemein geringer. Der Metabolit X₅ lag jedoch in unerwartet hoher Menge vor, besonders in der Bodenlösung.     

Controlled release of isoproturon from an alginate–bentonite formulation: water release kinetics and soil mobility

The herbicide isoproturon [3‐(4‐isopropylphenyl)‐1,1‐dimethylurea] was incorporated in alginate‐based granules to obtain controlled‐release (CR) properties. The basic formulation (sodium alginate (1.87%)–isoproturon (0.67%) in water) was modified by addition of different sorbents. The effect on isoproturon release rate, modified by the incorporation of natural and acid‐treated bentonite in alginate formulation, was studied by immersion of the granules in water while shaking. The release of isoproturon was diffusion‐controlled. The time taken for 50% of the active ingredient to be released into water, T₅₀, was longer for those formulations containing added bentonite (5.98 and 7.43 days, for natural and acid‐treated (1 M H₂SO₄) bentonite, respectively) than for the preparation without bentonite (3.78 days). The mobilities of non‐formulated technical grade (98%) and formulated isoproturon were compared using soil columns. The use of alginate‐based CR formulations containing bentonite reduced isoproturon movement compared with the technical product. Sorption capacity of the soil for isoproturon was measured using batch experiments (0.29 litre kg⁻¹) and the results obtained here in agreement with those obtained under dynamic conditions. © 2000 Society of Chemical Industry     

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.     

Fate of [14C]diflubenzuron on cotton and in soil

Aqueous suspensions and oil emulsions of a commercial [¹⁴C]diflubenzuron (N-[[(4-chlorophenyl)amino]carbonyl]-2,6-difluorobenzamide) formulation (Dimilin W-25) remained on the leaf surface of greenhouse-treated plant tissues. Absorption, translocation, and metabolism of the [¹⁴C]diflubenzuron were not significant. Less than 0.05% of the applied ¹⁴C was found in newly developed plant tissues 28 days after spray treatment. [¹⁴C]Diflubenzuron was degraded in soil. After 91 days, biometer flask studies showed that 28% of the ¹⁴C incorporated into the soil as [¹⁴C]diflubenzuron was recovered as ¹⁴CO₂. Major dichloromethane-soluble soil residues were identified as unreacted [¹⁴C]diflubenzuron and [¹⁴C]4-chlorophenylurea. A minor unknown degradation product cochromatographed with 2,6-difluorobenzoic acid. Insoluble ¹⁴C-residues increased with time and represented 67.8% of the residual ¹⁴C in the soil 89 days after treatment. Cotton plants grown for 89 days in [¹⁴C]diflubenzuron-treated soil contained only 3% of the ¹⁴C applied to the soil. Small quantities of acetonitrile-soluble [¹⁴C]4-chlorophenylurea were isolated from the foliar tissues. Root tissues contained small amounts of [¹⁴C]diflubenzuron and trace quantities of a minor ¹⁴C-product that chromotographed similarly to 2,6-difluorobenzoic acid. Most of the ¹⁴C in the plant tissues (84–93%) was associated with an insoluble residue fraction 89 days after treatment.     

Determination of extractable and non-extractable radioactivity from prairie soils treated with carboxyl- and ring-labelled [14C]2,4-D

Ring- and carboxyl-labelled [¹⁴C]2,4-D were incubated under laboratory conditions, at the 2 g/g level, in a heavy clay, sandy loam, and clay loam at 85% of field capacity and 20 1C. The soils were extracted at regular intervals for 35 days with aqaeous acidic acetonitrile, and analysed for [¹⁴C]2,4-D and possible radioactive degradation products. Following solvent extraction, a portion of the soil residues were combusted in oxygen to determine unextracted radioactivity as [¹⁴C]carbon dioxide. The remaining soil residues were then treated with aqueous sodium hydroxide, and the radioactivity associated with the fulvic and humic soil components determined. In all soils there was a rapid decrease in the amounts of extractable radioacitivity, with only 5% of that applied being recoverable after 35 days. All recoverable radioactivity was attributable to [¹⁴C]2,4-D, and no [¹⁴C]-containing degradation products were observed. This loss of extractable radioactivity was accompanied by an increase in non-extractable radioactivity. Approximately 15% of the applied radioactivity, derived from carboxyl-labelled [¹⁴C]2,4-D, and 30% from the ring-labelled [¹⁴C]2,4-D was associated with the soil in a non-extractable form, after 35 days of incubation. After 35 days, less than 5% of the radioactivity from the carboxyl-labelled herbicide, and less than 10% of the ringlabelled material, was associated with the fulvic components derived from the three soils. Less than 5% of the applied radioactivities were identifiable with any of the humic acid components. It was considered that during the incubation [¹⁴C]2,4-D did not become bound or conjugated to soil components, and that non-extractable radioactivity associated with the three soil types resulted from incorporation of radioactive degradation products, such as [¹⁴C]carbon dioxide, into soil organic matter.     

The adsorption and degradation of glyphosate in five Hawaiian sugarcane soils*

The rate of aerobic evolution of ¹⁴CO₂ from ¹⁴C-glyphosate labelled in the methylphosphonyl carbon, varied 100-fold within a group of five Hawaiian sugarcane soils. The rate depended inversely on the degree of soil binding, probably associated with the phosphonic acid moiety, and to a less certain extent on soil pH and soil organic matter. After an initial rapid degradation, the rate of ¹⁴CO₂ evolution in three soils reached a constant at 16–21 days which continued to the 60-day termination. The other two soils showed a continually decreasing rate throughout. Two soils released over 50% of the labelled carbon in 60 days, a third released 35%, while the remaining soils released 1.2 and 0.8% respectively. Labelled carbon in the soils after 60 days consisted of glyphosate and one metabolite, aminomethyl-phosphonic acid, with glyphosate predominating in high fixing soils. The ¹⁴C could be extracted almost completely with NaOH solution, and remained mainly in solution after acidification.     

Effect of ABT on the activity and rate of degradation of isoproturon in susceptible and resistant biotypes of Phalaris minor and in wheat

The effect of the monooxygenase inhibitor, 1-aminobenzotriazole (ABT) on isoproturon phytotoxicity and metabolism was studied in resistant (R) and susceptible (S) biotypes of Phalaris minor and in wheat (Triticum aestivum). Addition of ABT (2·5, 5 and 10 mg litre^-1) to isoproturon (0·25, 0·5, 1, 2 and 4 mg litre^-1) in the nutrient solution significantly enhanced the phytotoxicity of isoproturon against the R biotype. Isoproturon at 0·25 mg litre^-1 reduced the dry weight (DW) of the S biotype by 77%, whereas the R biotype required 4·0 mg litre^-1 for similar reduction. Addition of 10 mg litre^-1 of ABT to the 0·25 mg litre^-1 isoproturon caused 71 and 82% reduction in DW of R and S biotypes, respectively. Wheat was more sensitive to the mixture of isoproturon and ABT than the R biotype of P. minor. Reduced concentrations of ABT in the mixture from 10 to 2·5 mg litre^-1 increased the DW of the R biotype more than that of the S biotype. The R biotype metabolised [¹⁴C]isoproturon at a faster rate than the S biotype. ABT (5 mg litre^-1) inhibited the degradation of [¹⁴C]isoproturon in both biotypes of P. minor and in wheat. In the presence of ABT, about half of the applied [¹⁴C]isoproturon remained as parent herbicide in all the three species after two days. The metabolites were similar in the R and S biotypes and wheat as determined by co-chromatography with reference standards and mass spectroscopy (MS). ABT inhibited the appearance of the hydroxy and monomethyl metabolites and their conjugates in all the test plants. These results suggest that the activity of the enzymes responsible for the degradation of isoproturon is greater in the R than in the S biotype of P. minor, resulting in its rapid detoxification. Incorporation of the monooxygenase inhibitor ABT into the nutrient solution greatly inhibited the degradation of [¹⁴C]isoproturon in the R biotype and increased its phytotoxicity. Both hydroxylation and N-dealkylation reactions were found to be sensitive to ABT; inhibition of hydroxylation was greater than that of demethylation. Since ABT could not completely suppress isoproturon degradation, it is possible that more than one monooxygenase is involved. © 1998 SCI     

Enhanced degradation of iprodione and vinclozolin in soil

Residues of iprodione and vinclozolin were measured following repeated application of the fungicides to a sandy loam soil in the laboratory. There was a progressive increase in rates of degradation with successive treatments. With iprodione, for example, the times for 50% loss of the first and second applications were about 23 and 5 days respectively. When treated for the third time, less than 10% of the applied dose remained in the soil after just 2 days. Similar results were obtained with vinclozolin in the same soil, and with both compounds in a second soil. In a third soil, which had relatively low pH, degradation of both compounds occurred only slowly and the rate of degradation of a second application was the same as that of the first. Degradation rates in this soil were increased by addition of 100 g kg^?1 of a soil in which degradation occurred more readily, and they were markedly increased by addition of 100 g kg^?1 of a soil in which enhanced degradation had been previously induced. Residues of both fungicides were also measured following repeated application in the field. When iprodione was applied to previously untreated plots, about 3% of the initial dose remained in the soil after 77 days. When applied to plots treated once before, less than 1% remained after 18 days, and when applied to plots treated twice previously less than 1% remained after 10 days. Similar results were obtained with vinclozolin. Enhanced degradation of subsequent soil treatments was also observed following a sequence of low-dosage sprays in the field.     

Mineralization of 2,4‐D,mecoprop, isoproturon and terbuthylazine in a chalk aquifer

The potential to mineralize 2,4‐dichlorophenoxyacetic acid (2,4‐D), mecoprop, isoproturon and terbuthylazine was studied in soil and aquifer chalk sampled at an agricultural field near Aalborg, Denmark. Laboratory microcosms were incubated for 258 days under aerobic conditions at 10 °C with soil and chalk from 0.15–4.45 m below the surface. The [ring‐U‐¹⁴C]‐labeled herbicides were added to obtain a concentration of 6 µg kg^?1 and mineralization was measured as evolved [¹⁴C]carbon dioxide. The herbicides were readily mineralized in soil from the plough layer, except for terbuthylazine, which was mineralized only to a limited extent. In the chalk, lag periods of at least 40 days were observed, and a maximum of 51%, 33% and 6% of the added 2,4‐D, mecoprop and isoproturon, respectively, were recovered as [¹⁴C]carbon dioxide. Large variations in both rate and extent of mineralization were observed within replicates in chalk. No mineralization of terbuthylazine in chalk was observed. As a measure of the general metabolic activity towards aromatic compounds, [ring‐U‐¹⁴C]‐benzoic acid was included. It was readily mineralized at all depths. © 2000 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     

Capacity of model biobeds to retain and degrade mecoprop and isoproturon

Biobeds are used to increase the adsorption and degradation of pesticide spillage on sites used for mixing and loading and for cleaning of sprayers. The adsorption and the rate of degradation of 14C-labelled isoproturon and mecoprop (MCPP) at concentrations from 0.0005 to 25 000 mgkg(-1) were determined in biobed soil. Further leaching of the two herbicides was determined in a model biobed with a surface area of 2 m2. The biobed material showed enhanced ability to adsorb the two herbicides. Kd was 5.2 litre kg(-1) for isoproturon and 1.6 litre kg(-1) for MCPP in biobed material, which is higher than in natural soil. In different experiments with natural soil, Kd ranges from 0.07 to 0.6 litrekg(-1) for MCPP and from 1.5 to 4.6 litre kg(-1) for isoproturon in soils with varying organic carbon content. Degradation of MCPP was rapid at concentrations from 0.0005 to 500 mg kg(-1), delayed at 5000 mg kg(-1), and very slow at 25 000 mg kg(-1). For isoproturon, the relative degradation was most rapid at the lowest concentration and decreasing with increasing concentrations. After 120 days, between 55% and 8% 14C was evolved as 14CO2 at concentrations between 0.0005 and 25 000 mg kg(-1). The rate of evolution of 14CO2 indicated that degradation rates at low concentrations were of first-order and at higher concentrations of zero-order. Leaching of MCPP and isoproturon was determined in a newly established model biobed during a 2-year period. About 13% of applied MCPP and 1.4% of applied isoproturon leached out during the winter following the first autumn application (worst-case scenario). Leaching was completely prevented when the biobed had a well-developed grass cover and was covered during the winter.     

Accelerated degradation of thiocarbamate herbicides in Israeli soils following repeated use of vernolate

Accelerated degradation of vernolate, EPTC and butylate but not of cycloate was detected in soils from three locations in Israel which were treated annually with vernolate. Repeated application of EPTC to soils with and without a history of vernolate application, under laboratory conditions, resulted in a progressive increase in its rate of dissipation with each application. Accelerated degradation of EPTC was also rapidly induced by mixing small amounts (5%) of soil with a history of vernolate treatment with soil that had never received vernolate. Liberation of ¹⁴CO₂ from [¹⁴C]EPTC was more rapid in vernolate-treated soils than in untreated soils, indicating a development of microbial populations in soil capable of rapidly degrading the EPTC. Degradation of [¹⁴C]EPTC was faster in soil previously cropped with maize than in non-cropped soil, but slower in soils cropped with cotton or peanuts.     

Differentiating between physical and chemical constraints on pesticide and water movement into and out of soil aggregates

A laboratory experiment comparing the movement of ³H₂O and [¹⁴C]isoproturon into and release from soil aggregates is described. Small aggregates (2.0–2.4 mm) were prepared from a clay topsoil and maintained at three different initial moisture conditions. A small volume of the radioisotope solution was introduced prior to bathing the aggregates in a 2 mM CaCl₂ solution to represent new rainwater. Whilst the ³H₂O was imbibed by the air-dry aggregates, the pesticide did not follow the water but remained on the surface of the aggregates. This may be related to its sorptive properties and an excess of sorption sites on the sorbent with respect to the sorbate. Increasing the length of exposure of the moist aggregates to [¹⁴C]isoproturon reduced the initial release of the compound into the bathing solution, probably due to diffusion (retarded by sorption) into the aggregates. The diffusion model described by Crank and a non-equilibrium desorption model were used to analyse the ³H₂O and [¹⁴C]isoproturon release curves. This showed that the release of ³H₂O from the dry aggregates was controlled by diffusion. The release of isoproturon was probably controlled by non-equilibrium sorption/desorption from air-dry aggregates and by a combination of non-equilibrium sorption/desorption and diffusion from wet aggregates. © 1999 Society of Chemical Industry