Impact of repeated pesticide applications on the binding and release of methyl 14C-monocrotophos and U-ring labelled 14C-carbaryl to soil matrices under field conditions

The dissipation of (O-methyl-14C) monocrotophos and U-ring labelled 14C-carbaryl was monitored for over two years in absence and presence of other insecticides using in situ soil columns. The dissipation of 14C-monocrotophos from soil treated with methomyl and carbaryl showed a faster rate of downward movement than in a control column tagged with the labelled insecticide alone. The same trend was observed in experiments with 14C-carbaryl that dissipated more readily in soil treated with non-labelled monocrotophos and methomyl. In the presence of other insecticides the percentage of bound residues was generally lower than in control experiments. The bound residues at the top of the column are released at a low rate under conditions prevailing in the field. The overall time required for dissipation of 50% of monocrotophos and carbaryl (t50) as estimated from control experiment was approximately 20 and 24 weeks, respectively. The data indicate that repeated applications of pesticides might enhance the release of 14C-bound residues.     

Dissipation and Leaching of 14C-Monocrotophos in Soil Columns under Subtropical Climate

Dissipation and leaching behavior of ¹⁴C-monocrotophos was studied for 365 days under field conditions using PVC cylinders. The first set (24 cylinders) was spiked with 1.0 μCi ¹⁴C-labeled monocrotophos along with 1.06 mg unlabeled monocrotophos to give a concentration of 2 mg kg ^?1 in the soil up to 15 cm depth. The second set (24 cylinders) received ¹⁴C-labeled monocrotophos along with other non-labeled insecticides viz., dimethoate @ 300 g a.i ha^?1, deltamethrin @ 12.5 g a.i ha^?1, endosulfan @ 750 g a.i ha^?1, cypermethrin @ 60 g a.i ha^?1, and triazophos @ 600 g a.i ha^?1 at an interval of 15 days each as recommended for the cotton crop. ¹⁴C-monocrotophos dissipated faster, up to 45% in first 90 days in columns treated with only monocrotophos compared to 25% in columns that received monocrotophos along with other insecticides. However, both the columns showed similar residues 180 days onward. After 180 days of treatment, 46% radiolabeled residues were observed, which reduced up to 39.6% after 365 days. Leaching of ¹⁴C-monocrotophos to 15–30 cm soil layer was observed in both the experimental setups. In the 15–30 cm soil layer of both soil columns, up to 0.19 mg ¹⁴C-monocrotophos kg^?1d. wt. soil was detected after 270 days.     

Effect of organic amendment on degradation and formation of bound residues of methabenzthiazuron in soil under constant climatic conditions

Abstract

The degradation of [phenyl‐U‐¹⁴C]methabenzthiazuron (MBT) and formation of bound residues in the surface soil of an orthic luvisol were studied under constant climatic conditions (20°C, 40 % of maximum water holding capacity). In two treatments (with and without preincubation in the soil) maize straw was amended at a rate of 1.5 g/100 g dry soil in addition to the application of MBT. The mineralization of uniformly labeled maize straw was studied simultaneously. In additional flasks, MBT was incubated at 0, 10 and 30°C with and without addition of maize straw.

The turnover of the amended maize straw led to an enhanced dissipation of MBT which was mainly due to the formation of bound residues. This corresponded to a higher microbial activity in the soil after straw amendment and the intensive mineralization of the radiolabeled maize straw. About 2–3 % of the applied radioactivity from the radiolabeled maize straw was measured in the soil microbial biomass 10 and 40 days after application whereas ¹⁴C from MBT was only incorporated into soil microbial biomass in the treatments with straw amendment.

Within the bound residue fractions relatively more radioactivity was measured in fulvic and humic acids after straw amendment. Increasing temperatures promoted the dissipation of MBT and the formation of bound residues in both treatments, but without amendment of maize straw these effects were far less pronounced. The laboratory scale degradation experiment led to similar results as were found in a corresponding lysimeter study. Differences that were observed could be explained by different temperature regimes of the experiments and time of aging in soil.     

Fate in soil of 14C-sulfadiazine residues contained in the manure of young pigs treated with a veterinary antibiotic

The fate of ¹⁴C-labeled sulfadiazine (¹⁴C-SDZ) residues was studied in time-course experiments for 218 days of incubation using two soils (A_p horizon of loamy sand, orthic luvisol; A_p horizon of silt loam, cambisol) amended with fresh and aged (6 months) ¹⁴C-manure [40 g kg^?1 of soil; 6.36 mg of sulfadiazine (SDZ) equivalents per kg of soil], which was derived from two shoats treated with ¹⁴C-SDZ. Mineralization of ¹⁴C-SDZ residues was below 2% after 218 days depending little on soil type. Portions of extractable ¹⁴C (ethanol-water, 9:1, v/v) decreased with time to 4–13% after 218 days of incubation with fresh and aged ¹⁴C-manure and both soils. Non-extractable residues were the main route of the fate of the ¹⁴C-SDZ residues (above 90% of total recovered ¹⁴C after 218 days). These residues were high immediately after amendment depending on soil type and aging of the ¹⁴C-manure, and were stable and not remobilized throughout 218 days of incubation. Bioavailable portions (extraction using CaCl₂ solution) also decreased with increasing incubation period (5–7% after 218 days). Due to thin-layer chromatography (TLC), 500 μg of ¹⁴C-SDZ per kg soil were found in the ethanol-water extracts immediately after amendment with fresh ¹⁴C-manure, and about 50 μg kg^?1 after 218 days. Bioavailable ¹⁴C-SDZ portions present in the CaCl₂ extracts were about 350 μg kg^?1 with amendment. Higher concentrations were initially detected with aged ¹⁴C-manure (ethanol-water extracts: 1,920 μg kg^?1; CaCl₂ extracts: 1,020 μg kg^?1), probably due to release of ¹⁴C-SDZ from bound forms during storage. Consistent results were obtained by extraction of the ¹⁴C-manure-soil samples with ethyl acetate; portions of N-acetylated SDZ were additionally determined. All soluble ¹⁴C-SDZ residues contained in ¹⁴C-manure contributed to the formation of non-extractable residues; a tendency for persistence or accumulation was not observed. SDZ's non-extractable soil residues were associated with the soluble HCl, fulvic acids and humic acids fractions, and the insoluble humin fraction. The majority of the non-extractable residues appeared to be due to stable covalent binding to soil organic matter.     

Comparative studies of the fate of atrazine‐14C and pentachlorophenol‐14C in various laboratory and outdoor soil‐plant systems

Abstract

Mass balance and fate of atrazine‐ ¹⁴C and pentachlorophenol‐ ¹⁴C (PCP‐ ¹⁴C) were studied in short‐term tests in a closed aerated laboratory soil‐plant system, using two concentrations in soil and two plant species, as well as under outdoor conditions for one vegetation period. In the laboratory, for both pesticides bioaccu‐mulation factors of radiocarbon taken up by the roots into plants were low. They were higher for lower (1 ppm) than for higher soil concentrations (6 ppm for atra‐zine, 4 ppm for pentachlorophenol) and varied with the plant species. Mineralization to ¹⁴CO₂ in soil was negatively related to soil concentration only for PCP‐ ¹⁴C. Conversion rates in soil including the formation of soil‐bound residues were higher for the lower concentrations of both pesticides than for the higher ones; conversion rates in plants were species‐dependent. In 14 terms of CO₂ formation and of conversion rates, PCP was less persistent in soil than was atrazine. For both pesticides, laboratory data on conversion and mineralization gave a rough prediction of their persistence in soil under long‐term outdoor conditions, whereas bio‐accumulation factors in plants under long‐term outdoor conditions could not be predicted by short‐term laboratory experiments.     

Deltamethrin degradation and effects on soil microbial activity

Deltamethrin [(S)-cyano-3-phenoxybenzyl-cis-(1R,3R)-2,2-dimethyl) cyclo–propane carboxylate),1] labelled at gem-dimethyl groups of the cyclopropane ring was applied on two Egyptian soils at a level of 10 mg/kg soil for a laboratory incubation experiment under aerobic and anaerobic conditions. A steady decrease of soil extractable¹⁴C-residues, accompanied by a corresponding increase of non- extractable bound ¹⁴C-residues was observed over a 90-day incubation period. The percentage of evolved ¹⁴CO₂ increased with time under aerobic and anaerobic conditions in both soils. The effect of deltamethrin on soil microorganisms as well as the counter effect of microorganisms on the insecticide was also investigated. As the incubation period increased, the inhibitory effect of the insecticide on the microorganisms decreased and the evolution of carbon dioxide depended on the applied dose. The nature of soil methanol soluble residues was determined by chromatographic analysis which revealed the presence of the parent insecticide as the main product in addition to four metabolites: 3-(2′,2′-dibromovinyl)-2,2-dimethylcyclopropane carboxylic acid (II); 3-phenoxybenzaldehyde (III); 3-phenoxybenzoic acid (IV); 3-phenoxybenzyl alcohol (V).     

Chemical and biological release of 14C‐bound residues from soil treated with 14C‐p,p'‐DDT

Abstract

¹⁴C‐p,p'‐DDT‐bound residues in soil can be released by treatment with concentrated sulphuric acid at ambient temperatures. Within 6 days, about 70% of the bound residues was released. Bound residues released after 9 months incubation with ¹⁴C‐DDT showed the presence of DDT and DDE only while bound residues released after 18 months, contained in addition 13% DDD.

Release of bound ¹⁴C‐residues also occurs readily following inoculation of the soil‐bound residues with fresh soil or with individual microorganisms. Almost complete release of bound residues was observed after incubation for 45 days. The rate of release was rapid during the first two weeks and decreased thereafter. TLC and HPLC analysis showed that the released residues contained DDE (about 80%) and a smaller amount of DDD. The disappearance of DDT from the released residues may be attributed to its microbiological degradation to DDE and DDD, shortly after its release.     

Fate of 14C‐allylalcohol herbicide in soils and crop residues

Abstract

Residue disappearance and leaching of ¹⁴C‐allyl‐alcohol from different soils were studied in laboratory experiments. Additionally, the uptake of residues by lettuce and carrots was investigated in the greenhouse. In laboratory experiments, residue disappearance and leaching from soils was correlated negatively to the organic matter content. In greenhouse experiments with a sandy loam soil at an application rate normally used in practice, an average of 12.5 % of the applied radioactivity was recovered after an eight day interval between application and sowing. Furthermore, an average of 8 % (sum in soil and plants) of the applied radioactivity was recovered after lettuce or carrot growing. Uptake of residues was higher by carrots than by lettuce, and higher by lettuce roots than by lettuce tops. No bioaccumulation was observed. The residues in soils and plants were, to a high percentage, unextractable and, to a smaller extent, fully water‐soluble products. Unchanged allylalcohol could not be detected by the analytical methods used.     

Soil bound residues of carbaryl and 1‐naphthol: Release and mineralization in soil,and uptake by plants

Abstract

¹⁴C‐carbaryl and ¹⁴C‐1‐naphthol form soil bound residues which get partially released when barley was grown. ¹⁴C‐residues could be detected in both shoot and root in the case of carbaryl treatment while only roots showed ¹⁴C‐residues in the case of 1‐naphthol. Flooding enhanced release of the bound residues while soil amendment did not. There was greater mineralization of bound residues of carbaryl than that of 1‐naphthol. Rice straw amendment enhanced mineralization.     

Laboratory studies on volatilization and mineralization of 14c‐p,p'‐DDT in soil,release of bound residues and dissipation from solid surfaces

Abstract

In support of field data, laboratory studies were conducted on volatilization, mineralization and binding of ¹⁴C‐p,p'‐DDT in soils at Sao Paulo. Incubation of soil for 6 weeks did not result in volatilized organics or mineralization; with >95% extractable radiocarbon in the form of p,p'‐DDT. Small amounts of bound residues (1.8%) were detected in soil. These data confirm the very slow dissipation of DDT in the field which presumably relates to the acidic pH of soil (4.5–4.8).

Bound ¹⁴C‐residues in soils treated with ¹⁴C‐p,p'‐DDT at Praia Grande and Sao Paulo could be released (5–21%) by sulphuric acid treatment. The released residue had the composition: 69–90% DDT, 7–32% DDD and 0–3% DDE. Incubation of soil bound ¹⁴C‐residues with fresh inoculum for 3 months did not result in release of ¹⁴C.

Dissipation from wooden surfaces was fairly slow. After 20 weeks, 74% of the applied radioactivity could be recovered; 44% hexane‐non‐extractable.     

Biological activity and bioavailability of grain bound 14C‐malathion residues in rats

Abstract

Paddy (unmilled rice), milled rice and maize‐bound ¹⁴C residues were prepared using ¹⁴C‐succinate‐labelled malathion at 10 and 152 ppm. After 3 months, the bound residues accounted for 12%, 6.5% and 17.7% of the applied dose in paddy, milled rice and maize respectively in the grains treated at 10 ppm. The corresponding values for the 152 ppm were 16.6%, 8.5% and 18.8%. Rats fed milled rice ‐ bound ¹⁴C‐residues eliminated 61% of the ¹⁴C in the faeces and 28% in the urine. The corresponding percentages for paddy and maize were 72%, 9% and 53%, 41% respectively; indicating that bound residues from milled rice and maize were moderately bioavailable. When rice‐bound malathion residues (0.65 ppm in feed) were administered to rats in a 5 week feeding study, no signs of toxicity were observed. Plasma and RBC cholinesterase activities were slightly inhibited: blood urea nitrogen was significantly elevated in the test animals. Other parameters examined showed no or marginal changes.     

Studies of the effects of temperatures and solar radiation on volatilization,mineralization and binding of 14c‐DDT in soil under laboratory conditions

Abstract

The effects of temperatures and solar radiation on the dissipation of ¹⁴C‐p,p'‐DDT from a loam soil was studied by quantifying volatilization, mineralization and binding. The major DDT loss occurred by volatilization, which was 1.8 times more at 45^oC than at ambient temperature (30°C). Mineralization of DDT slowly increased with time but it decreased slightly with increase in temperature. Binding of DDT to soil was found to be less at higher temperatures (35 and 45°C) as compared to ambient temperature. Degradation of DDT to DDE was faster at higher temperatures.

Exposure of non‐sterilized and sterilized soils treated with ¹⁴C‐DDT to sunlight in quartz and dark tubes for 6 weeks resulted in significant losses. Volatilization and mineralization in quartz tubes were more as compared to dark tubes. The volatilized organics from the quartz tubes contained larger amounts of p,p'‐DDE than the dark tubes. Further, higher rates of volatilization were found in non‐sterilized soils than in sterilized soils. The results suggest that faster dissipation of DDT from soil under local conditions relates predominantly to increased volatilization as influenced by high temperature and intense solar radiation.     

Fate of 14C-ethyl prothiofos insecticide in canola seeds and oils

Canola plants were treated with ¹⁴C- prohiofos under conditions simulating local agricultural practices. ¹⁴C-residues in seeds were determined at different time intervals. At harvest time about 32 % of ¹⁴C-activity was associated with oil. The methanol soluble ¹⁴C-residues accounted for 12 % of the total seed residues after further seeds extraction, while the cake contained about 49 % of the total residues. About 69 % of the ¹⁴C-activity in the crude oil could be eliminated by simulated commercial processes locally used for oil refining. Chromatographic analysis of crude and refined oil revealed the presence of the parent compound together with three metabolites which were identified as prothiofos oxon, O-ethyl phosphorothioate and O-ethyl S-propyl phosphorothioate, besides one unknown compound. While methanol extract revealed the presence of despropylthio prothiofos and O-ethyl phosphoric acid as free metabolites acid hydrolysis of the conjugated metabolites in the methanol extract yielded 2, 4-dichlorophenole which was detected by color. When rats were fed the extracted cake for 72 hours, the bound residues were found to be bioavailable. The main excretion route was via the expired air (42 %), while the ¹⁴C-residues excreted in urine and feces were 30 % and 11 %, respectively. The radioactivity detected among various organs accounted to 7.5 %.Chromatographic analysis of urine indicated the presence of prothiofos oxon, O-ethyl phosphoric acid and 2, 4-dichlorophenole as main degradation products of prothiofos in free and conjugated form.     

Biodegradation of Trifluralin in Harran Soil

Abstract

Degradation of trifluralin (α,α,α-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine) was investigated in soils taken from three different locations at Harran region of Turkey under laboratory conditions. Surface (0–10 cm) soils, which were taken from a pesticide untreated field Gürgelen, Harran-1 and Ikizce regions in the Harran Plain, were incubated in biometer flasks for 350 days at 25°C. Ring-UL-¹⁴C-trifluralin was applied at the rate of 2 µg g^?1 with 78.7 kBq radioactivity per 100 g soil flask. Evolved ¹⁴CO₂ was monitored in KOH traps throughout the experiment. Periodically, soil sub-samples were removed and extracted by supercritical fluid extraction (SFE). Unextractable soil-bound ¹⁴C residues were determined by combustion. During the 350 days incubation period 6.6, 5.4, and 3.3% of the applied radiocarbon was evolved as ¹⁴CO₂ from the Harran-1, Gürgelen, and Ikizce soil, respectively. At the end of 350 days the SFE-extractable and bound ¹⁴C-trifluralin residues were 39.0 and 29.2% of the initially applied herbicide in Gürgelen soil. The corresponding values for Harran-1 and Ikizce soils were 36.2, 28.4% and 41.6, 18.5% respectively.     

Fate of the veterinary antibiotic 14C-difloxacin in soil including simultaneous amendment of pig manure with the focus on non-extractable residues

The fate of ¹⁴C-labeled difloxacin (¹⁴C-DIF) was studied in time course experiments after application on soil (Ap horizon of silt loam) and amendment of authentic DIF containing pig manure (146 mL kg^?1; 4.17 MBq kg^?1; 0.85 mg kg^?1) or water (124 mL kg^?1; 0.42 MBq kg^?1; 0.09 mg kg^?1) for 56 and 120 days of incubation, respectively.

Mineralization of ¹⁴C-DIF was below 0.2% in both experiments after 56 days or 120 days. In the course of the experiments, portions of extractable radioactivity (Accelerated Solvent Extraction (ASE); acetonitrile-water) decreased to 19–21% depending only little on manure amendment. Non-extractable residues of ¹⁴C-DIF increased to 70–74% after 56 days and 120 days, respectively, and therefore were the main route of ¹⁴C-DIF in soil. According to radioanalytical HPLC and LC-MS/MS, only the parent compound was found in all extracts over the whole time of the experiment. According to fractionation of the non-extractable residues (NER) into particle size fractions, ¹⁴C portions were associated to the water used for fractionation, the silt and clay fractions, whereas no radioactivity was detected in the sand fraction. The majority of ¹⁴C was found within the clay fractions.

Fractionation of humic components showed that radioactivity derived from ¹⁴C-DIF was associated with humic acids, fulvic acids, humins and minerals and very little with soluble, non-humic HCl fraction. The highest portions of radioactivity were found in the fulvic acid fraction. Results obtained by size exclusion chromatography (SEC) of the purified fulvic acids were similar for every sample analyzed. One large portion of ¹⁴C co-eluted with fulvic acids of a molecular weight below 910 g mol^?1. Both fractionation methods demonstrated that the parent compound DIF or initial metabolites were rapidly integrated into humic materials and, thus, were major components of NER.     

Metabolism of 14C‐carbaryl and 14C‐1‐naphthol in moist and flooded soils

Abstract

The metabolism of ¹⁴C‐carbaryl and ¹⁴C‐1‐naphthol in moist and flooded soils was studied in a continuous flow‐through system over a period of 28 days permitting ¹⁴C‐mass balance. The percent distribution of radiocarbon in organic volatiles, carbon dioxide, extractable and non‐extractable (bound) fractions of soils were determined. Organic volatiles could not be detected in both carbaryl and 1‐naphthol treated soils. More of ¹⁴CO₂ (25.6%) was evolved from moist than flooded soil (15.1%) treated with carbaryl. However, the mineralization of ¹⁴C‐1‐naphthol was negligible. The extractable radiocarbon was more in flooded soil (28.9%) than moist soil (5.5%) from carbaryl treatment. Less than one percent was present as parent compound, whereas carbaryl was mainly metabolized to 5‐hydroxy carbaryl in moist soil and to 4‐ and 5‐hydroxy carbaryl in flooded soil. The extractable radiocarbon amounted to 18.2 and 24.3% in moist and flooded soils respectively and the parent compound was less than one percent with 1‐naphthol treatment. Most of the radiocarbon was found as soil bound residues; the formation being more with 1‐naphthol than carbaryl. Humin fraction of the soil organic matter contributed most to soil bound residues of both carbaryl and 1‐naphthol.     

Bioavailability and toxicological potential of sunflower-bound residues of 14C-chlorpyrifos insecticide in rats

Sunflower plants were treated with ¹⁴C-chlorpyrifos under conditions simulating local agricultural practice. Residues present in the oil, methanol extract and cake of the treated sunflower seeds were 7.2, 2.8, and 12 ppm, respectively. When rats fed on sunflower cake containing bound residues for three days, the animals eliminated 46 % of the radioactivity in urine, 25 % in feces and 10 % in the expired air. A further bioavailable amount of 8 % was found in selected organs indicating that the bound residues were highly bioavailable. Chromatographic analysis of urine extract revealed the presence of the parent compound, its oxon, desethyl chlorpyrifos and desethyl chlorpyrifos oxon as free metabolites in addition to a conjugated metabolite. It was liberated by acid hydrolysis and identified as 3,5,6-trichloro-2-hydroxypyridine. Bound residues were found to have biological effects such as inhibition of rat plasma ChE, elevations of liver parameters (ALT, AST, and ALP), decrease in total protein and albumin content suggesting a hepatotoxic potential. A significant increase in the values of creatinine, urea, cholesterol, triglycerides and significant decrease in Catalase and Glutathion-S-Transfrase were observed in treated rats.     

Fate of the 14C-labeled herbicide prosulfocarb in a soil and in a sediment-water system

The fate of ¹⁴C-labeled herbicide prosulfocarb was studied in an agricultural soil and in a sediment-water system, the sediment part of which was derived from Yangtze Three Gorges Reservoir, China. Time-course studies were performed for 28 d and 49 d, respectively. Main transformation routes of ¹⁴C-prosulfocarb were mineralization to ¹⁴CO₂ and formation of nonextractable residues amounting to 12.13% and 10.43%, respectively, after 28 days (soil), and 9.40% and 11.98%, respectively, after 49 d (sediment-water system). Traces of prosulfocarbsulfoxide were detected by means of TLC, HPLC, and LC-MS; other transformation products were not found. Initial extraction of soil assays using 0.01 M CaCl₂ solution showed that the bioavailability of the herbicide was considerably low; immediately after application (0.1 d of incubation), only 4.78% of applied radioactivity were detected in this aqueous fraction. DT₅₀ values of ¹⁴C-prosulfocarb estimated from radio-TLC and -HPLC analyses were above 28 d in soil and ranged between 29 d and 49 d in the sediment-water system. Partitioning of ¹⁴C from water to sediment phase occurred with DT₅₀ slightly above 2 d. With regard to the sediment-water system, adsorption occurred with log K_oc = 1.38 (calculated from 2 day assays) and 2.35 (49 d assays). As similarly estimated from portions of ¹⁴C found in CaCl₂ extracts of the 0.1 d assays, ¹⁴C-prosulfocarb's log K_oc in soil was 2.96. With both experiments, similar portions of nonextractable radioactivity were associated with all soil organic matter fractions, i.e. nonhumics, fulvic acids, humic acids, and humin/minerals. Throughout all sample preparation, the experiments were severely impaired by losses of radioactivity especially with concentration of samples containing water in vacuo. All findings pointed to volatility of parent prosulfocarb in presence of water rather than volatility of transformation products. According to literature data, this behavior of prosulfocarb was not expected, though volatility was demonstrated under field conditions.     

Dissipation and leaching of (14)C-monocrotophos in Soil Columns under subtropical climate

Dissipation and leaching behavior of 14C-monocrotophos was studied for 365 days under field conditions using PVC cylinders. The first set (24 cylinders) was spiked with 1.0 microCi 14C-labeled monocrotophos along with 1.06 mg unlabeled monocrotophos to give a concentration of 2 mg kg -1 in the soil up to 15 cm depth. The second set (24 cylinders) received 14C-labeled monocrotophos along with other non-labeled insecticides viz., dimethoate @ 300 g a.i ha-1, deltamethrin @ 12.5 g a.i ha-1, endosulfan @ 750 g a.i ha-1, cypermethrin @ 60 g a.i ha-1, and triazophos @ 600 g a.i ha-1 at an interval of 15 days each as recommended for the cotton crop. 14C-monocrotophos dissipated faster, up to 45% in first 90 days in columns treated with only monocrotophos compared to 25% in columns that received monocrotophos along with other insecticides. However, both the columns showed similar residues 180 days onward. After 180 days of treatment, 46% radiolabeled residues were observed, which reduced up to 39.6% after 365 days. Leaching of 14C-monocrotophos to 15-30 cm soil layer was observed in both the experimental setups. In the 15-30 cm soil layer of both soil columns, up to 0.19 mg 14C-monocrotophos kg-1d. wt. soil was detected after 270 days.     

Studies on bound 14C-prometryn residues in soil and plants

A high-temperature distillation technique was developed for determining and chemically identifying the bound (nonextractable) residues of ¹⁴C-prometryn in an organic soil and plants. A considerable portion of the bound ¹⁴C residues in the incubated organic soil was identified as prometryn. These residues were absorbed by plants grown in the soil. Hono-and di-$\text{N}$-dealkylated metabolites of prometryn were present in the plant bound ¹⁴C residues and a major portion of bound residues as associated with lignin. Soil-bound ¹⁴C residues were also released from soil by microbes. The bound ¹⁴C residues in soil were associated with humin, humic acid, and fulvic acid fractions. Thermoanalytical methods were used to obtained information on the nature and location of ¹⁴C bound residues in soil and humic materials.