Desorption of diuron and isoproturon from undispersed clay loam soil

Studies were conducted to investigate the desorption of diuron and isoproturon adsorbed on undispersed clay loam soil, and the influence of residence time in soil on desorption. The soil was treated at 0·6 or 3 mg kg^-1, at 70% moisture content and in the presence of sodium azide to prevent degradation. Measurement of herbicide concentrations in soil solution sampled by means of glass microfibre filters showed that adsorption mainly occurred for one day but long-term sorption proceeded for >two weeks. After a one-day or three-week residence time, soil solution was partly replaced (28%). Measurement of concentrations in solution showed rapid desorption, with equilibria being achieved within 1 h (diuron) or a few hours (isoproturon). After 16 successive desorptions done at 30-min or 12-h intervals, equilibration times tended to be longer. For the short residence time, desorption and long-term sorption could occur simultaneously and equilibration might be faster. Residence time had no significant effect on desorption kinetics nor on the small hysteresis observed for diuron. The aging effect, involving long-term sorption only, decreased the proportion of diuron removed from the soil by successive desorptions but, for isoproturon, desorption frequency and desorption kinetics were more important. © 1997 SCI     

Influence of soil moisture on short-term adsorption of diuron and isoproturon by soil

The adsorption of diuron and isoproturon by a clay loam soil at 35% (3-16 kPa) and 62% (1 kPa) soil moisture content was studied by means of glass microfibre filters capable of sampling soil solution for herbicide analysis. Adsorption was rapid, with 40–80% of the final (24 h) sorption being achieved within 2 min. These equilibria were achieved more rapidly for diuron, which was also the more highly adsorbed. Adsorption of both herbicides was favoured by low soil moisture initially, but was enhanced by higher soil moistures at sorption times greater than 30 min. However, increasing the soil moisture from 31% (10 kPa) to 62% (1 kPa) had little effect on the final soil sorption capacity. Regarding the water status in the soil, it is thought that adsorption took place in small pores (<3 μm). Herbicides diffused rapidly into small pores and adsorption by wet soil was delayed for a short period of time (about 30 min).     

Adsorption of isoproturon,diuron and metsulfuron-methyl in two soils at high soil:solution ratios

Rates of degradation of isoproturon, diuron and metsulfuron-methyl were measured in two soils incubated at two temperatures (5 and 25 °C) with soil moisture at a matric potential of ?5 kPa. Rates of change in soil solution concentration were also measured after extraction of water from the soil using a centrifugation technique. The data, in general, indicated a more rapid rate of decline in aqueous-phase concentrations of herbicide than in total soil concentrations, and hence a progressive increase in partition coefficient in favour of the adsorbed phase^. In all of the incubations, however, adsorption of the herbicide was initially less than that measured using standard equilibration techniques that involved shaking with large volumes of solution relative to weights of soil. This may be explained by the ready availability of more adsorption sites in the shaken systems. With isoproturon and diuron, the changes in adsorption with time were similar at the two incubation temperatures. This indicates that the apparent changes in adsorption with these two compounds were not caused by preferential degradation in the soil solution, but by a slow equilibration with adsorption sites. The results with the weakly adsorbed compound metsulfuron-methyl, however, suggested the possibility of preferential degradation in the solution phase because, when degradation was slow, the absolute amounts adsorbed remained constant or increased slightly, even although solution concentrations declined. Implications of the results for pesticide behaviour in soils in the field are discussed.     

Study of diuron in soil solution by means of a novel simple technique using glass microfibre filters

Retention by a glass fibre filter of the liquid phase of a clay loam soil treated with ¹⁴C-diuron provides a novel method for analysis of the herbicide in soil solution. At 26.3% (w/w) soil moisture content, less than 10% of the applied diuron was found in solution, and this percentage decreased slightly with diuron dose. The herbicide was rapidly adsorbed on soil during the first day, but adsorption continued and the concentration of diuron in solution could be further reduced by 36–50% during the following 6 days. Drying the soil after treatment, with possible crystallization of herbicide applied at high doses, tended to fix the solution concentrations. Ethanol (3% v/v) in soil water favoured herbicide dissolution. Increasing soil moisture to 36.3% (w/w) slightly decreased the concentration of the herbicide in solution, but increased the percentage held in solution. Frost and a drying-rewetting cycle had little or no subsequent effect on diuron concentration in soil solution.     

Evidence for the Accelerated Degradation of Isoproturon in Soils

The herbicide isoproturon was degraded rapidly in a sandy loam soil under laboratory conditions (incubation temperature, 15°C; soil moisture potential, -33 kPa). Degradation was inhibited following treatment of the soil with the antibiotic chloramphenicol, but unaffected by treatment with cycloheximide, thus indicating an involvement of soil bacteria. Rapid degradation was not observed with other phenylurea herbicides, such as diuron, linuron, monuron or metoxuron incubated in the same soil under the same experimental conditions. Three successive applications of isoproturon to ten soils differing in their physicochemical properties and previous cropping history induced rapid degradation of the herbicide in most of them under laboratory conditions. There were, however, no apparent differences in ease of induction of rapid degradation between soils which had been treated with isoproturon for the last five years in the field and those with no pre-treatment history. A mixed bacterial culture able to degrade isoproturon in liquid culture was isolated from a soil in which the herbicide degraded rapidly.     

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     

Influence of adsorption–desorption phenomena on pesticide run-off from soil using simulated rainfall

The surface run-off of a number of pesticides (diuron, isoproturon, atrazine, alachlor, aclonifen, trifluralin, lindane and simazine), chosen for their range of adsorption behaviours, was studied using simulated rainfall applied to small plots over a short time (one hour). Pesticides were applied together onto bare soil using two different sandy loam soils from Jaillière and Coet Dan sites. The surface run-off samples were collected throughout the running of the event and concentrations of pesticides were measured in both liquid and solid phases. Sorption isotherms for isoproturon and diuron on Jaillière soil as well as eroded particles were measured under equilibrium conditions and compared to their partitioning during surface run-off. At the rainfall intensity used, both soils generated a large load of eroded particles. The average run-off flow rate increased with time for the Jalliére soil, while it remained relatively constant at a higher level for the Coet Dan soil. The concentrations of each pesticide in the run-off samples decreased as the experiments proceeded. The pesticides were classified into two types by their partitioning between the solid and liquid phases. Atrazine, simazine, diuron, isoproturon and alachlor were mainly transported in surface run-off water. By contrast, 90% of trifluralin and aclonifen was adsorbed onto eroded particles. Lindane was intermediate, with a 37% adsorption level. When the contribution of eroded particles was minor, the agrochemical concentrations were inversely proportional to the water flow rate. We have proposed a model that describes the mass of chemicals extracted from soil into surface water during a surface run-off event of a given average duration and flow rate. This model takes into account the dilution of the soil solution and the desorption of chemicals through two parameters called, respectively, the dilution factor and the extraction retardation factor. The desorption kinetic was the limiting step in the surface run-off of weakly sorbed chemicals, such as isoproturon. © 1999 Society of Chemical Industry     

The effect of soil moisture content on the sorption of five sterol biosynthesis inhibiting fungicides as a function of their physicochemical properties

We investigated the sorption of five widely used sterol biosynthesis inhibitor fungicides (SBIs: flusilazole, propiconazole, epoxiconazole, fenpropimorph and prochloraz) on a loam soil to assess availability of the SBI residues that are usually left in soil after crop treatments. We focused particularly on the soil moisture content effect, which is poorly documented and is difficult to investigate under realistic conditions. SBI sorption was determined (using diuron as a reference) at two soil moisture contents (26.1% and 46.6% w/w) over a period of 3 weeks using a direct soil solution sampling method. After 24 h of contact, <1% of each applied fungicide was recovered in the soil solution. Despite their low availability in the liquid phase, long‐term sorption was observed for all the compounds, reducing concentrations in the soil solution and doubling the value of the partition coefficient. Significant effects of soil moisture on long‐term sorption were observed, depending on the properties of the chemicals and the sorption mechanisms. Wershaw's humus model (humic substances have a membrane‐like structure) was adapted to fit our observations. Low soil moisture content is assumed to modify the structure of humic substances and to generate hydrophobic surfaces, which favour sorption of hydrophobic fungicides (flusilazole, propiconazole and epoxiconazole). This effect is likely to decrease with the increase in the hydrophobic character of non‐ionic pesticides. It becomes adverse for the more hydrophilic compounds (diuron), which are more sorbed at high soil moisture content due to their higher affinity for hydrophilic regions of humus and to diffusion. Soil moisture effects are more complex when compounds are likely to be protonated in soil. Weakly basic compounds (prochloraz) may partition rapidly into the liquid‐like interior of humus at low soil moisture content but increased diffusion at high soil moisture content may cause additional sorption by ion exchange at colloid surfaces. Strongly basic compounds (fenpropimorph) may essentially adsorb due to ionic interactions with colloids, and their sorption is enhanced at high soil moisture content due to diffusion. Consequences for environmental fate and biological activity of pesticides are briefly discussed. © 2000 Society of Chemical Industry     

Detection of triazine and urea herbicide residues by various characteristics of oat seedlings in bioassays

Atrazine, simazine, diuron, and linuron applied to soil increased the percentage moisture of oat (Avena sativa L.) shoots in bioassays at the lowest dose tested of 0·25 ppm. Further increases occurred up to 2 ppm but at higher concentrations the percentage moisture decreased. At all doses of each herbicide, shoot dry weight was decreased. In oats grown on soil collected from a peach orchard which had received repeated annual applications of these herbicides, the percentage moisture of the oat shoots was higher than the control value whenever the oat dry weight was decreased and provided a method of residue detection as sensitive as dry weight measurements. Treatment of oats by soil application of the above herbicides in bioassays also caused increases in the electrical conductivity of an aqueous extract of the oat shoots per mg of dry weight and this characteristic was slightly more sensitive than dry weight in detecting herbicides in orchard soil. The conductivity of the extract per mg of water in the shoots, however, only increased as percentage moisture decreased. The weight of neutral water-soluble material in oat shoots decreased much more rapidly than dry weight in bioassays with standard herbicide concentrations. Determination of the weight of neutral water-soluble material in oat plants grown on orchard soil samples indicated the presence of herbicide residues in 50% of the cases in which residues were not detectable by dry weight. The weight of neutral material as a percentage of dry weight was almost as sensitive. Chemical analysis of soil in which oat plants had a decreased level of neutral water-soluble compounds indicated that this characteristic had a lower limit of detection for herbicide residues of approximately 0.10 ppm.     

Preferential Flow Pathways and Their Capacity to Transport Isoproturon in a Structured Clay Soil

A field experiment was established to monitor preferential flow pathways and their capacity to transport isoproturon in a heavy clay soil. A hydrologically defined plot of 600 m² at a field site on the Oxford University Farm at Wytham was created with integral flow monitoring and sampling devices. Data are presented from two flow events which occurred in April and May 1994. The highest concentrations of isoproturon (130 μg litre⁻¹) were observed in the drainage system. The vast majority of the 0·7% of applied pesticide that left the plot was via the drainage system (75–90%) with lateral subsurface flow accounting for a smaller proportion (max 23%). Whilst high pesticide concentrations could be found in overland-flow water, the volumes of water moved by this route were small (max 3%). Less water was estimated to have left the field in response to rainfall than in the previous year. This was attributed to decay of the mole drain system. Consequently the amount of applied pesticide lost in runoff (0·7%) was less than that estimated for the first year (1·5%). The work has shown that, even when a farmer follows best practice in the application of a herbicide to a winter cereal in a drained clay field, high concentrations of the herbicide (relative to the EC drinking water limit) will contaminate surrounding watercourses.     

Characterization of triadimefon sorption in soils using supercritical fluid (SFE) and accelerated solvent (ASE) extraction techniques

Sorption–desorption of the fungicide triadimefon in field‐moist silt loam and sandy loam soils were determined using low‐density supercritical fluid extraction (SFE). The selectivity of SFE enables extraction of triadimefon from the soil water phase only, thus allowing calculation of sorption coefficients (K_d) at field‐moist or unsaturated conditions. Triadimefon sorption was influenced by factors such as soil moisture content and temperature; sorption increased with increased moisture content up to saturation, and decreased with increased temperature. For instance, K_d values for triadimefon on the silt loam and the sandy loam soils at 40 °C and 10% water content were 1.9 and 2.5 ml g⁻¹, respectively, and at 18% water content, 3.3 and 6.4 ml g⁻¹, respectively. Isosteric heats of sorption (ΔH_i) were −42 and −7 kJ mol⁻¹ for the silt loam and sandy loam soils, respectively. Sorption–desorption was also determined using an automated accelerated solvent extraction system (ASE), in which triadimefon was extracted from silt loam soil by 0.01 M CaCl₂. Using the ASE system, which is basically a fast alternative to the batch equilibration system, gave a similar ΔH_i value (−29 kJ mol⁻¹) for the silt loam soil (K_f = 27 µg^{1 − 1/n} ml^1/n g⁻¹). In order to predict transport of pesticides through the soil profile more accurately on the basis of these data, information is needed on sorption as a function of soil water content. © 2000 Society of Chemical Industry     

Diuron mineralisation in a Mediterranean vineyard soil: impact of moisture content and temperature

BACKGROUND: The diuron‐mineralising ability of the microbiota of a Mediterranean vineyard soil exposed each year to this herbicide was measured. The impact of soil moisture and temperature on this microbial activity was assessed. RESULTS: The soil microbiota was shown to mineralise diuron. This mineralising activity was positively correlated with soil moisture content, being negligible at 5% and more than 30% at 20% soil moisture content. According to a double Gaussian model applied to fit the dataset, the optimum temperature/soil moisture conditions were 27.9 °C/19.3% for maximum mineralisation rate and 21.9 °C/18.3% for maximum percentage mineralisation. The impact of temperature and soil moisture content variations on diuron mineralisation was estimated. A simulated drought period had a suppressive effect on subsequent diuron mineralisation. This drought effect was more marked when higher temperatures were used to dry (40 °C versus 28 °C) or incubate (28 °C versus 20 °C) the soil. The diuron kinetic parameters measured after drought conditions were no longer in accordance with those estimated by the Gaussian model. CONCLUSION: Although soil microbiota can adapt to diuron mineralisation, its activity is strongly dependent on climatic conditions. It suggests that diuron is not rapidly degraded under Mediterranean climate, and that arable Mediterranean soils are likely to accumulate diuron residues. Copyright © 2010 Society of Chemical Industry     

Aerobic Metabolism of Flupropacil in Sandy Loam Soil

The aerobic soil metabolism of [¹⁴C]flupropacil (isopropyl 2-chloro-5-(1,2,3,6-tetrahydro-3-methyl-2,6-dioxo-4-trifluoromethylpyrimidin-1-yl)benzoate) was determined in microbially active, sieved (2-mm) sandy loam soil with a soil moisture content of 75% at 1/3 bar. The soil was treated with [¹⁴C]flupropacil at 0·5 mg kg⁻¹ (twice the field use rate) and placed in incubation flasks connected to a series of traps (50 g litre⁻¹ NaOH, 0·5M H₂SO₄, ethylene glycol) and incubated at 25(±1)°C. Soil was sampled at 0, 3, 9, 20, 30, 48, 76, 120, 181 and 238 days of aerobic incubation. Volatiles were collected once every two weeks and on the day of soil sampling. Flupropacil metabolized with a half-life of 79 days under aerobic conditions. The major metabolite was flupropacil acid which accounted for up to 69·1% of the initially applied radioactivity at Day 238. Each of the two minor metabolites detected at the end of the study accounted for less than 0·5%. One of the minor metabolites was identified as C4242 acid (2-chloro-5-(1,2,3,6-tetrahydro-2,6-dioxo-4-trifluoromethylpyrimidin-1-yl)benzoic acid). Only a negligible portion (less than 0·3%) of the applied flupropacil was mineralized to [¹⁴C]carbon dioxide. Extractable radioactivity ranged from 78·9% to 95·5%, with bound residues accounting for 3·2%–23·4%. The material balance ranged from 91·6% to 104·4%.     

Degradation of atrazine and isoproturon in surface and sub-surface soil materials undergoing different moisture and aeration conditions

The influence of different moisture and aeration conditions on the degradation of atrazine and isoproturon was investigated in environmental samples aseptically collected from surface and sub-surface zones of agricultural land. The materials were maintained at two moisture contents corresponding to just above field capacity or 90% of field capacity. Another two groups of samples were adjusted with water to above field capacity, and, at zero time, exposed to drying-rewetting cycles. Atrazine was more persistent (t(1/2) = 22-35 days) than isoproturon (t(1/2) = 5-17 days) in samples maintained at constant moisture conditions. The rate of degradation for both herbicides was higher in samples maintained at a moisture content of 90% of field capacity than in samples with higher moisture contents. The reduction in moisture content in samples undergoing desiccation from above field capacity to much lower than field capacity enhanced the degradation of isoproturon (t(1/2) = 9-12 days) but reduced the rate of atrazine degradation (t(1/2) = 23-35 days). This demonstrates the variability between different micro-organisms in their susceptibility to desiccation. Under anaerobic conditions generated in anaerobic jars, atrazine degraded much more rapidly than isoproturon in materials taken from three soil profiles (0-250 cm depth). It is suggested that some specific micro-organisms are able to survive and degrade herbicide under severe conditions of desiccation.     

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.     

Rainfastness and adsorption of herbicides on hard surfaces

Herbicides are still used to control weeds on hard surfaces, including municipal, private and industrial sites. Used under unfavourable conditions, especially when rain occurs shortly after application, herbicides may run off to surface waters. Such losses of herbicides from hard surfaces are estimated to be much higher than for herbicides used in arable fields. In this study, three kinds of hard surface were evaluated: asphalt, concrete surface and gravel (fine and coarse). Three herbicides were applied: glyphosate, diuron and diflufenican. Adsorption isotherms of diuron and diflufenican to the three surfaces were determined. At different times after treatment with the herbicides, rainfall was simulated by use of a rain-droplet spray nozzle, and the run-off was collected for analysis. After this run-off event, the materials were immersed in water to measure desorption which, together with the compound in the run-off, gave a measure of the dislodgable residues. The apolar herbicides diuron and especially diflufenican adsorbed strongly to asphalt. The polar herbicide glyphosate lost 75% in run-off from asphalt but was adsorbed strongly to soil and concrete pavement.     

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     

Etude des phénomènes de sorption entre deux triazines herbicides et des acides humiques

A study of sorption phenomena between two triazine herbicides and humic acids Terbutryne is very readily adsorbed by humic acids while atrazine is only slightly adsorbed and this only in an acid environment. The influence of pH on adsorption and the competitive effect of the cations Ca²⁺, Al³⁺ and Fe³⁺ shows that the proton form of the molecules of the two herbicides can be adsorbed by an ion exchange-type mechanism; the neutral form of terbutryne molecules could be adsorbed by other mechanisms. Desorption of terbutryne is accompanied by a more marked hysteresis phenomenon in the case of neutral molecules, and, in an acid environment, calcium shows a weak capacity for displacement in relation lo the adsorbed herbicide.     

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.     

Adsorption de quelques phénylurées herbicides par des acides humiques

L'adsorption de plusieurs phcnyluré (diuron, linuron. mtox- uron, isoproturon) par des acides humiques augmente lorsque le pH diminue, Lëtablissement de liaisons hydrogène entre les molécules herbicides el les groupements fonctionnels non disso- ciés des acides humiques pourrait expliquer cet effet, Compars au calcium, au mème pH, les cations Fe³⁺, Al³⁺, Cu²⁺ ne modifient pas l'intensité de l'adsorption mais, compte tenu de la composition ionique des acides humiques, on peut supposer l'existence d'interactions entre les molcules herbicides et ces cations fixs sur les acides humiques.