Summary. The adsorption of N'‐(4‐chlorophenoxy)‐phcnyl‐NN‐dimethylurea (C‐1983) and N‐butyl‐N‐(3,4‐dichlorophenyl)‐ N‐methylurea (ref. comp.) by different soils was investigated with a slurry‐type procedure. Freundlich isothermal equilibrium plots demonstrate that both herbicides, but especially C‐1983, were quite strongly adsorbed, on soil particles. The following concentrations of C‐1983 adsorbed were found to be in equilibrium with 1 ppm soil solutions: 14 γ/g (sandy loam), 40 γ/g (clay loam) and 110 γ/g (humus soil) The corresponding values for the ref. comp. were 6.3, 21 and 60 γ/g. Adsorption of C‐1983 was strong on bentonite (400 γ/g) and less pronounced on cellulose (48 γ/g). Soil pH had no significant effect on adsorption. Desorption equilibrium plots indicate that about 50% of C‐1983 adsorbed was released into solution according to adsorption equilibrium conditions. The remaining 50% came off at a much lower rate. Leaching of C‐1983 and the ref. comp. was studied with soil columns to which 200 mm of simulated rain was applied during a period of 45 hours. At a low soil moisture level (wilting capacity), the bulk of C‐1983 was not removed from the top 1 cm section of all three soils. Leaching was somewhat more pronounced in sandy loam (80% recovery in top 1 cm layer) than in clay loam (90%) or in humus soil (95%). Only traces of C‐1983 were recovered from depths below 10 cm and in water percolating from the columns. The ref. comp. leached more easily than C‐1983, especially in sandy loam (bulk of herbicide below 5 cm level). Leaching of both herbicides, but especially of the ref. comp., was less pronounced at the higher soil moisture level (field capacity wet) than at the lower one (wilting capacity). It is suggested that leaching is not a major factor in removing C‐1983 from the surface soil layers. On the other hand, its very strong adsorption seems to be a decisive factor in controlling its availability insoil solution and thus determining both the initial toxicity and and the residual activity in some types of soils. The practical implications for field use are discussed. Le comportement de la N'‐(4‐chllorophénoxy)‐phényl‐NN‐diméthylurée (C‐1983) dans les terres et dans les plantes 1. Adsorption et lessivage dans divers types de terres

The pathway and the rate of breakdown of C-1983 in bacterial suspensions, soil samples and plants was investigated with the i^C-carbonyl-Iabelled herbicide.Bacterial suspensions derived from humus soil were demonstrated to metabolize C-1983 [I] to A"-(4-chlorophenoxy)-phenyl->'-methylurea [II], ^'-(4-chIorophenoxy)phenylurea [III] and (4-chlorophenoxy)-aniline [V]. In addition, at least two minor, unidentified metabolites were observed.In samples of humus soil and sandy loam, the pathway of degradation was found to be essentially the same as in bacterial suspensions. The rate of breakdown was slightly higher in sandy loam (35% loss in 8 weeks) than in humus soil (25% loss) under optimal growth conditions (temperature, humidity). In humus soil, which is highly adsorptive for C-1983, breakdown decreased with time, thereby suggesting that desorption of herbicide was too slow to maintain sufficient soil solution concentration. In autodaved humus soil, breakdown was considerably less pronounced (15% loss in 16 weeks) than in non-au toe laved samples.Under model conditions (irradiation with ultra-violet source of 300 W) photodestruction of C-1983 was found to be rapid (90% loss within 13 hours).From roots and leaves of .^ra mays, Viciafaba and Phaseolus vulgaris, the same metabolites as given above (compounds [II] and [HI]) were recovered in significant amounts. Furthermore, some minor, unidentified labelled compounds and varying amounts of residual radioactivity (noi extracted by acetone) were observed. Evolution of ^^COa was slow. Upon following the breakdown of C-1983 as a function of time in two weed species, Polygonum convolvulus was demonstrated to metabolize C-1983 more rapidly than Galinsoga parvifiora.The implications ofthe present results are discussed in terms ofthe biological breakdown of C-1983 in soils under field conditions, the susceptibility of different plant species towards C-i983, and the determination of residue values in plant tissues and soils. Le comportement de la W-{\-chlorophinoxy)-phenyl-N^-dimethylurie (C-1983) dans les sols et dans les plantes in. Degradation dans les sols et darts Us plantesR^sum^. La voie et la vitesse ded^gradation du C-1983 dans des suspensions microbiennes, des ^chantillons de terre et des plantes ont ti€ ^tudi^s a l'aide du compost marque par^C dans la fonction carbonyle. II a 616 d^montr^ que des suspensions bactiriennes pr6paries k partir d'^chantilions d'humus transforment le C-1983 [I] en 7V'-(4-chloroph6noxy)-ph6nyl-jV-m^thylurde [II], en 7V-(4-chIorophenoxy)-ph6nylur^e [III], et en (4-chloroph^noxy)-aniline [V], De plus, la presence de faibles quantit^s d'au moins deux autres substances marquies, non identifi^es. a ^t^ obaerv^e.La voie de degradation dans des ^chantillons d'humus et dc terre sableuse a iti essentiellement la meme que dans les suspensions bact^riennes. D'autre part, le taux de degradation a ^t6 ltgerement plus 61ev^ dans la terre sableuse (35% de pcrte en 8 semaines) que dans I'humus (25% de perte), Dans i'humus, qui a une forte capacity d'adsor...

Summary. The pathway and the rate of breakdown of C‐1983 in bacterial suspensions, soil samples and plants was investigated with the 14C‐carbonyl‐labelled herbicide. Bacterial suspensions derived from humus soil were demonstrated to metabolize C‐1983 [I] to N'‐(4‐chloropheuoxy)‐phenyl‐N'‐methylurea [II], N'‐(4‐chlorophenoxy)‐phenylurea [III] and (4‐chlorophenoxy)‐aniline [V]. In addition, at least two minor, unidentified metabolites were observed. In samples of humus soil and sandy loam, the pathway of degradation was found to be essentially the same as in bacterial suspensions. The rate of breakdown was slightly higher in sandy loam (35% loss in 8 weeks) than in humus soil (25% loss) under optimal growth conditions (temperature, humidity). In humus soil, which is highly adsorptive for C‐1983, breakdown decreased with time, thereby suggesting that desorption of herbicide was too slow to maintain sufficient soil solution concentration. In auloclaved humus soil, breakdown was considerably less pronounced (15% loss in 16 weeks) than in non‐autoclaved samples. Under model conditions (irradiation with ultra‐violet source of 300 W) photo‐destruction of C‐1983 was found to be rapid (90% loss within 13 hours). From roots and leaves of Zea mays, Vicia faba and Phaseolus vulgaris, the same metabolites as given above (compounds [II] and [III]) were recovered in significant amounts. Furthermore, some minor, unidentified labelled compounds and varying amounts of residual radioactivity (not extracted by acetone) were observed. Evolution of 14CO2 was slow. Upon following the breakdown of C‐1983 as a function of time in two weed species, Polygonum convolvulus was demonstrated to metabolize C‐1983 more rapidly than Galinsoga parviflora. The implications of the present results are discussed in terms of the biological breakdown of C‐1983 in soils under field conditions, the susceptibility of different plant species towards C‐1983, and the determination of residue values in plant tissues and soils. Le comportement de la N'‐(4‐chlorophénoxy)‐phényl‐NN‐diméthylurée (C‐1983) dans les sols et dans tes plantes III. Dégradation dans tes sais et dans les plantes

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