Adsorption and degradation rates of chlorsulfuron and metsulfuron-methyl were measured in soil taken from depths of 0-20, 20-40 and 40-60 cm at eight sites. Adsorption of both herbicides was negatively correlated with soil pH, and positively correlated with soil organic matter content. When two soils with very high organic matter were excluded from the calculations, the correlations with organic matter content were no longer statistically significant but those with soil pH were affected only .slightly. Degradation rates of both herbicides generally decreased with increasing depth in the soil and were positively correlated with microbial biomass and negatively correlated with soil pH. The possible significance of the results to persistence of the herbicides in the field is discussed. Adsorption et degradation du chlorsulfuron methyl dans des sols issus de profondeurs differentesLes niveaux d'adsorption et de degradation du chlorsulfuron ct du metsulfuron-methyl ont ete mesures dans des sols preleves a des profondeurs de 0-20; 20-40 et 40-60 cm dans huit sites.L'adsorption des deux herbicides exprimait une correlation negative avec le pH du sol, et positive avec la teneur en matieres organiques. Quand les deux sols a haute teneur en matieres organiques etaient exclus des calculs, la correlation avec la teneur en matiere organique n'etait plus statistiquement significative, tandis que celle avec le pH etait seulement peu afFectee. Les taux de degradation des deux herbicides baissent en general avec I'augmentation de la profondeur du sol, et exprimaient une correlation positive avec la biomasse microbienne et negative avec le pH du sol. La transposition possible de ces resultats a la persistance des herbicides au champ est discutee. Adsoption und Abbau von Chlorsulfuron und Metsulfuron in verschiedenen BodentiefenDie Adsorptions-und Abbauraten von Chlorsulfuron und Metsulfuron in Bodenproben von 8 verschiedenen Orten aus 0-20, 20-40 und 40-60 cm Tiefe wurden gemessen. Die Adsorption der beiden Herbizide war mit dem pH-Wert negativ und mit der organischen Substanz positiv korreliert. Bei Herausnahme der Werte von 2 Boden mit besonders hohem Gehalt an organischer Substanz lieB sich die Korrelation zur organischen Substanz nicht mehr statistisch sichern, wahrend die Berechnung fCir den pH-Wert nur wenig beeinfluBt wurde. Die Abbaurate der beiden Herbizide nahm allgemein mit zunehmender Bodentiefe ab; sie war mit der mikrobiellen Biomasse positiv und mit dem pH-Wert negativ korrelierl. Die Bedeutung der Ergebnisse fur die Persistenz der Herbizide im Freiland wird diskutiert. IutroductionChlorsulfuron and metsulfuron-methyl are members of the sulphonylurea group of herbi-

Summary. Adsorption and degradation rates of triasulfuron in 8 different soils were negatively correlated with soil pH and were generally lower in subsoils than in soils from the plough layer. The half‐life at 20°C varied from 33 days in a top soil at pH 5·8 to 120 days in a subsoil at pH 7·4. Adsorption distribution coefficients in these two soils were 0·55 and 0·19, respectively. Movement and persistence of residues of chlorsulfuron, triasulfuron and metsulfuron‐methyl were compared in a field experiment prepared in spring 1987. Triasulfuron was less mobile in the soil than the other two compounds. Residues of all three herbicides were largely confined to the upper 40–50 cm soil 148 days after application. With an initial dose of 32 g ha−1, residues in the surface soil layers were sufficient to affect growth of lettuce and sugar‐beet sown approximately one year after application. Laboratory adsorption and degradation data were used with appropriate weather data in a computer model of herbicide transport in soil. The model gave good predictions of total soil residues during the first five months following application, and also predicted successfully the maximum depth of penetration of the herbicides into the soil during this period. However, more herbicide was retained close to the soil surface than was predicted by the model. The model predicted extensive movement of the herbicides in the soil during winter but did not predict that residues sufficient to affect crop growth could be present in the upper 15–20 cm soil after one year.

Alachlor degradation in soils followed first‐order reaction kinetics. Half‐lives in moist soil varied from 11.30 to 34.8 days at 25°C to 95.9 to 279.6 days at 5°C, and at 15°C varied from 15.7 to 83.1 days at 5 kPa soil water stress (field capacity) to 82.8 to 281.4 days at 1500 kPa (permanent wilting point). Degradation rates in laboratory incubations with fluctuating temperatures were predicted with reasonable accuracy from the constant‐temperature data. The degradation rate and extent of adsorption were lower in subsoils than in soils from the plough layer. Degradation rate was positively correlated with microbial biomass and microbial respiration, and adsorption was positively correlated with soil organic matter content. Persistence of alachtor in field plots was correlated well with variations in weather pattern during the period September 1990 to July 1991, with an effective half‐life varying from 20 to 60 days. Persistence in the field plots was predicted accurately by a computer model of herbicide behaviour.

Abstract: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.

A stable mixed bacterial culture which degrades the herbicide linuron was isolated from soil by enrichment with linuron in a liquid mineral medium. Radio-respirometry studies showed that the culture mineralised linuron completely. No intermediate degradation products were detected in the medium. The culture was able to utilise linuron as a source of both nitrogen and carbon and was also able to degrade the related herbicides monolinuron and chlorbromuron and the possible intermediate degradation products of linuron: 3,4-dichlorophenyl-1methylurea, 3,4-dichlorophenylurea and 3,4-dichloroaniline. The culture was unable to degrade the 1,l-dimethyl substituted ureas monuron, diuron or metoxuron. The culture contained Gram-negative aerobic rods, and Grampositive aerobic non-spore-forming rods and cocco-bacilli. Of 124 ,isolates from the mixed culture, none degraded linuron in pure culture, indicating that a consortium of organisms is involved. Further investigation suggested that Pseudomonas spp. were important components of the population responsible for degradation.