Persistence in soil of the two herbicides glyphosate and 2,4-D was investigated after application for brush control in conifer reforestation areas. Field experiments were carried out at five sites in southern Sweden and six in northern Sweden. Initially, glyphosate disappeared faster in northern soils than in southern soils. This was probably a result of the higher biological activity in the northern soils. However, small amounts of glyphosate were detected in the northern area long after all traces had disappeared in the south, presumably because of the long period during which the soil remained frozen in the northern area and because of the slow release of vegetation-bound herbicide. One metabolite of glyphosate, aminomethylphosphonic acid (AMPA), persisted longer than glyphosate itself. After 2 years, 8% of the theoretical amount was found in the northern area and, after 1 year, 1% in the southern area. 2,4-D disappeared rapidly from all sites, although minor amounts persisted for several years, probably because of slow release from vegetation-bound residues.

SummaryThe effects of soil temperature and soil moisture content on the rate of simazine degradation were measured in the laboratory in soils from sixteen sites located in several different countries. First‐order half‐lives under standard incubation conditions were significantly correlated with clay content, organic carbon content and soil pH in a multiple linear regression. The temperature dependence of degradation was similar in the different soils whereas the moisture dependence showed considerable variation between soils. Persistence of simazine was also measured in the same soils in the field and at live additional sites. Weather records from the different sites for the periods of the Held experiments were used in conjunction with constants derived from the laboratory data in a computer program to simulate persistence in the field. In general, the model overestimated residues in the field. About half of the calculated residues were within 25% of those observed, an accuracy sufficient for practical purposes, but on several occasions the discrepancies between calculated and observed residues were greater than 50%. Possible reasons for the discrepancies and requirements for further experiments are discussed.

Summary: Résumé: ZusammenfassungThe effect of repeated applications of 2,4‐D and MCPA in a long‐term field experiment on their subsequent degradation was investigated. The herbicides had been applied annually since 1954, Comparisons were made with plots treated once and then retreated the following year. Repealed applications of the two herbicides resulted in a reduction in degradation lime from 10 weeks for 2,4‐D and 20 weeks for MCPA after one application to 4 and 7 weeks respectively after 19 years of annual application. In spite of an increased capability of the soil microflora lo degrade 2,4‐D and MCPA 9 months after a previous treatment. no change in numbers of organisms capable of utilizing the substances as carbon sources were found in treated plots as compared with the control. It is obvious that there is a cross adaptation between 2,4‐D and MCPA. It seems most likely that two different ‘enrichment’ been induced, each capable of decomposing both herbicides, but with different efficiencies.Effet d'applications répétée de 2,4‐D et de MCPA sur leur degradation dans le solL'effet d'applications répétée de 2.4‐D et de MCPA sur leur degradation uléerieure a étéétudié dans une expérience au champ de longue durée. Les herbicides ont été appliqués chaque année depuis 1954. Des comparaisons ont été effectuées avec des parcelles traiées une fois et traitées à nouveau l'année suivante. Les applications répétées des deux herbicides ont provoqué une réduction du temps de dégradation qui est passé de 10 semaines pour le 2,4‐D et 20 semaines pour le MCPA après une seule application, à 4 et 7 semaines respectivement, après 19 années d'applications annuelles. En dépit d'une capacité accrue de la microflore du sol à dégrader le 2.4‐D et le MCPA, neuf mois après un traitement préalable, aucun changement n'a été constaté dans le nombre des organismes capables d'utiliser les produits comme sources de carbone dans les parcelles traitées, par rapport au témoin. II est évident qu'il y a une adaptation croisée entre le 2,4‐D et le MCPA. II apparait très probable que deux ‘enrichissements’ de Hores ont été induits, chacun d'eux étant capable de décomposer les deux herbicides mais avec des efficacités différentes. Die Wirkung wiederholter Behandlungen von 2.4‐ D und MCPA auf ihren Abbau im Boden Die Wirkung wiederholter Anwendungen von 2,4‐D und MCPA auf ihren Abbau wurden in einem Freiland‐Versuch untersucht. Die Herbizide wurden jährlich, sell 1954 ange‐wendet. Es wurden Vergleiche mil Parzellen angcstellt, die zum erstenmal und im folgenden Jahr nochrnaK bchandelt worden waren. Die widerholien Behandlungen mit den beiden Herbiziden bewirkte elne Verringerung der Ab‐bauzeit von 10 Wochen fur 2,4‐D und 20 Wochen fiir MCPA nach einer Behandlung, auf 4 bzw. 7 Wochen nach 19 Jahren jahrlicher Anwendung. Trotz der erhohten Ffihigkeit der Bodenmikroflora 2,4‐D und MCPA neun Monate nach einer vorhergehenden Behandlung abzubauen, war die Zahl der Organismen, die diese Herbizide als Kohlensloffquelle verwerten konnen, in den behandelten und unbehandelten Parzellen gleich gross. Zwischen 2.4‐D und MCPA existiert ofVensichtlich elne Kreuz‐Adaplalion. Sehr wahrscheinlich wurden zwei versehiedene ‘enrichment’‐Floren induziert, von der jede in der Lage ist beide Herbizide, aber mit unterschiedlicher Effizienz, Hbzubauen.