The introduction of crops resistant to the broad spectrum herbicide glyphosate, N-(phosphonomethyl)glycine, may constitute an answer to increased contamination of the environment by herbicides, since it should reduce the total amount of herbicide needed and the number of active ingredients. However, there are few published data comparing the fate of glyphosate in the environment, particularly in soil, with that of substitute herbicides. The objective of this study is to compare the fate of glyphosate in three soils with that of four herbicides frequently used on crops that might be glyphosate resistant: trifluralin, alpha,alpha,alpha-trifluoro-2,6-dinitro-N,N-dipropyl-p-toluidine, and metazachlor, 2-chloro-N-(pyrazol-1-ylmethyl)acet-2',6'-xylidide for oilseed rape, metamitron, 4-amino-4,5-dihydro-3-methyl-6-phenyl-1,2,4-triazin-5-one for sugarbeet and sulcotrione, 2-(2-chloro-4-mesylbenzoyl)cyclohexane-1,3-dione for maize. The distribution of herbicides between the volatilized, mineralized, extractable and non-extractable fractions was studied, along with the formation of their metabolites in laboratory experiments using 14C-labelled herbicides, over a period of 140 days. The main dissipation pathways were mineralization for glyphosate and sulcotrione, volatilization for trifluralin and non-extractable residues formation for metazachlor and metamitron. The five herbicides had low persistence. Glyphosate had the shortest half-life, which varied with soil type, whereas trifluralin had the longest. The half-lives of metazachlor and sulcotrione were comparable, whereas that of metamitron was highly variable. Glyphosate, metazachlor and sulcotrione were degraded into persistent metabolites. Low amounts of trifluralin and metamitron metabolites were observed. At 140 days after herbicide applications, the amounts of glyphosate and its metabolite residues in soils were the lowest in two soils, but not in the third soil, a loamy sand with low pH. The environmental advantage in using glyphosate due to its rapid degradation is counterbalanced by accumulation of aminomethylphosphonic acid specifically in the context of extensive use of glyphosate.

A comprehensive review of quantitative structure-activity relationships (QSAR) allowing the prediction of the fate of organic compounds in the environment from their molecular properties was done. The considered processes were water dissolution, dissociation, volatilization, retention on soils and sediments (mainly adsorption and desorption), degradation (biotic and abiotic), and absorption by plants. A total of 790 equations involving 686 structural molecular descriptors are reported to estimate 90 environmental parameters related to these processes. A significant number of equations was found for dissociation process (pKa), water dissolution or hydrophobic behavior (especially through the KOW parameter), adsorption to soils and biodegradation. A lack of QSAR was observed to estimate desorption or potential of transfer to water. Among the 686 molecular descriptors, five were found to be dominant in the 790 collected equations and the most generic ones: four quantum-chemical descriptors, the energy of the highest occupied molecular orbital (EHOMO) and the energy of the lowest unoccupied molecular orbital (ELUMO), polarizability (α) and dipole moment (μ), and one constitutional descriptor, the molecular weight. Keeping in mind that the combination of descriptors belonging to different categories (constitutional, topological, quantum-chemical) led to improve QSAR performances, these descriptors should be considered for the development of new QSAR, for further predictions of environmental parameters. This review also allows finding of the relevant QSAR equations to predict the fate of a wide diversity of compounds in the environment.

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Retention partly controls the behaviour of pesticides in the environment, and is usually characterized with sorption and desorption isotherms. However, this approach suffers several limitations: pesticide sorption in soils is time dependent, and desorption-related parameters are scant and difficult to use. The objective of this study was to maximize the exploitation of sorption and/or desorption data to characterize desorption along with the effect of aging on retention. The experiments involved three soils and five herbicides (a broad-spectrum herbicide, glyphosate, and four commonly used selective herbicides, trifluralin, metazachlor, metamitron and sulcotrione). Sorption isotherms were not linear and herbicide desorption was markedly hysteretic. Desorption was inversely related to adsorption, being small when sorption was great (glyphosate and trifluralin), and great when sorption was small (metazachlor, metamitron and sulcotrione). Single, different desorption isotherms are obtained that depend on initial sorbed herbicide concentration. A theoretical approach allowed calculation of adapted desorption parameters for different sorption concentrations from only one desorption isotherm. Soil retention of metazachlor, metamitron and sulcotrione increased with time, while that of trifluralin remained stable. The sorption parameter values increased with time, which is explained by a decrease in the herbicide concentration due to degradation, the non-linear nature of the sorption isotherms, hysteretic desorption, and the formation of bound residues. The relative contributions of these phenomena, which depend on the nature of herbicide and soil type, were estimated. Generalized equations were derived to describe sorption and desorption, and these equations could be implemented in pesticide-fate models to take into account sorption and desorption parameters as well as their time dependence.Désorption des herbicides et dépendance temporelle de leur sorption dans les sols. RésuméLe comportement des pesticides dans l'environnement de´pend en grande partie de leur re´tention dans les sols. La re´tention est ge´ne´ralement caracte´rise´e par des isothermes d'adsorption et de de´sorption, mais cette approche souffre d'un certain nombre de limites: le temps de re´sidence dans le sol des re´sidus de pesticide modifie leurs proprie´te´s de re´tention et les parame`tres de de´sorption sont limite´s et difficiles aì nterpre´ter. L'objectif de cette e´tude est d'optimiser l'exploitation des donne´es d'adsorption/de´sorption pour caracte´riser la de´sorption des pesticides et l'effet du temps sur leur re´tention. Les e´tudes ont e´teŕ e´alise´es avec trois sols et cinq herbicides: un herbicide a`large spectre d'action, le glyphosate, et quatre herbicides se´lectifs fre´quemment utilise´s: la trifluraline, le me´tazachlore, la me´tamitrone et la sulcotrione.Les re´sultats ont montre´que les isothermes de sorption ne sont pas line´aires et que la de´sorption des herbicides pre´sente une forte hyste´re´sis. La de´sorption est inversem...

Because of the detection of AMPA in the deep soil layer, the replacement of both trifluralin and metazachlor with glyphosate might not contribute to decreasing environmental contamination by herbicides. PRZM may be used to evaluate and to compare other weed control strategies for herbicide-resistant as well as non-resistant crops.