title: Adsorption and degradation of the weak acid mesotrione in soil and environmental fate implications.
abstract: The ability of soils to adsorb and degrade pesticides strongly influences their environmental fate. This paper examines the adsorption and degradation of a weak acid, a new herbicide mesotrione 12-[4-(methylsulfonyl)-2-nitrobenzoyl]-1,3-cyclohexanedione], in 15 different soils from Europe and the USA. Experiments were conducted to understand the influence of soil properties, covering a wide range of soil textures, soil pH values (4.4 to 7.5), and organic carbon contents (0.6 to 3.35%). Mesotrione adsorption (Kd values ranged from 0.13 to 5.0 L/kg) was primarily related to soil pH, and to a lesser extent by percent organic carbon (%OC). As soil pH rose. mesotrione Kd values got smaller as mesotrione dissociated from the molecular to anionic form. Mesotrione degradation (half-lives ranged from 4.5 to 32 d) was also related to soil pH, getting shorter as soil pH rose. Simple regression of mesotrione adsorption against soil pH and %OC and against degradation provided a close fit to the data. The correlation between mesotrione adsorption and degradation means that Kd and half-life values are only relevant for use in environmental fate assessment if these values are "paired" for the same soil pH and %OC. The implications were as illustrated for leaching, raising important issues about combining pesticide adsorption and degradation behavior in environmental fate assessments.
authors.affiliation: Syngenta, Jealott's Hill International Research Station, Bracknell, Berkshire, UK. jeremy.dyson@syngenta.com
authors.forename: J S
authors.initials: JS
authors.lastname: Dyson
authors.forename: S
authors.initials: S
authors.lastname: Beulke
authors.forename: C D
authors.initials: CD
authors.lastname: Brown
authors.forename: M C G
authors.initials: MC
authors.lastname: Lane
mesh_terms: D000327:Adsorption; D001682:Biological Availability; D003512:Cyclohexanones / Q000737:chemistry* / Q000378:metabolism; D006207:Half-Life; D006540:Herbicides / Q000737:chemistry* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D012988:Soil Microbiology; D012989:Soil Pollutants / Q000378:metabolism*
chemical_list: D003512:Cyclohexanones; D006540:Herbicides; D012989:Soil Pollutants; C432907:mesotrione