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Imazaquin Sorption in Highly Weathered Tropical Soils

Centro de Energia Nuclear na Agricultura/Universidade de São Paulo, Caixa Postal 96, 13400-970, Piracicaba, São Paulo, Brazil;
Escola Superior de Agricultura "Luiz de Queiroz"/Universidade de São Paulo, Caixa Postal 09, 13418-900, Piracicaba, São Paulo, Brazil.
Universidade Federal de São Carlos, Caixa Postal 153, 13600-970, Araras, São Paulo, Brazil.

^* Corresponding author (regitano{at}cena.usp.br).

ABSTRACT

Imazaqnin is a herbicide used intensively in Brazil to control a large number of weeds associated with soybean [Glycine max (L.) Merr.]. Most of the Brazilian soils are highly weathered (50–60% are Oxisols). The extent of sorption and desorption of imazaquin in these soils will dictate its mobility and environmental fate once microbial degradation is negligible. Sorption and desorption of imazaquin were studied in nine samples (seven surface and two subsurface) from highly weathered soils and in two other soils. Sorption of imazaquin was low in all soils, except in the soil with high organic carbon content (OC) content and low soil-solution pH (Typic Humaquept, GH soil). The OC and the soil-solution pH were the soil attributes that correlated better with the amount of sorbed imazaquin in the studied highly weathered soils [r = 0.91 (significant at the 0.01 probability level) and –0.62 (significant at the 0.05 probability level), respectively]. The Fe oxide content was an important soil attribute affecting sorption of imazaquin when OC was low. In general, a model based on hydrophobic interactions was capable of predicting sorption of imazaquin. However, this model and others based on multivariate regression were not capable of predicting sorption of imazaquin on highly weathered soils with pH-dependent charges and low OC, such as the subsurface acric soils. Low sorption implies a high potential for mobility and, therefore, imazaquin may reach important sources of drinking water. This potential for mobility may be diminished in highly weathered soils due to their acidic nature and high Fe and Al oxide contents.

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