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Environmental fate and behavior of mesotrione alone and mixed with S-metolachlor and terbuthylazine in Brazilian soils

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Author(s):
Kassio Ferreira Mendes
Total Authors: 1
Document type: Doctoral Thesis
Press: Piracicaba.
Institution: Universidade de São Paulo (USP). Centro de Energia Nuclear na Agricultura (CENA/STB)
Defense date:
Examining board members:
Valdemar Luiz Tornisielo; Plinio Barbosa de Camargo; Caio Antonio Carbonari; Patrícia Andréa Monquero; Ricardo Victoria Filho
Advisor: Valdemar Luiz Tornisielo
Abstract

The mixture of herbicides is a widely used technique in weed control in several crops, including maize. However, interactions that might potentially result from mixing herbicides are a matter of constant concern and research. Soils are ideal settings to study transport and behavior of herbicides along gradients of physicochemical properties. The aim of this study was to evaluate sorption-desorption, leaching mineralization, biodegradation, and microbial respiration of mesotrione applied alone and mixed with S-metolachlor + terbuthylazine in samples from Brazilian sites cultivated with maize. The sorption-desorption of 14C-labeled [cyclohexane-2-14C] mesotrione was evaluated using the batch method, leaching was in glass columns, mineralization and degradation experiments were conducted using biometric flask, and microbial respiration was established according to the soil microorganisms: carbon transformation test with 14C-glucose solution in biometric flasks as well. Sorption of mesotrione applied alone and mixed had Kd sorption coefficient) values ranging from 0.08 to 5.05 kg L-1 and from 0.09 to 5.20 kg L-1, respectively, with similar behavior across soils. Mesotrione sorption was influenced primarily by the clay mineral (CM) content and the soil pH. Leaching of mesotrione is relatively high in the tropical soils and correlates with the pH (R2 = -0.84) and CM content (R2 = 0.75) and may pose a potential groundwater contamination risk. From the 49 d laboratory incubation data, increased mineralization half-life of mesotrione were observed for the mixture of herbicides, ranging from a 4 d increase for the sandy loam soil to a 1 d increase in the sandy clay texture soils. Mesotrione degradation rate had a 2-fold increase in the sandy loam compared to the sandy clay soil. Two metabolites can be identified from mesotrione degradation, 4-methyl-sulfonyl-2-nitrobenzoic acid (MNBA) and 2-amino-4-methylsulfonyl benzoic acid (AMBA). For unamended soil - control (without herbicide), microbial activity followed similar behavior to amended soil with herbicides in total 14CO2 released and accumulated, ranging from 23 to 27%. The mode of application of mesotrione did not influence sorption, desorption or leaching. Mesotrione sorption was relatively low in soils, which creates the high potential for leaching in maize producing areas. Thus, recommendations for mesotrione application, without the prior knowledge of the soil physical and chemical properties can result in an inefficient weed control. Mesotrione biotransformation was relatively quick, indicating this herbicide has low persistence, and consequently, low residual effect on crops and weeds when present in similar soils to this present study. Microbial respiration for all treatments was slightly higher in the sandy clay compared with sandy loam soil; although soil samples with application of herbicides (alones and in a mixture) did not have decreased basal microbial respiration or mineralization rate of glucose (AU)