Article pubs.acs.org/JAFC
Leaching of Diuron, Hexazinone, and Sulfometuron-methyl Applied Alone and in Mixture in Soils with Contrasting Textures Fabricia Cristina dos Reis* College of Agriculture “Luiz de Queiroz”, University of São Paulo, Piracicaba, Brazil
Valdemar Luiz Tornisielo Center for Nuclear Energy in Agriculture, University of Sao Paulo, Piracicaba, Brazil
Rodrigo Floriano Pimpinato Center for Nuclear Energy in Agriculture, University of Sao Paulo, Piracicaba, Brazil
Bianca A. B. Martins Center for Nuclear Energy in Agriculture, University of Sao Paulo, Piracicaba, Brazil
Ricardo Victória Filho College of Agriculture “Luiz de Queiroz”, University of São Paulo, Piracicaba, Brazil ABSTRACT: When herbicides are applied in mixture, interactions among them could potentially promote changes in herbicide behavior in the soil. Thus, application mode (isolated or in mixture) and soil texture (sandy or clayey) were investigated in the total leaching of the commercial mixture diuron + hexazinone + sulfometuron-methyl, and of each isolated compound. Experiments in soil columns also evaluated the movement of each herbicide and mixtures across soil layers. In the sandy soil, the greatest total leaching was observed with hexazinone compared to diuron and sulfometuron. Most of the applied diuron remained at the top layer of the soil, indicating that this herbicide has low soil mobility. Overall, our results show that hexazinone has greater leaching potential and mobility along the soil profile compared to diuron and sulfometuron. Our data can be used in assessing the fate of diuron, hexazinone, and sulfometuron alone or in mixture on natural ecosystems, under different soil types and application modes. KEYWORDS: mobility, radiolabeled herbicides, environmental fate, tank mixture
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INTRODUCTION The use of herbicides is a well-established and effective practice to control weeds in agricultural systems; however, environmental concerns are a reality because herbicide molecules have been detected in several surfaces and groundwater quality studies.1 No matter if applied pre- or postemergence, herbicides ultimately reach the soil surface. Herbicide leaching along the soil profile is favored by rain or irrigation water, and depends on the physicochemical properties of both herbicide and soil. It is important that the applied herbicide leaches to the zone where weed seeds are germinating; however, if the herbicide leaches deeper than where it should be concentrated, herbicide may be lost and may reach the groundwater level.2,3 The sorption of herbicides to the organic and inorganic colloids of soils is directly correlated with the leaching, and its intensity depends on the physicochemical characteristics of the compound, soil attributes, and weather.4 When the herbicide is bound to soil colloids, its availability will decrease and, consequently, its mobility,3 so there is an inverse relationship © XXXX American Chemical Society
between mobility of an herbicide and its sorption capacity by soil colloids.5 The mobility of herbicides and other pesticides can be studied in the field by the use of lysimeters, or in laboratory conditions with disturbed or undisturbed soil columns.6,7 Leaching studies under laboratory conditions not only allow determining herbicide leaching or mobility potential in the soil but also can be used to compare different herbicides and treatments.8 The commercial herbicide mixture diuron + hexazinone + sulfometuron-methyl is recommended preemergence in sugarcane production systems. Hexazinone is a weak acid herbicide (ionization constant of 2.2),9 presenting high solubility in water (33000 mg·L−1 at 25 °C) and moderately long residual period, Received: November 15, 2016 Revised: February 10, 2017 Accepted: February 27, 2017
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DOI: 10.1021/acs.jafc.6b05127 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry with a typical field half-life of 90 days. Diuron is a neutral herbicide that has low solubility in water (42 mg·L−1 at 25 °C) and an average field half-life of 90 days.10 The weak acid sulfometuron-methyl (ionization constant of 5.2) has moderate solubility in water (300 mg·L−1 at pH 7) and has low persistence in the environment (half-life = 20−28 days).10 Weed control with herbicide tank mixtures and commercial mixtures has been widely studied regarding mixture effectiveness, antagonism, and/or synergism. However, there are few studies on herbicide formulations containing more than one active ingredient or tank mixtures of herbicide formulations and their interactions with some soil properties, including texture, because most studies evaluate behavior of individual molecules.11 Some studies indicate that application in mixture could affect leaching of the herbicides that are present in the mixture. For example, greater control of Ipomoea triloba was observed when diuron, hexazinone, and sulfometuron-methyl were applied in mixture compared with each herbicide applied alone, at 17 cm of layer column in sand soil.12 Percent control of the bioindicator species Cucumis sativus was 20% when the commercial mixture of diuron and hexazinone was applied, compared to 10% control when hexazinone was applied alone.13 Therefore, this study was conducted to evaluate the leaching behavior of the herbicides diuron, hexazinone, and sulfometuron-methyl applied in mixture or alone, in two Brazilian agricultural soils, with contrasting textures.
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Table 1. Soil Physicochemical Attributes attributes pH (KCl) pH (H2O) OMa P K Ca Mg Al H + Al SBb CECc sand silt clay a
units
clayed (dark-red latosol)
sandy (typic quartzipsamments)
g·kg−1 mg·kg−1 mmolc·kg−1 mmolc·kg−1 mmolc·kg−1 mmolc·kg−1 mmolc·kg−1 mmolc·kg−1 mmolc·kg−1 g·kg−1 g·kg−1 g·kg−1
5.2 6.4 41 28 9.2 40 16 0.05). Percentages of 14C-sulfometuron differed between the two soil types (F = 107.65; p < 0.001). Although 14C-sulfometuron-methyl percentages were greater in the sandy soil than in the clayey soil, the amount of this herbicide detected in the total leachates was nearly zero (Table 2). There was a significant interaction among soil depth (layers) and soil type (p < 0.05) (Figure 1) on sulfometuron-methyl
Diuron. No differences within application mode were observed for diuron leachates detected in the sandy soil. However, in the clayey soil, the percentage of 14C-diuron detected was greater when applied in mixture compared to when diuron was applied alone (Table 2). Nonetheless, the percentage of diuron detected in both soils did not exceed 0.13%. Thus, differences in 14C-diuron percentages in the leachate among soils and modes of application were very small ( 0.05). This result is surprising, because in the sandy soil, due to its low organic matter and CEC, we expected slightly more diuron leaching, but instead, this herbicide remained in the first layer in the sandy soil as well. It is worth mentioning that the dose of diuron applied (1387 g a.i. ha−1) was higher than the recommended dose for sandy soils (1025 g a.i ha−1),21 which would increase diuron leaching potential across the soil layers.
leaching along the soil profile. For the clayey soil, sulfometuron leached through the first 2 layers, with greater amounts of the herbicide detected in the first layer (0−5 cm) compared to the second one (5−10 cm) for both modes of application (Figure 1). Sulfometuron amounts deeper in the clayey soil column (10 cm on) were nearly zero. For the sandy soil, amounts of sulfometuron detected in the top layer (0−5 cm) were the same as the amounts found in the top layer of the clayey soil, regardless the mode of application. In addition, there was less sulfometuron present in the second layer in the sandy soil compared to the second layer of the clayey soil because sulfometuron leached until the third soil layer in the sandy soil. Sulfometuron was equally detected in the third layer of the sandy soil for both modes of application (Figure 1). This indicates that sulfometuron-methyl leaching is greater in sandy soils than in clayey soils. Sulfonylurea herbicides, such as sulfometuron, are weak acids,22 which means that under neutral to alkaline conditions the molecules are negatively charged, and as herbicide mobility increases, so does soil pH.23 Because sulfometuron is a weak acid, its leaching is expected to be different between soil types with different pHs. Harvey et al.23observed that the difference D
DOI: 10.1021/acs.jafc.6b05127 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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Journal of Agricultural and Food Chemistry
participated in the definition of the methodology, and revised the paper.
in affinity of sulfometuron-methyl with the different soils appears to be due to differences in soil organic matter content and soil pH. Indeed, the organic matter content between the sandy and clayey soils in this study strongly differed, with the clayey soil possessing greater amounts of organic matter than the sandy one (Table 1). The greater amounts of sulfometuron-methyl were retained at the upper two layers of the soils (Figure 1). In a field study, sulfometuron-methyl was determined to be immobile, i.e., confined to the upper soil depth, (0−15) (Trubey et al.24). Lym and Swenson25 found that sulfometuron-methyl leached through the soil column to a depth of 65 cm after 48 h of rainfall simulation (451 mm), but the highest amounts of sulfometuron-methyl were found in the top 5 cm layer of the soil columns. When the soil columns were leached during 9 weeks (51 mm of rainfall simulation per week), sulfometuronmethyl leaching along the soil was less compared to 48 h rainfall simulation. Despite detection of differences in diuron leaching along the soil between modes of application within the first layer (0−5 cm), no differences were observed in the deeper layers, so application mode probably did not influence diuron mobility along the soil. In addition, no influence of soil texture was observed for total diuron leaching. Hexazinone amounts detected in the leachates were greater when this herbicide was applied in the sandy soil than in the clayey soil. Both soil type and depth affected hexazinone leaching through the soil column. For sulfometuron-methyl, soil type affected the total leaching of this herbicide. In this study, we aimed to evaluate the leaching of the studied herbicides in different soil types and application modes, as well as along the soil profile, applying the herbicides at their recommended rates and at a high and continuous rain simulation, for 48 h. Such conditions were considered to be conservative in terms of herbicide leaching potential and because high and continuous rain represents tropical regimes during the growing season. Our results help in the comprehension of the environmental fate of the herbicides studied here in natural ecosystems. For a more complete understanding about the behavior of the herbicide molecules studied here, when they are applied alone or in mixture to different soils, studies on adsorption and desorption should be performed.
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Author Contributions
This work is part of the PhD Dissertation of the first author Fabrı ́cia Cristina dos Reis. Funding
The first author thanks CNPq for the scholarship granted for the first author. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors are deeply grateful to Dupont Crop Protection for providing herbicides for this study. The authors would like to thank Dr. Katherine Carson for the valuables inputs on this manuscript.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel: +55 19 983004006. Mailing address: College of Agriculture “Luiz de Queiroz”, University of São Paulo, Av. Pádua Dias, 11, Cx. Postal 9, Piracicaba-SP, 13418-900, Brazil. ORCID
Fabricia Cristina dos Reis: 0000-0002-6201-4163 Author Contributions
F.C.d.R. carried out the experiments and wrote the initial project and the paper. V.L.T. was coadviser of the first author, was responsible for the training, and participated in the definition of the methodology. B.A.B.M. carried out the statistical analysis and wrote and revised the paper. R.F.P. helped the first author to carry out the research and participated in the definition of the methodology. R.V.F. was adviser of the first author, idealized the initial project, E
DOI: 10.1021/acs.jafc.6b05127 J. Agric. Food Chem. XXXX, XXX, XXX−XXX
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