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Leaching of Herbicides Commonly Applied to Sugarcane in Five Agricultural Soils

Lixiviação de Herbicidas Comumente Aplicados na Cana-de-Açúcar em Cinco Solos Agricultáveis

ABSTRACT:

Leaching intensity depends on the physicochemical properties of soils and herbicides. Consequently, a good understanding of this process is essential to determine mitigation measures to reduce or eliminate the risk of water contamination around areas with sugarcane crops. Therefore, the objective of this study was to analyze the leaching of ametryn, diuron, hexazinone, and metribuzin by using columns in five soils with different physicochemical properties cultivated with sugarcane. The radiolabeled herbicides with 14C were evaluated at six soil depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m) and in the leachate after 200 mm rainfall simulation for 48 h. Herbicide leaching was mostly affected by soil type. Ametryn and diuron were reported in the highest amount (>87%) on the topsoil (0-0.05 m), which was indicative of the low leaching of these two herbicides in the soil profile. Thus, these herbicides can contaminate surface water by runoff. Leachate percentage was always <0.3% for ametryn, diuron, and metribuzin; this was also the case for hexazinone in two out of the five soils. Hexazinone proved to be a potential contaminant of groundwater and metribuzin presented high leaching in the soil profile. Previous knowledge of the physico-chemical properties of soils cultivated with sugarcane is essential to recommend the use of these herbicides in weed management.

Keywords:
soil behavior; pre-emergent; leachate; mobility

RESUMO:

A intensidade da lixiviação depende das propriedades físico-químicas dos solos e dos herbicidas. Consequentemente, uma boa compreensão desse processo é essencial para a determinação de medidas de mitigação para reduzir ou eliminar o risco de contaminação de águas próximas às áreas cultivadas com cana-de-açúcar. Portanto, o objetivo deste estudo foi analisar a lixiviação de ametryn, diuron, hexazinone e metribuzin usando colunas de cinco solos cultivados com cana-de-açúcar. Os herbicidas radiomarcados com 14C foram avaliados em seis profundidades do solo (0-0,05; 0,05-0,10; 0,10-0,15; 0,15-0,20; 0,20-0,25; e 0,25-0,30 m) e no lixiviado após uma simulação de 200 mm de chuva por 48 horas. A lixiviação dos herbicidas foi principalmente afetada pelo tipo de solo. Ametryn e diuron foram encontrados em maior quantidade (>87%) na camada superficial do solo (0-0,05 m), mostrando a baixa lixiviação desses dois herbicidas no perfil do solo; assim, esses herbicidas podem contaminar as águas superficiais por escoamento superficial. A porcentagem do lixiviado foi sempre <0,3% para ametryn, diuron e metribuzin, o que também ocorreu para o hexazinone em dois dos cinco solos. O hexazinone mostrou ser um potencial contaminante das águas subterrâneas, e o metribuzin apresentou alta lixiviação no perfil do solo. O conhecimento prévio das propriedades físico-químicas dos solos cultivados com cana-de-açúcar é essencial para recomendar o uso desses herbicidas no manejo das plantas daninhas.

Palavras-chave:
comportamento no solo; pré-emergente; lixiviado; mobilidade

INTRODUCTION

Among the existing problems in the sugarcane industry that affect production is weed control, which may account for losses from 74.1 to 90.5% of production without control (Yirefu et al., 2012Yirefu F, Tana T, Tafesse A, Zekarias Y. Competitive ability of sugarcane (Saccharum officinarum L.) cultivars to weed interference in sugarcane plantations of Ethiopia. Crop Prot. 2012;32:138-43.). Such negative effects caused by the presence of weeds can be minimized by control practices, such as mechanical, cultural and chemical methods. Under the current production conditions in Brazil, the chemical method, which involves the application of herbicides, is the most widely used (Carvalho et al., 2010Carvalho FT, Castro RM, Otsubo RI, Pereira FAR. Controle de dez espécies daninhas em cana-de-açúcar com o herbicida mesotrione em mistura com ametryn e metribuzin. Planta Daninha. 2010;28(3):585-90.; Kuva and Salgado, 2014Kuva MA, Salgado TP. Manejo de plantas daninhas nas principais culturas agrícolas perenes do Brasil - Seção I - Manejo de plantas daninhas da cultura de cana-de-acucar. In: Monquero PA, organizador. Manejo de plantas daninhas nas culturas agrícolas. São Carlos: RiMa; 2014. p.82-112.) because of extensive cultivated areas, limited availability of manual labor, ease of application, cost and treatment efficacy.

Herbicides play a vital role in modern agriculture. However, concerns about food safety and the environmental impact of residues of these molecules have increased (Zhang et al., 2010Zhang CZ, Zhang ZY, Liu XJ, Jiang W, Wu YD. Dissipation and environmental fate of herbicide H-9201 in carrot plantings under field conditions. Food Chem. 2010;119(3):874-9.; Kubo et al., 2012Kubo T, Ohno M, Nagasawa S, Kose T, Kawata K. Behavior of herbicide pyrazolynate and its hydrolysate in paddy fields after application. Bull Environ Contam Toxicol. 2012;89(5):985-89.). Because of agricultural herbicide applications, surface and ground waters have been contaminated as a result of runoff and leaching into the soil (Rozemeijer and Broers, 2007Rozemeijer JC, Broers HP. The groundwater contribution to surface water contamination in a region with intensive agricultural land use (Noord-Brabant, The Netherlands). Environ Pollut. 2007;148(3):695-706.). The leaching process is the main form of descending transport of non-volatile and water-soluble molecules into the soil. These molecules move through the soil profile, along with the water flow. The water potential difference between two points and leaching intensity depend on the physicochemical properties of soils and herbicides (Inoue et al., 2003Inoue MH, Oliveira Jr RS, Regitano JB, Tormena CA, Tornisielo VL, Constantin J. Critérios para avaliação do potencial de lixiviação dos herbicidas comercializados no estado do Paraná. Planta Daninha. 2003;21(2):313-23.). Consequently, a good understanding of this process is essential to determine mitigation measures to reduce or eliminate water contamination risk (Dores et al., 2013Dores EF, De Souza L, Villa RD, Pinto AA. Assessment of metolachlor and diuron leaching in a tropical soil using undisturbed soil columns under laboratory conditions. J Environ Sci Health Part B. 2013;48(2):114-21.) in the areas surrounding sugarcane cultivation.

A wide variety of herbicides are used in Brazil, and they may have a strong environmental impact because of their high leaching potential in soils (Dores et al., 2013Dores EF, De Souza L, Villa RD, Pinto AA. Assessment of metolachlor and diuron leaching in a tropical soil using undisturbed soil columns under laboratory conditions. J Environ Sci Health Part B. 2013;48(2):114-21.; Toniêto et al., 2016Toniêto TA, Pierri L, Tornisielo VL, Regitano JB. Fate of tebuthiuron and hexazinone in green-cane harvesting system. J Agric Food Chem. 2016;64(20):3960-6.). Among these herbicides, ametryn [1,3,5-triazine-2,4-diamine, N-ethyl-N’-(1-methylethyl)-6-(methylthio)], diuron [3-(3,4-dichlorophenyl)-1,1-dimethylurea], hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2,4 (1H,3H) dione], and metribuzin [4-amino-6-(1,1-dimethylethyl)-3-(methylthio)-1,2,4-triazin-5(4H)-one] are pre-emergent herbicides heavily used for weed control in sugarcane. Indeed, they have been frequently reported in groundwaters (Kubilius and Bushway, 1998Kubilius DT, Bushway RJ. Determination of hexazinone and its metabolites in groundwater by capillary electrophoresis. J Chromatogr A. 1998;793(2):349-55.; Kim and Feagley, 2002Kim JH, Feagley SE. Leaching of trifluralin, metolachlor, and metribuzin in a clay loam soil of Louisiana. J Environ Sci Health Part B. 2002;37(5):393-403.; Caracciolo et al., 2005Caracciolo AB, Giuliano G, Grenni P, Guzzella L, Pozzoni F, Bottoni P, et al. Degradation and leaching of the herbicides metolachlor and diuron: a case study in an area of Northern Italy. Environ Pollut. 2005;134(3):525-34.; Santos et al., 2015Santos EA, Correia NM, Silva JRM, Velini ED, Passos ABRJ, Durigan J.C. Herbicide detection in groundwater in Córrego Rico-SP watershed. Planta Daninha. 2015;33(1):147-55.).

Therefore, the objective of the study was to evaluate the leaching of ametryn, diuron, hexazinone, and metribuzin using soil columns in five classes of soils from areas cultivated with sugarcane.

MATERIAL AND METHODS

Four leaching experiments were conducted, using 14C-labeled ametryn, diuron, hexazinone and metribuzin according to the method described in the guidelines “Leaching in Soil Columns” by the OECD (2004Organisation for Economic Co-Operation and Development - OECD. OECD guidelines for testing of chemicals. Paris: Leaching in Soil Columns; 2004. 15p. (Test number, 312)).

The five soils used in the experiments were collected in sugarcane fields in the region of Piracicaba, São Paulo, Brazil, on Iracema Farms, from the 0-0.10 m depth layer after a layer of vegetation covering the soil was removed. The soil samples were air dried, sieved on a 1.7-mm mesh and stored at room temperature in labeled plastic bags. The main physicochemical properties of soils are shown in Table 1.

Table 1
Physicochemical properties of sugarcane areas of soils used in the experiments in Piracicaba, São Paulo, Brazil

The stock solutions were prepared by using non-radiolabeled analytical standards of ametryn (4,000 g ha+1), diuron (4,000 g ha-1), hexazinone (500 g ha-1), and metribuzin (1,920 g ha-1), with purities of 99.0, 98.7, 99.5, and 99.7%, respectively (Sigma Aldrich, Saint Louis, MO, USA), at a concentration of 2.0 µg µL-1 in acetone for ametryn, diuron, and hexazinone; and 10.0 µg µL-1 in acetronitrile for metribuzin prior to preparing the herbicides for the work solution. The analytical standards 14C-ametryn, 14C-diuron, 14C-hexazinone, and 14C-metribuzin (Izotop, Budapest, Hungary) showed 100.0, 98.7, 99.7, and 97.0% radiochemical purity and 1.4, 2.4, 3.1, and 2.3 MBq mg-1 of specific activity, respectively. Radiolabeled and non-radiolabeled standards were carefully mixed in 0.01 mol L-1 CaCl2 to the work solution, and then 200.0 µL of each solution was added to each soil column, directly applied with an automatic pipette to the moist soil at the top of each column. Thus, 785.0, 785.0, 98.0, and 380.0 µg a.i. (active ingredient) of ametryn, diuron, hexazinone, and metribuzin, respectively, were added to the glass column. Herbicide rates were calculated in accordance with a gathering depth of 0.1 m and column area equal 19.625 x 10-4 m2.

A 0.50 m tall glass column (two repetitions) packed with samples of five types of soils was used for the experiment. The soil columns were prepared by closing the tip with quartz wool, filling the conical part with washed quartz sand dried in an oven at 100 oC and packing the column to a height of 0.30 m, placing small dry soil air at the bottom portions, and vibrating the set to accommodate the soil sample to avoid the formation of air bubbles. The soil samples which were conditioned on the columns were weighed to check the performance of the packaging process and contained 637.9 g of Clay-1; 696.1 g of Clay-2; 917.17 g of Loam-1; 853.3 g of Loam-2, and 951.9 g of Sand.

Soil columns were placed inside a 2.0 L beaker and wetted slowly with upward flow of a 0.01 mol L-1 CaCl2 solution so that the solution level was not greater than 0.10 m front wetting of the soil sample. The soil sample was flooded for approximately 30 min. After flooding of the column, when the CaCl2 solution reached the top of the upward flow, the columns were removed from the beaker and installed on an iron support to hold the soil columns. After 1 to 2 h, the CaCl2 solution was drained.

After application, the surface of the soil sample was covered with a quartz wool disk, fitting an inverted funnel connected to a tube by which the 0.01 mol L-1 CaCl2 solution was passed. A flow of approximately 8 mL h-1 for 48 h was simulated while using the 0.01 mol L-1 CaCl2 solution, resulting in a rain simulation of approximately 200 mm in 48 h.

Every 12 h, three aliquots of 10 ml of leachate were collected and added to 10 mL of Insta-Gel for measurement by liquid scintillation spectrometry (LSS), using a Tri-Carb 2910 TR counter (LSA PerkinElmer, Waltham, MA, USA). After 48 h of radiolabeled herbicide application, the glass columns were removed from the support, the soil samples were withdrawn from the columns, and air was injected at the tip of the column to force the soil outlet, which was cut into six sections of equal-size depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m). The soil samples were air dried, weighed, homogenized and ground in a mechanical mill (Marconi MA330, Piracicaba, SP, Brazil). Three sub-samples (0.2 g) of each dried layer of soil were biologically oxidized (OX500, RJ Harvey Instrument Corporation, Tappan, NY, USA) for quantification of total radioactivity.

The results were expressed as the percentage of radioactivity found in the leachate and in each segment of the column relative to the radioactivity initially applied. The recovery of the studies (sum of the percentages of radiolabeled herbicide found in the soil depth and leachate) should range between 90 and 110% for the radiolabeled substances according to the OECD (2004Organisation for Economic Co-Operation and Development - OECD. OECD guidelines for testing of chemicals. Paris: Leaching in Soil Columns; 2004. 15p. (Test number, 312)). Additionally, to check the repeatability and sensitivity of the analytical method, the samples of oxidized soil and leachate were analyzed in triplicate.

To analyze herbicide leaching along the soil column, an analysis was made of the effect of two factors, soil types and total leaching (herbicides found at six depths plus leachate), as well as their interactions. All measurements were performed in replicates, with arithmetic means and standard desviation of means (means ± SD) calculated from the repeated measurements. The entire data set was tested to ensure that the principles of the basic analysis of variance were met for each herbicide. In the case of significant effects, Tukey’s test (p<0.05) for multiple comparisons of the means was used. Figures were plotted in Sigma Plot (version 10.0 for Windows, Systat Software Inc., Point Richmond, CA, USA).

RESULTS AND DISCUSSION

Mass balance was conducted to calculate the amount of 14C found in the soil depth which was transformed into 14C-CO2 plus 14C found in the leachate for all herbicides. Considering an efficiency of 98% of the biological oxidizer where determinations were performed, total recovery of 14C ranged from 103.16 to 108.69%, 98.15 to 109.86%, 100.86 to 107.44%, 92.52 to 106.74%, of the initial applied dose of ametryn, diuron, hexazinone, and metribuzin, respectively, for all soil types, which indicates appropriate mass recoveries for the methods used in our studies.

The interaction between the five soils (Clay-1, Clay-2, Loam-1, Loam-2, and Sand) and total leaching (herbicides found in six depths plus leachate) was assessed for diuron (F = 19.22; p<0.01), hexazinone (F = 23.09; p<0.01), and metribuzin (F = 4.02; p<0.01) (Figures 2,3, and4, respectively), but not for ametryn (F = 1.47; p>0.05) (Figure 1).

Figure 1
Recovery percentage of ametryn applied in a glass column with different depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m) of Clay-1 (Oxisol Typic Hapludox), Clay-2 (Oxisol Typic Hapludox), Loam-1 (Nitosol Eutrophic), Loam-2 (Udult soil), and Sand (Typic Quartzipsaments), with simulated rainfall of 200 mm for 48 h in the soil depth and leachate.

Figure 2
Recovery percentage of diuron applied in a glass column with different depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m) of Clay-1 (Oxisol Typic Hapludox), Clay-2 (Oxisol Typic Hapludox), Loam-1 (Nitosol Eutrophic), Loam-2 (Udult soil), and Sand (Typic Quartzipsaments), with simulated rainfall of 200 mm for 48 h in the soil depth and leachate.

Figure 3
Recovery percentage of hexazinone applied in a glass column with different depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m) of Clay-1 (Oxisol Typic Hapludox), Clay-2 (Oxisol Typic Hapludox), Loam-1 (Nitosol Eutrophic), Loam-2 (Udult soil), and Sand (Typic Quartzipsaments), with simulated rainfall of 200 mm for 48 h in the soil depth and leachate.

Figure 4
Recovery percentage of metribuzin applied in a glass column with different depths (0-0.05, 0.05-0.10, 0.10-0.15, 0.15-0.20, 0.20-0.25, and 0.25-0.30 m) of Clay-1 (Oxisol Typic Hapludox), Clay-2 (Oxisol Typic Hapludox), Loam-1 (Nitosol Eutrophic), Loam-2 (Udult soil), and Sand (Typic Quartzipsaments), with simulated rainfall of 200 mm for 48 h in the soil depth and leachate.

Radioactivity of ametryn was reported only for superficial layers, and deeper layers (0.15-0.30 m) accounted for less than 0.2%; the highest percentage (>87%) of this herbicide was found on the topsoil (0-0.05 m) (Figure 1), indicating limited leaching of this herbicide in the soil profile. This hypothesis was conformed, based on the fact that no ametryn was found in the leachate. Corroborating the data, Vivian et al. (2007Vivian R, Queiroz MELR, Jakelaitis A, Guimarães AA, Reis MR, Carneiro PM, et al. Persistência e lixiviação de ametryn e trifloxysulfuron-sodium em solo cultivado com cana-de-açúcar. Planta Daninha. 2007;25(1):111-24.) reported that most ametryn remained in the layer from 0 to 0.10 m depth in a Typic Haplufalf soil (pH = 6.6, organic carbon (OC) = 0.69-0.92%, and clay = 29-39%). Paula et al. (2016Paula RT, Abreu AB, Queiroz ME, Neves AA, da Silva AA Leaching and persistence of ametryn and atrazine in red-yellow latosol. J Environ Sci Health Part B. 2016;51(2):90-95. ) also reported that, after the first rain simulation, 80% of the mass of applied ametryn remained at a depth of 0.05 m in a Red-Yellow Latosol (pH = 4.73, OC = 3.4%, and clay = 60%). Leaching of ametryn can be influenced by different physico-chemical soil properties; however, in our study, this influence was not found, probably because of the low number of study soils (n= 5). Leaching of ametryn was also influenced by rainfall intensity, OC content and pH of the Red-Yellow Ultisol (pH = 5.9, OC = 1.48%, and clay = 25%) and the Red-Yellow Latosol (pH = 4.9, OC = 0.98%, and clay = 44%) from pasture areas in Brazil (Andrade et al., 2010Andrade SRB, Silva AA, Lima CF, D’Antonino L, Queiroz MELR, França AC, et al. Lixiviação do ametryn em Argissolo Vermelho-Amarelo e Latossolo Vermelho-Amarelo, com diferentes valores de pH. Planta Daninha. 2010;28(3):655-63. ).

Ametryn is a very weak acid with 200 mg L-1 of water solubility at 20 oC, showing low leachability and dissociation coefficient (pKa) of 10.07 (PPDB, 2017Pesticide Properties Database - PPDB. Agriculture & Environment Research Unit (AERU). University of Hertfordshire; 2017. [accessed on: Jun. 2017]. Available in: Available in: http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm .
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), although ametryn was present in a high amount in molecular (neutral) form within the pH range of soils (4.45-5.93) in this study (Table 1). On the other hand, Paula et al. (2016Paula RT, Abreu AB, Queiroz ME, Neves AA, da Silva AA Leaching and persistence of ametryn and atrazine in red-yellow latosol. J Environ Sci Health Part B. 2016;51(2):90-95. ) reported that ametryn has a high leaching potential (1-7% detected in the leachate after 15 cm) in the long term because it is very persistent (over 60% of the herbicide remained in soil after 70 days of application). However, Mitchell et al. (2005Mitchell C, Brodie J, White I. Sediments, nutrients and pesticide residues in event flow conditions in streams of the Mackay Whitsunday region, Australia. Mar Pollut Bull. 2005;51:23-36.) identified ametryn waste (0.3 µg L-1) in river water in Australia, close to soils cultivated with sugarcane, probably carried with rainwater flow by runoff, which is usually inversely proportional to leaching.

Diuron was also concentrated on the topsoil (0-0.05 m), with more than 90% of the total amount of herbicided applied, but the highest diuron concentration was found in Sand soil (Figure 2). This fact can be justified by the higher OC content (2.0%) in Sand soil (Table 1). As well as ametryn, diuron also demonstrated less leaching (<2%) in the soil profile (0.05-0.30 m), and only about 0.3% of herbicide was reported in the leachate from all soils. This is similar to limited diuron leaching found in other studies (Caracciolo et al., 2005Caracciolo AB, Giuliano G, Grenni P, Guzzella L, Pozzoni F, Bottoni P, et al. Degradation and leaching of the herbicides metolachlor and diuron: a case study in an area of Northern Italy. Environ Pollut. 2005;134(3):525-34.; Guzzella et al., 2006Guzzella L, Capri E, Di Corcia A, Barra Caracciolo A, Giuliano G. Fate of diuron and linuron in a field lysimeter experiment. J Environ Qual. 2006;35(1):312-23.; Dores et al., 2013Dores EF, De Souza L, Villa RD, Pinto AA. Assessment of metolachlor and diuron leaching in a tropical soil using undisturbed soil columns under laboratory conditions. J Environ Sci Health Part B. 2013;48(2):114-21.). The behavior of diuron in soil can be attributed to low water solubility (35.6 mg L-1 at 20 oC) (PPDB, 2017Pesticide Properties Database - PPDB. Agriculture & Environment Research Unit (AERU). University of Hertfordshire; 2017. [accessed on: Jun. 2017]. Available in: Available in: http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm .
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) and to high sorption coefficient, Kd = 14.3 mL g-1 in Brazilian soils (Dores et al., 2013) of this herbicide. Mendes et al. (2016Mendes KF, Inoue MH, Goulart MO, Pimpinato RF, Tornisielo VL. Leaching of a mixture of hexazinone, sulfometuron-methyl, and diuron applied to soils of contrasting textures. Water Air Soil Pollut. 2016:227:1-9.) found that diuron was detected only in the superficial layer (0-0.05 m) of two soils, Alfisol - Paleudult (pH = 6.4, OC = 1.80%, and clay = 37.6%) and Ultisol - Typic Hapludalf (pH = 6.9, OC = 0.52%, and clay = 15.1%).

Several studies have indicated that diuron behavior is positively correlated with OC contents and cation exchange capacity (CEC) (Spurlock and Biggar, 1994Spurlock FC, Biggar JW. Thermodynamics of organic chemical partition in soils. Environ Sci Technol. 1994;28(6):996-1002.; Troiano, 2001Troiano J, Weaver D, Marade J, Spurlock F, Pepple M, Nordmark C, et al. Summary of well water sampling in California to detect pesticide residues resulting from nonpoint-source applications. J Environ Qual. 2001;30(2):448-59.); thus, soils with low OC content exhibit high leaching of this herbicide. When studying leaching in soil columns of diuron in an Oxisol Typic Hapludox (OC = 0.34-0.72% and clay = 24-30%) and Typic Quartzipsaments (OC = 0.20-0.30% and clay = 6-11%) representative of the Guarani Aquifer recharge areas, Matallo et al. (2003Matallo MB, Luchini LC, Gomes MAF, Spadotto CA, Cerdeira AL, Marin GC. Lixiviação dos herbicidas tebutiuron e diuron em colunas de solo. Pestic Rev Ecotoxicol Meio Amb. 2003;13:83-90.) concluded that diuron leached through the 0.50 m layer, and the OC content of these soils determined the leaching capacity of this herbicide.

On the other hand, Chen and Young (2008Chen W, Young TM. NDMA formation during chlorination and chloramination of aqueous diuron solutions. Environ Sci Technol. 2008;42(4):1072-77.) reported that diuron is a commonly used herbicide in California (USA) and it has been frequently detected in the drinking water of the state. They also found that diuron was probably carried with rainwater flow by runoff, as already described above. The study suggests that diuron may be a precursor to the formation of nitrosodimethylamine (NDMA). NDMA is a member of the N-nitrosamine family and a potentially strong carcinogen. The authors also report that increased concern about the occurrence of NDMA in drinking water has resulted from the reactions during chlorination or contamination with industrial effluents. In this study, the formation of diuron metabolites was not evaluated, but preventing this herbicide from going into the water sources is obviously of concern.

In general, radioactivity of hexazinone was found at different depths, with higher herbicide concentration (<39%) in the 0.15-0.25 m layer for all soils (Figure 3), indicating significant leaching of this herbicide in soil profile, as also reported by Mendes et al. (2016Mendes KF, Inoue MH, Goulart MO, Pimpinato RF, Tornisielo VL. Leaching of a mixture of hexazinone, sulfometuron-methyl, and diuron applied to soils of contrasting textures. Water Air Soil Pollut. 2016:227:1-9.) and Reis et al. (2017Reis FC, Tornisielo VL, Pimpinato RF, Martins BAB, Victória Filho R. Leaching of diuron, hexazinone, and sulfometuron-methyl applied alone and in mixture in soils with contrasting textures. J Agric Food Chem. 2017;65(13):2645-50.). Roy et al. (1989Roy DN, Konar SK, Charles DA, Feng JC, Prasad RP, Campbell RA. Determination of persistence, movement and degradation of hexazinone in selected Canadian boreal forest soils. J Agric Food Chem. 1989;37:443-47.), who studied the behavior of hexazinone in clay soils, also reported high herbicide concentrations in the first 0.15 m of soil (approximately 98%). Toniêto et al. (2016Toniêto TA, Pierri L, Tornisielo VL, Regitano JB. Fate of tebuthiuron and hexazinone in green-cane harvesting system. J Agric Food Chem. 2016;64(20):3960-6.) reported that about 42% of the applied hexazinone was retained in the first 0.025 m of the straw layer of sugarcane and ∽43% was leached (>0.30 m), with low sorption (Kd ≤ 1.74 mL g-1) in the clay Oxisol (pH = 4.3, OC = 2.3%, and clay = 59.3%). These authors also concluded that rainwater distribution is a crucial factor which dictates herbicide environmental fate because it is the vehicle by which the molecule is transferred from the straw to the soil in green-cane systems.

In this research, hexazinone leached <0.1% only for the Clay-2 and Loam-1 soils, following Clay-1 and Loam-2 (<3%), whch are not statistically different between soil types. However, higher leaching (∽32%) was found in Sand soil as compared to other soils (Figure 3). According to Nicholls (1988Nicholls PH. Factors influencing entry of pesticides into soil water. Pestic Sci. 1988;22:123-37.), leaching is higher in sandy soils than in silty or clay soils. The data also agree with those found by Reis et al. (2017Reis FC, Tornisielo VL, Pimpinato RF, Martins BAB, Victória Filho R. Leaching of diuron, hexazinone, and sulfometuron-methyl applied alone and in mixture in soils with contrasting textures. J Agric Food Chem. 2017;65(13):2645-50.), who found that averages for hexazinone detected in the leachate were 0.03 and 5.01% for the clay soil (Dark-Red Latosol) and the sandy soil (Typic Quartzipsamments), respectively.

According to PPDB (2017Pesticide Properties Database - PPDB. Agriculture & Environment Research Unit (AERU). University of Hertfordshire; 2017. [accessed on: Jun. 2017]. Available in: Available in: http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm .
http://sitem.herts.ac.uk/aeru/ppdb/en/in...
), hexazinone is a weakly basic herbicide with pKb = 2.2 and high solubility in water (33 g L-1 at 20 oC). The pH values of all soils used in this experiment are higher than this value (Table 1). This indicates that the molecules are predominantly in the molecular form and more available in the soil solution, hence they are more susceptible to leaching, which contributes to the increasing number of reports of contamination of surface water and groundwater sources (Queiroz et al., 2009Queiroz SCN, Ferracini VL, Gomes MAF, Rosa MA. Comportamento do herbicida hexazinona em área de recarga do aqüífero Guarani cultivada com cana-de-açúcar. Quím Nova. 2009;32(2):378-81.).

In the case of metribuzin, radioactivity was found at different depths, except at a depth of 0.25-0.30 m, and higher amounts (<75%) of herbicide were retained in the first 0.05 m layer in Clay-1 soil, and only <0.2% of metribuzin was detected in the leachate from all soils (Figure 4). Kim and Feagley (2002Kim JH, Feagley SE. Leaching of trifluralin, metolachlor, and metribuzin in a clay loam soil of Louisiana. J Environ Sci Health Part B. 2002;37(5):393-403.) reported that the portion of metribuzin within the top 0.0-0.15 m soil depth was 75.4% of the portion found at the top 0-0.60 m soil depth about 30 days after application; as such, metribuzin was also mainly found at the top 0.15 m soil depth because of its high solubility in water (11.65 g L-1 to 20 ºC), its weak sorption force (Kfoc = 37.92 mL g-1), its pKb of 0.99 (PPDB, 2017Pesticide Properties Database - PPDB. Agriculture & Environment Research Unit (AERU). University of Hertfordshire; 2017. [accessed on: Jun. 2017]. Available in: Available in: http://sitem.herts.ac.uk/aeru/ppdb/en/index.htm .
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), and its soil pH ranging from 4.45 to 5.93 (Table 1). Therefore, it is worth noting that the presence of OC in soils may also encourage increased sorption; hence, less leaching of metribuzin is found (López-Piñeiro et al., 2013López-Piñeiro A, Albarrán DP, Becerra AD, Sánchez-Llerena J. Sorption, leaching and persistence of metribuzin in Mediterranean soils amended with olive mill waste of different degrees of organic matter maturity. J Environ Manage. 2013;122:76-84.). Triazine herbicides, such as metribuzin, are strongly acid in nature and their leaching is influenced by variation in soil pH (Ladlie et al., 1976Ladlie JS, Meggitt WF, Penner D. Effect of soil pH on microbial degradation, adsorption, and mobility of metribuzin. Weed Sci. 1979;24(5):477-81.). However, in their studies, Savage (1976Savage KE. Adsorption and mobility of metribuzin in soil. Weed Sci. 1976;24(5):525-28.), Peter and Weber (1985Peter CJ, Weber JB. Adsorption, mobility and efficacy of metribuzin as influenced by soil properties. Weed Sci. 1985;33(6):868-73.) found that metribuzin leaching was also influenced by sand, clay and soil OC content.

Therefore, as previously described by Mendes et al. (2016Mendes KF, Inoue MH, Goulart MO, Pimpinato RF, Tornisielo VL. Leaching of a mixture of hexazinone, sulfometuron-methyl, and diuron applied to soils of contrasting textures. Water Air Soil Pollut. 2016:227:1-9.), recommendations for the use of herbicides for weed management in areas cultivated with sugarcane without prior knowledge of the physico-chemical properties of the soil can result in inefficient control of weeds and possible contamination of groundwater and surface water.

With these findings, the following conclusions can be inferred about the potential leaching of ametryn, diuron, hexazinone, and metribuzin in soils cultivated with sugarcane. Leaching percentage was always <0.3% for ametryn, diuron, and metribuzin; for hexazinone, this was also the case in two out of the five soils. Hexazinone proved to be a potential contaminant of groundwater; metribuzin presents high leaching in soil profile whereas ametryn and diuron have low leaching in the short term and high intensity rainfall because of physicochemical properties; thus, these herbicides can contaminate surface water by runoff. For a more complete understanding about the bioavailability of these herbicides when applied in different soils, sorption-desorption and persistence studies should be performed.

ACKNOWLEDGEMENTS

The authors would like to thank the São Paulo State Research Foundation (FAPESP) and the National Council for Scientific and Technological Development (CNPq) for their financial support.

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Publication Dates

  • Publication in this collection
    06 May 2019
  • Date of issue
    2019

History

  • Received
    17 June 2017
  • Accepted
    30 Oct 2017
Sociedade Brasileira da Ciência das Plantas Daninhas Departamento de Fitotecnia - DFT, Universidade Federal de Viçosa - UFV, 36570-000 - Viçosa-MG - Brasil, Tel./Fax::(+55 31) 3899-2611 - Viçosa - MG - Brazil
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