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Fate of tebuthiuron and hexazinone in green-cane harvesting system Thiago Antônio Pinheiro Toniêto, Letícia Pierri, Valdemar Luiz Tornisielo, and Jussara Borges Regitano J. Agric. Food Chem., Just Accepted Manuscript • DOI: 10.1021/acs.jafc.5b04665 • Publication Date (Web): 05 Jan 2016 Downloaded from http://pubs.acs.org on January 10, 2016

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Journal of Agricultural and Food Chemistry

Fate of tebuthiuron and hexazinone in green-cane harvesting system

Thiago Antônio Pinheiro Toniêto†, Letícia de Pierri†, Valdemar Luiz Tornisielo‡, Jussara Borges Regitano*† †

Department of Soil Science, University of São Paulo (USP/ESALQ), Av. Pádua Dias 11, 13418-260,

Piracicaba (SP), Brazil, Tel: +55 019 34172126, [email protected]; ‡ Laboratory of Ecotoxicology, University of São Paulo (USP/CENA), Av. Centenário 303, 13400-970, Piracicaba (SP), Brazil.

ACS Paragon Plus Environment

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ASTRACT

2

In Brazil, fire prior to sugarcane harvesting has to be phased out by 2017, but it was

3

already phased out in up to 85-90 % of the cropped area. This new system is called

4

green cane and entirely changed weed management practices. The main goal of this

5

study was to evaluate the effects of the straw presence as well as humic acid (HA),

6

formulation, soil type, and aging on the sorption and leaching of

7

hexazinone. Both herbicides presented low sorption for all treatments (Kd,app ≤ 3.25 L

8

kg-1), but it was higher for tebuthiuron in the clayer soil (LVd). Straw and aging only

9

slightly enhanced sorption. The HA effects were not clear. Sorption was mostly affected

10

by herbicide and soil type. Straw may promote physical trapping (∼31% of applied

11

amount) which cannot be accessed by "batch" sorption (∼15 % of the applied amount is

12

sorbed) attenuating leaching of highly mobile herbicides in green cane systems. In order

13

to properly access leaching through straw residues under laboratory condition, rainfall

14

distribution is very important.

15

Keywords

16

Straw, humic acid, fire, pesticides, mobility, groundwater pollution.


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C-tebuthiuron and

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INTRODUCTION

18

Sugarcane (Saccharum officinarum) became particularly important in Brazil

19

after the National Alcohol Program (PROALCOOL) creation in the 70’s. The goal was

20

to replace fossil fuels by ethanol on large scale. Currently (2013/2014), sugarcane

21

represents ∼2 % of Brazilian GDP and yield surpasses 680 million tons.1 However,

22

according to the new environmental protocol signed,2 the use of fire prior to harvesting

23

should be phased out by 2017 (this will be called “green-cane” harvesting system). It

24

should change weed control management practices that correspond to one of the major

25

costs involved on sugarcane production. As a matter of fact, the use of herbicides

26

increased abruptly in the last ten years. It matched with the beginning of the green-cane

27

likely due to lower herbicide transposition through the straw resulting in lower efficacy3

28

and natural selection of large seed weeds (e.g., Ipomoea spp.) having sufficient reserves

29

(energy) to emerge.4

30

The green-cane system provides about 5 to 20 Mg ha-1 year-1 of straw on the soil

31

top, which increases soil organic carbon content and agrochemicals interception, mainly

32

fertilizers and pesticides.4-7 A straw layer of 7 Mg ha-1 is capable to retain up to 200 L

33

of a herbicide spray in tropical systems8 and herbicide transposition decreases with

34

straw amounts.3,9,10 Moreover, sugarcane straw degrades slowly due to its high C/N

35

ratio, i.e. only ∼30 % of the original dry matter degraded 3 y after harvesting.11 The

36

straw ability to cover the soil for long periods may favor chemical or physical

37

entrapment of the herbicide decreasing its efficacy or degradation.5

38

Tebuthiuron (N-[5-(1,1-dimethylethyl)-1,3,4-thiadiazol-2-yl]-N,N’-dimethylurea) and

39

hexazinone [3-cyclohexyl-6-(dimethylamino)-1-methyl-1,3,5-triazine-2-4(1H,3H)dione] are


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pre-emergent herbicides heavily used for weed control in sugarcane (Table 1). Both of

41

them show high persistence (half-lives = 12-15 mo for tebuthiuron and up to 9 mo for

42

hexazinone), moderate to extremely high toxicity,12 low sorption (Kd = 0.002-3.6 L kg-1

43

for tebuthiuron and 0.07-1.65 L kg-1 for hexazinone),13-15 and high water solubility (Sw

44

= 2,500 mg L-1 for tebuthiuron and 33,000 mg L-1 for hexazinone), which point out

45

their high leaching potential in soils.16 Indeed, they have been frequently reported in

46

groundwaters.17-19

47

Pesticide interactions with soil particles enhance with aging.20-24 Aging refers to

48

pesticide residence time in the soil. Although not yet fully understood, it happens due to

49

enhancement in binding energy and diffusion over time5,22. These interactions may

50

become so strong that is no longer possible to extract part of them by traditional

51

methods (this fraction is called “non-extractable”).25 Thus, aging before the first rain

52

may considerably decrease leaching to groundwater,5,20 mainly for those herbicides

53

with high solubility and low sorption, such as tebuthiuron and hexazinone. In other

54

words, herbicide leaching is usually overestimated when aging is disregarded.21

55

Humic acids (HA) have been applied to agricultural areas to enhance soil

56

environmental quality and crop yieds.26,27 However, HA effect on the environmental

57

behavior of pesticides are not yet fully elucidated. HA functional groups may interact

58

with pesticides serving as sorption sites thus reducing their mobility;28-31 but they may

59

also act as co-solvent28 or hydrophobic sites for partitioning in the liquid phase thus

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enhancing their mobility28,32, mainly in soils with high permeability. In addition, HA

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may compete with pesticides for sorption sites in the soils,31,32 which also enhance

62

leaching. To the best of our knowledge, there are no reports regarding HA effects on the

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environmental fate of herbicides in green-cane harvesting systems.


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Therefore, the main goals of this work were to evaluate the effects of sugarcane

65

straw as well as of HA, formulation, and aging on the sorption and leaching of

66

tebuthiuron and hexazinone in contrasting tropical soils under laboratory conditions.

67

The presence of the straw is critical for weed control in the new green cane harvesting

68

system adopted in Brazil.

69 70

MATERIALS AND METHODS

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The surveys were conducted at the Center for Nuclear Energy in Agriculture

72

(CENA-USP), Piracicaba-SP, Brazil, using sorption33 and leaching34 protocols with

73

small modifications.

74 75

Chemicals, soils and treatments

76

Analytical standards and radiolabeled forms (14C-UL) of tebuthiuron (specific

77

activity = 3.01 MBq mg-1, purity > 98%) and hexazinone (specific activity = 3.14 MBq

78

mg-1, purity > 98%) were adopted.

79

Soil samples of a sandy clay loam and a clay Oxisols (LVA and LVd according

80

to Brazilian Soil Classification System, respectively)35 (coordinates 22° 40’20'' S and

81

47° 37’ 31” W, respectively) were selected because they are very representative of the

82

sugarcane areas in São Paulo state, Brazil, and present contrasting texture. The soil

83

samples were collected at 0-20 cm layer, air-dried, sieved in 2-mm mesh, and

84

conditioned at room temperature. Particle size was measured by the hydrometer method

85

and chemical attributes according to Raij et al.36 Briefly, pH was determined in

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CaCl2 0.01 mol L-1 (soil:solution ratio = 1:2.5); calcium, magnesium and potassium

87

were extracted by an ion exchange resin and Ca2+ and Mg2+determined by atomic

88

absorption spectrophotometry and K+ by flame photometry; potential acidity (H + Al)


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was determined by pH-SMP; and total soil organic carbon by combustion in an

90

elementary autoanalyzer.

91

Sugarcane straw samples (variety SP 90-2032) were collected after third ratoon

92

harvesting, oven dried at 65 °C, and cut into 2.5-cm pieces. HA source and application

93

rate followed manufacturer’s recommendations (150 L ha-1). Soils, straw, and HA

94

attributes are presented on table 2.

95 96

Sorption experiment

97

The treatments comprehended two straw rates (0 and 10 Mg ha-1), two HA rates

98

(0 and 150 L ha-1), two herbicides (tebuthiuron and hexazinone), two incubation periods

99

(0 and 21 d), two soils (LVA and LVd), and two formulations (with commercial

100

formulation (CF) and without (AI = active ingredient only)), which were completely

101

randomized and arranged in a factorial design, in duplicate.

102

About 60 mg of sugarcane straw (= 10 Mg ha-1 straw, assuming bulk density =

103

1.2 g cm-3 and soil depth = 10 cm) were added to 5 g soil (air dried) in centrifuge tubes

104

(Teflon, 50 mL) and soil moisture adjusted to 75 % of the field capacity. For time zero

105

(t0), 10-ml aliquots of 0.005 mol L-1 CaCl2 solution were added to the tubes immediately

106

after herbicides application to attain maximum recommended field rates (tebuthiuron =

107

1000 g a.i. ha-1 and hexazinone = 500 g a.i. ha-1, both with radioactive concentration ∼

108

135 Bq mL-1), which were weighed, shaken horizontally (160 rpm) for 24 h, and

109

centrifuged (830 g for 15 min). Then, 1-mL aliquots of the supernatants were analyzed

110

by liquid scintillation spectrometry (LSS) to determine soil-solution equilibrium

111

concentration (Ce). For 21 d (t21), the tubes were placed in a semi-dark room at 25 ± 2°C

112

and the soil moisture maintained by gravimetry. Then, the previous procedure was

113

repeated to evaluate sorption. This period was adopted because most pesticide reaction


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114

with soil particles happens within this period, although sugarcane cycle comprehends

115

12-15 mo.

116

Then, the supernatants were discarded and the tubes placed in a ventilation oven

117

(40°C, 48 h). After drying, a few tubes were weighed and the slurries removed,

118

macerated, and oxidized (0.20 g) in a biological oxidizer (900°C for 3 min), in

119

triplicates, to determine herbicides sorbed concentration to the soil (S) and mass

120

balance. The 14CO2 released was captured by the monoethanolamine solution and the

121

radioactivity determined by LSS. The recovery ranged from 90 to 110 % of the applied

122

radioactivity, which allowed calculating S by the difference between initial (Ci) and

123

equilibrium (Ce) concentrations. Afterwards, the apparent sorption distribution

124

coefficients (Kd,app= S / Ce) were calculated for a single concentration.24 It is reasonable

125

since our previous work showed that initial concentrations for both herbicides were

126

within linear isotherm range and therefore allow comparing soil’s sorption.37

127 128

Leaching experiment

129

This experiment was completely randomized in a factorial design, in triplicates,

130

comprehending two HA rates (0 and 150 L ha-1), two herbicides (tebuthiuron and

131

hexazinone), and two herbicide formulations (AI and CF), but just in the sandier soil

132

(LVA) in order to represent the worst case scenario, totalizing 24 columns.

133

Glass columns (length = 30 cm, diameter of inner conical sphere = 5 cm) were

134

used for soil packaging. Fiberglass and sterilized sand with HCl were added to the

135

column bottoms to serve as a support. Soil packaging was manually performed (height

136

= 5 cm and soil density ∼ 1.78 g cm-3) and then aliquots of 11-g of straw were added to

137

the topsoils (= 16 Mg ha-1 of straw (82 kg m-3) determined based on field

138

measurements). This straw mass was subdivided into three even layers of about 2.5-cm


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139

separated by fiber glasses (packing density = 0.08 g cm-3). At the very top, another thin

140

layer of fiber glass was added in order to promote homogeneous dispersion of the

141

aqueous solution and avoid surface disturbance. Afterwards, the soil columns were

142

saturated by capillarity with 0.005 mol L-1 CaCl2, the “aqueous solution” excess let

143

percolate for ∼ 2 h, and then the herbicides solutions applied to attain the maximum

144

recommended field rates (radioactivity ∼ 30 Bq g-1 of soil). The columns were placed in

145

a semi-dark room at 25 ± 2oC and the rainfall was simulated in two stages of 50 mm

146

with 0.005 mol L-1 CaCl2 solution (6 and 30 h after herbicide application), applied in

147

continuous flow for 2 h with the aid of a peristaltic pump. Since rainfall distribution

148

affects leaching, it was simulated using one- (flow rate = 0.8 mL min-1 channel-1) and

149

four-drop (flow rate = 0.2 mL min-1 channel-1) channels per column, always totalizing

150

100 mm of rain.

151

Leachates were collected immediately after each rain simulation, and 10-mL

152

aliquots were analyzed by LSS. Further, straw and soil samples were removed from the

153

columns using pressurized air and placed into aluminum containers to air dry. These

154

materials were further ground and homogeneously mixed. Subsequently, subsamples of

155

0.10-g of straw and 0.20-g of soil were oxidized in a biological oxidizer (900 °C for 3

156

min), in triplicate, and radioactivity determined by LSS. Recovery ranged from 89 and

157

117 %.

158 159

Statistical analysis

160

All experimental data were submitted to variance analysis within each herbicide

161

and soil type. When interactions were observed, the factors were outspread and their

162

significance compared by Tukey test (p ≤ 0.05).

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RESULTS AND DISCUSSION

165 166

Sorption of tebuthiuron and hexazinone

167

Apart of the treatment, tebuthiuron and hexazinone showed low sorption in all

168

soils (Kd,app ≤ 3.25 and 1.76 L kg-1, respectively) (Table 3), which is consistent with

169

previous studies.15,37,38 The greater sorption of tebuthiuron compared to hexazinone was

170

expected due to intrinsic molecule properties, such as its lower water solubility.

171

Moreover, sorption was considerably higher in the clay soil (LVd, Table 3).

172

Across all treatments, 24 and 14% of the applied tebuthiuron and hexazinone were

173

sorbed to the LVA whereas 56 and 41 % of them were sorbed to the LVd, respectively.

174

In other words, the LVd presented Kd,app even 16 times higher than the LVA. It is well

175

known that clay soils have the ability to accumulate more carbon in relation to sandy

176

soils39 since clay promotes physical protection and chemical stabilization of SOC,40

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especially in tropical soils, as corroborated by its higher initial SOC (Table 2). It

178

contributes to higher soil CEC and buffer capacity. In addition, clayer tropical soils

179

have considerable amounts of Fe and Al oxides and hydroxides that may work as

180

sorption sites for these molecules. Our previous work has already shown that organic

181

carbon content was the soil attribute that better correlated with these herbicides

182

sorption, but the addition of Fe oxide contents helped in their sorption prediction.37

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These should favor hidrophobic partitioning and sorption processes.24

184

Straw addition only slightly enhanced sorption of both herbicides, but the

185

increase was more abrupt for tebuthiuron in the sandier soil (LVA) when freshly applied

186

(Table 3). Across all treatments, sorption in this soil increased from 15 to 24% for

187

tebuthiuron and from 9 to 12% for hexazinone when freshly applied. On the other side,

188

it increase from 28 to 30% for tebuthiuron, but slightly decreased from 18 to 17 % for


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hexazinone when aged for 21 d (Table 3). In the LVd, the influence of straw addition

190

was most noticeable for hexazinone when freshly applied. In this case, sorption

191

enhanced from 36 to 42% in the presence of the straw whereas it enhanced only from 42

192

to 44 % when aged. For tebuthiuron, independent of the residence time, sorption

193

enhanced by 2% (from 55 to 57%), on average, in the presence of the straw.

194

Although literature reports increase in pesticides sorption with HA addition,29-31

195

that was not the case here. HA addition did not appear to significantly influence these

196

herbicides sorption. It only slightly reduced tebuthiuron sorption (Kd,app) when freshly

197

applied to the LVd (Table 3). In this case, sorption reduced from 55 to 52% of the

198

applied amount (Table 3). Sorption reversibility due to soluble organic compounds

199

addition to soils is observed primarily when molecule sorption is low, but also depends

200

on herbicide properties.41 In other words, interactions between pesticides and soluble

201

organic materials are conditioned by hydrophobicity/hydrophilicity properties of

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both,32,42 but may also involve specific or non-specific sorption mechanisms. The high

203

solubilities and moderately polar structures of these herbicides and HA co-solvent

204

properties and competition for the sorption sites31-32 should favor desorption and explain

205

part of our results, especially when the herbicide is freshly applied, although

206

competition for sorption sites was proved for compounds different than ours.

207

Aging enhanced sorption of both herbicides in all treatments and soil types.

208

Overall, sorption enhanced from 20 to 29 and from 53 to 58% for tebuthiuron in the

209

LVA and LVd, respectively, and from 10 to 18 and from 39 to 43% for hexazinone,

210

respectively. It is clear that the aging effects were most significant for the herbicide with

211

lower sorption (hexazinone) in the soil with lower buffer capacity (LVA). However, to

212

better predict pesticides fate in soils, the residence time must be considered.20,24

213

Moreover, in the absence of sugarcane straw, aged Kd,app values increased mostly after


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HA addition (Table 3), which may preclude leaching of highly soluble herbicides,

215

particularly hexazinone. This result was most prominent for commercial formulation,

216

mainly in the LVA, likely due to the presence of certain vehicles on its formulation that

217

facilitate diffusion to less accessible sorption sites and increase herbicide sorption over

218

time. 22

219

Regardless of treatment and soil type, it is possible to conclude that tebuthiuron

220

and hexazinone always showed high leaching potential (Kd,app < 5.0 L kg-1) (Table 3).

221

However, the methodology adopted (“batch” experiment) disregards the physical barrier

222

created by the straw,24 which may overestimate pesticide mobility in green-cane

223

systems. Considering a completely different molecule and another substrate, it was

224

observed ∼50% reduction in simazine leaching by the presence of wheat straw on the

225

soil surface.43 The authors were unable to identify if leaching reduction was held by

226

herbicide sorption or by its interception to the straw, or both. Our results suggest that it

227

was likely due to wheat straw interception.

228 229

Leaching of hexazinone and tebuthiuron

230

According to the literature, the adoption of one drop channel at the center of the

231

soil columns to simulate rainfall events, as recommended by OECD protocol (for soils),

232

underestimated leaching of both herbicides, most likely due to lack of rainfall

233

interception. Across all treatments, in this case, ∼57 and 42 % of the applied amounts of

234

tebuthiuron and hexazinone were retained in the first 2.5 cm straw layer and ∼28 and 43

235

% were leached, respectively (Figure 1). Rainwater distribution is a crucial factor

236

dictating pesticides environmental fate since it is the vehicle by which the molecule is

237

transferred from the straw to the soil in green-cane systems.3 The intensity and shape of

238

the rain and the aging effects directly influence molecules transfer into soil and, hence,


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239

its efficacy and fate.22,44,45 Furthermore, the ligno-cellulosic nature of the sugarcane

240

straw and its lack of capillarity (differently from soils) favor water preferential flow at

241

the center of the column,3 avoiding herbicide contact with water and, therefore, its

242

subsequent percolation.24 In addition, water into contact with lignin induces stress in the

243

glassy core of the polymer and, at the same time, promotes expansion in rubbery region,

244

resulting in a pressure between the lignin subunits and, consequently, cracking its

245

structure.46 These cracks eventually may also facilitate water preferential flow itself.3

246

Therefore, leaching experiments were repeated dividing rain simulation into four

247

dropping channels per column in order to reproduce rainfall distribution more

248

realistically (Figure 2). At this scenario, also across all treatments, ∼ 33 and 31 % of

249

tebuthiuron and hexazinone applied amounts were retained in the first 2.5 cm straw

250

layer, respectively, and ∼ 44 % were equally leached for both molecules (Figure 2).

251

However, ∼ 19 and 24 % of the applied tebuthiuron and hexazinone reached the soil,

252

respectively. In the previous approach (one drop), only ∼ 9 and 6 % of the applied

253

amounts reached the soil, respectively. Although tebuthiuron presented higher sorption,

254

it showed similar soil profile distribution to hexazinone when rainfall was better

255

distributed (four drops) (Figure 2). We may not forget that tebuthiuron sorption was

256

higher, but still very low mainly in the LVA. Even after 100 mm of rain, ∼ 40 % of both

257

applied herbicides remained intercepted in the straw, which differs from sorption

258

expectations, mainly for hexazinone (Figure 2). Therefore, the straw left on soil surface

259

in green-cane harvesting systems seems advantageous from the environmental point of

260

view since it attenuates groundwater contamination risks by highly mobile herbicides,

261

such as tebuthiuron and hexazinone.47 These results were in disagreement with those

262

from sorption suggesting that straw presence did not considerably affect our herbicides

263

mobility.


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264

HA application appeared to have increased hexazinone retention to the straw

265

and, as a result, decreased its leaching apart of the formulation (although differences

266

were not statistically significant). Hexazinone straw retention was enhanced by about

267

11-12 % in the presence of HA (it changed from 35 to 47 % and from 29 to 40 % of the

268

applied amount for the AI and CF, respectively) (Figure 2). For tebuthiuron, it seems

269

that the presence of HA may have increased AI solubility and; therefore, decreased

270

straw retention (from 49 to 36 %) and increased its leaching (from 37 to 50 %), but the

271

same was not observed for the CF (Figure 2). Tebuthiuron results were in accordance to

272

those of the sorption study (Kd,app was lower in the presence of HA when tebuthiuron

273

was freshly applied) and agreed with those attained by other authors.28,32 Nevertheless,

274

such contrasting results do not allow drawing a final conclusion regarding the

275

treatments employed. Therefore, many times here the data was treated as an average

276

value across all treatments.

277

We could conclude that sugarcane straw only slightly enhanced tebuthiuron and

278

hexazinone sorption, which was always low apart of chemical formulation, HA

279

application, pesticide aging, and soil type. HA application and CF seems to have little

280

influence on sorption of highly soluble herbicides, such as ours. Aging enhanced

281

sorption mainly for hexazinone in the soil sandier soil (LVA), but soil type was the

282

parameter that mostly affected sorption. In order to properly access leaching through

283

straw residues under laboratory condition, rainfall distribution is very important. One

284

drop channel to simulate rainfall event, as proposed by OECD Guideline for soils,

285

underestimated leaching. The presence of sugarcane straw worked as a physical barrier

286

for herbicides trapping, capable to retain ∼ 30 % of the applied herbicides, thus either

287

avoiding or retarding leaching. Although tebuthiuron presented higher sorption,

288

leaching amounts were similar to those for hexazinone. Considering that we worked


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289

with the worst case scenario regarding leaching purposes, i.e. a sandy soil with low OC

290

content, heavy rain, very mobile herbicides, and no aging, these results would be

291

expected.

292 293

Aknowledgements

294 295

The authors thank FAPESP for the research financial support (Process Number:

296

2012/15843-0); CNPq for granting the first author a Master’s degree scholarship;

297

Fabrício G. Giore for reviewing the project ideas; and Carlos Alberto Dorelli and

298

Rodrigo Pompinato for their technical support.


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Page 15 of 27


LITERATURE CITED (1)

IBGE – Brazilian Institute of Geography and Statistics. Systematic Survey of

Agricultural

Production.

2015.

Rio

de

Janeiro,

29,

1-76.

ftp://ftp.ibge.gov.br/Producao_Agricola/Levantamento_Sistematico_da_Producao_Agri cola_[mensal]/Fasciculo/lspa_201505.pdf. (accessed Jul 01, 2015). (2)

UNICA - Industry Union of Sugarcane. Institutional folder. 2011. São Paulo.

http://www.unica.com.br/content/show.asp?cntCode={BEE1 D0D54264B1B37E0C7D4031D6}>

http://www.unica.com.br/

06FFuserFiles/

Protocolo_Assinado_Agro ambiental.pdf. (accessed Nov 14, 2013.) (3)

Maciel, C.D.G.; Velini, E.D. Simulação do caminhamento da água da chuva e

herbicidas em palhas utilizadas em sistemas de plantio direto. Planta Daninha. 2005, 23, 471- 481. (4)

Correia, N.M.; Durigan, J.C. Emergência de plantas daninhas em solo coberto

com palha de cana-de-açúcar. Planta Daninha. 2004, 22, 11-17. (5)

Locke, M.A.; Bryson, C.T. Herbicide-soil interactions in reduced tillage and

plant residue management systems. Weed Sci. 1997, 45, 307-320. (6)

Ochsner, T.E.; Stephens, B.M.; Koskinen, W.C.; Kookana, R.S. Sorption of a

hydrophilic pesticide: Effects of soil water content. Soil Sci. Soc. Am. J. 2006, 70, 1991–1997. (7)

Czycza, R.V. Quantidade e qualidade da matéria orgânica do solo em sistemas

de colheita com e sem queima da cana-de-açúcar (Quantity and quality of soil organic matter in crop systems with and without burning of sugarcane). Master thesis, University of São Paulo, 2009. (8)

Rodrigues, B.N. Influência da cobertura morta no comportamento dos herbicidas

imazaquin e clomazone. Planta Daninha. 1993, 11, 21-28.


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Schmitz, G.L.; Witt, W.W.; Mueller, T.C. The effect of wheat (Triticum

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FIGURE CAPTIONS Figure 1 – Percentages of tebuthiuron and hexazinone leached after 100-mm rainfall simulation with one-drop channel per column. Bars represent the significant difference by Tukey test (p ≤ 0.05, n = 3).

Figure 2 – Percentages of tebuthiuron and hexazinone leached after 100-mm rainfall simulation with four-drop channels per column. Bars represent the significant difference by Tukey test (p ≤ 0.05, n = 3).


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TABLES Table 1. Structural formulas and chemical classifications of tebuthiuron and hexazinone. Attributes

Tebuthiuron

Hexazinone

C9H16N4OS Substituted urea

C12H20N4O2 Triazine

Structural formula Molecular formula Chemical family

Table 2. Soils, straw, and humic acid’s chemical and physical attributes. ----------- Soil atributes ----------a b LVA LVd c pH 4.5 4.3 d

SOC

-3

g dm

-3

P

mg dm mmolc dm-3

+

K

2+

Ca

2+

Mg Al

3+

H + Al SB CEC

Sand

23

3

10

0.7

-1

g kg

1.3

5

37

2

7

3.0

6.6

11

f

g

8

45

18.9

124 284

Silt

18

123

Clay

226

593

a c

a

pH

C/N ratio CEC

5.9

-

34/1 -1

mmolc kg

density e

WHC

-

g dm ³ %

350 0.08 215

---------- Humic acid attributes ----------

79

8.2 756

------------- Straw attributes -------------

pH C/N ratio

7.2

-

2/1 -1

density

g mL

total C

-1

gL

1.09 14

b

LVA = sandy clay loam Oxisol; LVd = clay Oxisol pH CaCl2 0.01 mol L-1; dsoil organic carbon; ewater holding capacity; fsum of bases; fcation exchange capacity.


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Table 3. Effects of soil type (LVA and LVd), sugarcane straw (0 and 10 Mg ha-1), formulation (AI and CF), humic acid (0 and 150 L ha-1), and aging (0 and 21 d) on the apparent sorption distribution coefficients (Kd,app) of tebuthiuron and hexazinone. Values inside parenthesis refer to sorbed herbicide amounts (% of applied). Values followed by same uppercase in the columns and lowercase in the rows do not differ statistically (Tukey test, p ≤ 0.05). Treatments formulation

straw (Mg ha-1)

CF CF + HA AI AI + HA

0 10 0 10 0 10 0 10

a

without straw with straw c without HA d with HA e aging b

CF CF + HA AI AI + HA a

without straw with straw c without HA d with HA e aging b

0 10 0 10 0 10 0 10

Tebuthiuron 0d

Hexazinone

21 d

Mean 0d 21 d Mean -1 Kd,app (L kg ) -------------------------------------- LVA --------------------------------------

0.36 (15) 0.79 (28) 0.09 (4) 0.34 (15) 0.56 (22) 0.93 (32) 0.18 (8) 0.42 (17) 0.61 (23) B 0.37 (14) 0.24 (11) 0.61 (23) 0.18 (8) 1.00 (30) 0.62 (23) 0.78 (28) 0.34 (15) 0.38 (16) 0.44 (18) 0.83 (29) 0.28 (12) 0.32 (14) 0.66 (25) 0.88 (31) 0.72 (26) A 0.32 (14) 0.46 (19) 0.33 (14) 0.83 (29) 0.22 (10) 0.35 (15) 0.43 (18) 0.77 (28) 0.92 (31) 0.26 (11) 0.41 (17) 0.37 (15) Bb 0.76 (28) Ba 0.57 (22) 0.19 (9) 0.50 (18) 0.35 (13) 0.27 (12) 0.42 (17) 0.35 (14) 0.65 (24) Ab 0.88 (30) Aa 0.77 (27) 0.51 (20) 0.86 (30) 0.68 (25) 0.22 (10) 0.39 (16) 0.30 (13) 0.52 (20) 0.79 (28) 0.65 (24) 0.25 (11) 0.34 (19) 0.29 (15) 0.51 (20) 0.82 (29) 0.66 (25) 0.23 (10) b 0.46 (18) a 0.34 (14) -------------------------------------- LVd -------------------------------------2.24 (52) 2.84 (58) 1.13 (35) 1.76 (46) 2.62 (56) 2.87 (58) 2.54 (55) B 1.69 (45) 1.61 (44) 1.45 (41) 1.95 (48) 2.76 (57) 1.13 (35) 1.22 (35) 2.27 (52) 2.79 (57) 1.35 (39) 1.74 (46) 2.52 (55) 2.92 (58) 1.23 (37) 1.54 (43) 2.58 (55) 3.17 (60) 2.78 (57) A 1.41 (41) 1.64 (44) 1.49 (42) 2.20 (51) 2.99 (59) 1.20 (37) 1.72 (45) 2.58 (56) 3.25 (61) 1.65 (44) 1.55 (43) 2.23 (52) 2.88 (58) 2.56 (55) B 1.18 (36) 1.56 (42) 1.37 (39) B 2.51 (55) 3.02 (59) 2.77 (57) A 1.52 (42) 1.64 (44) 1.58 (43) A 1.37 (40) 1.64 (44) 1.50 (42) 2.49 (55) Ab 2.95 (59) Aa 2.72 (57) 2.25 (52) Bb 2.95 (59) Aa 2.60 (56) 1.33 (39) 1.56 (42) 1.44 (40) 2.37 (53) 2.95 (58) 2.66 (56) 1.35 (39) b 1.60 (43) a 1.47 (41)

a

mean values for all treatments without straw addition (0 Mg ha-1); mean values for all treatments with straw addition (10 Mg ha-1); c mean values for all treatments without HA application; d mean values for all treatments with HA application; e mean values for all treatments for each incubation time (0 and 21 d). b


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FIGURES (Figure 1)


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FIGURES (Figure 2)


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TABLE OF CONTENTS GRAPHIC


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The authors declare no competing financial interest in publishing this paper.


27

Fate of Tebuthiuron and Hexazinone in Green ... - ACS Publications

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