Mineralization of 14C-atrazine in an entic haplustoll as affected by selected winter weed control strategies

Soil & Tillage Research 96 (2007) 234–242 www.elsevier.com/locate/still

Mineralization of 14C-atrazine in an entic haplustoll as affected by selected winter weed control strategies S. Hang a,*, S. Houot b, E. Barriuso b a

b

University of Co´rdoba, CC 509, 5000 Co´rdoba, Argentina National Institute of Agronomical Research, Environment and Arable Crops, BP 01, 78850 Thiverval-Grignon, France Received 17 March 2006; received in revised form 14 May 2007; accepted 6 June 2007

Abstract The effect of winter weed control (WWC) management on 14C-atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4diamine) mineralization was investigated in an Entic Haplustoll in Argentina. Three WWC managements were selected: Chemical Fallow (CF) and Cereal Cover Crop (CCC), both under no-tillage, and Reduced Tillage (RT) with chisel and moldboard plow. Soil was sampled at two depths: 0–5 and 5–10 cm, to evaluate the soil stratification induced by the tillage system. To distinguish differences in atrazine degradation in soils with and without previous history of atrazine application two crop sequences were selected: continuous soybean [Glycine max L., Merr.] (CS) without previous atrazine exposure, and soybean–maize (Zea mays L.) rotation (SM) with atrazine application every winter and in alternate springs. The release of 14C-CO2 during laboratory incubations of soils treated with ring labelled 14C-atrazine was determined. Soil organic matter (SOM) distribution was determined with depth and among three soil size fractions: 200–2000 mm, 50–200 mm and <50 mm. Previous atrazine application enhanced atrazine degrading microorganims. Atrazine mineralization was influenced by both WWC management and the tillage system. Chemical fallow showed the highest atrazine mineralization in the two crop sequences. Depth stratification in atrazine degradation was observed in the two WWC treatments under the no-tillage. Depth stratification in the content of soil organic C and relative accumulation of organic C in coarsest fractions (200–2000 and 50–200 mm) were observed mainly in no-till systems. Depth stratification of atrazine degrading activity was mainly correlated to the stratification of fresh organic matter associated with the coarsest fractions (200–2000 mm). Atrazine persistence in soil is strongly affected by soil use and management, which can lead to safe atrazine use through selection of appropriate agricultural practices. # 2007 Elsevier B.V. All rights reserved. Keywords: Crop sequence; Non-tillage; Reduced tillage; Soil organic matter stratification; Herbicides

1. Introduction Atrazine (6-chloro-N2-ethyl-N4-isopropyl-1,3,5-triazine-2,4-diamine) is an herbicide widely used in Argentina in crops such as sorghum (Sorghum bicolour) or maize and also during fallow between crops. In spite of the resistance developed by some weeds, the low efficacy

* Corresponding author. E-mail address: [email protected] (S. Hang). 0167-1987/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.still.2007.06.004

observed during atrazine control could be caused by its accelerated mineralization in soils. It has been proven that previous atrazine applications stimulate the development of a soil microbial community capable of using atrazine as a source of C, N, and energy (Barriuso and Houot, 1996). Atrazine mineralization can be 50% in periods lower than 28 d during laboratory incubations (Hang et al., 2003). Secondary factors, such as pH, organic C content or cationic exchange capacity, may stimulate or reduce atrazine mineralization (Sparling et al., 1998; Yassir et al., 1999; Houot et al., 2000). Soil

S. Hang et al. / Soil & Tillage Research 96 (2007) 234–242

practices such as tillage system, crop sequence or intermediate cover crop may alter the efficacy of atrazine and its evolution in the soil. Winter weed control (WWC) is a matter of concern in agricultural production, since the selected methods should guarantee adequate water accumulation and adequate soil cover under non-tillage systems. Besides, they should include strategies to maintain weeds at the lowest populations possible (Young and Thorne, 2004). WWC can be carried out with tillage implements such as chisel and moldboard plow or with chemical fallow or cereal cover crops, which can be killed with herbicides just prior to planting of the primary crop. The quantity and diversity of SOM and the microbial community in the rhizosphere can be influenced by the type of vegetation developed in each WWC (Biederbeck et al., 2005). Chemical fallow may affect SOM with a heterogeneous composition of weeds typically having high turnover rate (Wardle et al., 1999), whereas a cover crop contributes with only one species. Fang et al. (2001) suggested that the upper soil receives different types of organic substances originating from roots, microbial activity and root exudates and may be selectively enriched with specific degraders. In addition, the set of agronomic practices such as crop rotation, cover crops and conservation tillage may modify a wide range of soil properties (Emmerling et al., 2001), including the quantity, quality, and distribution of SOM (Franzluebbers, 2002). These changes may affect the microbial activity in the first few centimetres of soil supplied with herbicide (Wardle et al., 1999). SOM is the primary edaphic property associated with atrazine behaviour in soil. Barriuso and Koskinen (1996) determined that the atrazine retention capacity of SOM increased with fresh or non-humified SOM. The aim of this work was to study the effect of WWC management on atrazine decomposition. Three WWC management systems were evaluated by means of laboratory incubations with 14C- atrazine. 14C-atrazine mineralization was used as an indicator of atrazine decomposition and the presence of specific microbial activity able to mineralize the triazinic ring. 2. Materials and methods 2.1. Soils and management systems This work was conducted at the Manfredi Experimental Station of the Instituto Nacional de Tecnologı´a Agropecuaria (INTA) (318490 S; 638360 W, altitude: 292 m), located 70 km south of Co´rdoba, Argentine. Annual average temperature of 16.8 8C and a mean annual precipitation of 736 mm. The study was carried

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out in a trial established in 1992, on an Entic Haplustoll (Soil Survey Staff, 1998) with a prior history of 7 years under grass. Two crop sequences were investigated: continuous soybean (Glycine max L., Merr.) (CS) and soybean–maize (Zea mays L.) (SM) rotation. These cropping sequences were factorally arranged with three WWC management systems: chemical fallow (CF), cereal cover crop (CCC), and reduced tillage (RT). With the CF treatment, weeds were controlled between the beginning of May and the middle of June with glyphosate (1.5 kg ha1) + metsulfuron methyl (3–4 g ha1). A second application of glyphosate was applied in CS as a preplant or early post-emergence application. In the SM rotation, the second herbicide application was glyphosate + atrazine (2.5 kg ha1 + 1 kg ha1). In the CCC treatment, oat (Avena sativa) was sown after soybean harvest and treated at the end of September with glyphosate (1.5 kg ha1) in CS, and glyphosate + atrazine (1.5 kg ha1 + 1 kg ha1) in SM. In the RT regime, tillage was carried out through the entire trial with a chisel (0–20 cm) followed by a moldboard plow 10 cm deep. Atrazine (1 kg ha1) was applied to the maize crop and glyphosate (1.5 kg ha1) to the soybean crop at the end of the fallow or post-emergence. Soil was sampled at 0–5 and 5–10 cm. The soil samples were air-dried and sieved through a 2-mm sieve. All samples were characterized by clay content determined by sedimentation, soil pH in water (soil/ water, 1:1), and soil organic C content by wet combustion (Table 1). All techniques used are described in Sparks (1996). Soil size fractionation was done by dispersion of soils in water (soil:water, 50 g:100 mL) with 24-h shaking in 250-mL centrifuge bottles with 20 glass balls (0.5 cm of diameter). The fractions 2000– 200 mm, 200–50 mm and <50 mm were recovered from the dispersed suspension by sieving and dried at 50 8C. Soil weight and organic carbon concentration in each fraction (OCf) were quantified (Table 1). 2.2. Atrazine behaviour in laboratory conditions Ring-U-labelled 14C-atrazine (radiopurity >98%, specific activity: 7.77  108 Bq mmol1) was purchased from Sigma. Isotopic dilution with unlabelled atrazine was done in water to give a final concentration of 11.2 mg L1 and 2.95  106 Bq L1 of radioactivity. Triplicate incubations were performed in hermetically closed glass jars. One millilitre of the 14C-atrazine solution was added to 10 g of dry soil. The soil water content was adjusted to 80% of the water holding capacity (WHC) (1.5 mL g1) with Milli-Q water

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Table 1 Main characteristics of the soils, tillage system and weed control management Clay (g kg1)

SOCa (g kg1)

OCfb (g kg1) 2000–200 mm

200-50 mm

<50 mm

6.8

10.0

1.6

0.12

0.22

1.14

5–10

6.5

8.2

1.2

0.05

0.10

1.23

0–5

7.0

6.2

1.9

0.20

0.19

1.31

5–10

6.2

10.8

1.4

0.05

0.11

1.17

0–5

6.4

12.0

1.2

0.05

0.16

0.99

5–10

5.4

12.8

1. 2

0.06

0.12

1.00

0–5

6.5

11.4

1.7

0.12

0.22

1.17

5–10

6.2

7.2

1.3

0.05

0.16

1.01

0–5

6.2

11.0

1.9

0.16

0.21

1.33

5–10

6.2

11.4

1.2

0.03

0.10

1.13

0–5

6.3

10.4

1.4

0.07

0.13

0.96

5–10

6.0

10.6

1.3

0.04

0.16

1.13

Crop Sequence

Tillage system

Weed control

Herbicide applications

Depth

Continuous soybean (CS)

No-tillage

Chemical Fallow (CF)

1st Glyphosate + metsulfuron methyl, 2nd Glyphosatec

0–5

Cereal Cover Crop (CCC) Avena sativa L. Chisel and Moldboard Soybean–maize rotation (SM)

No-tillage

Chisel and Moldboard

a b c

Reduced Tillage (RT) Chemical Fallow (CF)

Glyphosate

Glyphosate

1st Glyphosate + metsulfuron methyl, 2nd Glyphosate + atrazine

Cereal Cover Crop (CCC) Avena sativa L.

Glyphosate + atrazine

Reduced Tillage (RT)

Glyphosate (before soybean) Atrazine (before maize)

pH

SOC: soil organic carbon. OCf: organic carbon in each soil size fractions fraction (200–2000, 50–200 and <50 mm). 1st and 2nd applications during winter Chemical Fallow.

(Millipore) taking into account the atrazine solution. The final concentration was 1.7 mg g1of soil (approximately 0.9 kg a.i. atrazine ha1). The 14C-CO2 evolved during the incubation was trapped in 2 mL of 2 M NaOH. The vials containing the NaOH were sampled and replaced after 3, 7, 15, and 23 days of incubation. Atrazine mineralization data were obtained by measuring 14C-CO2 in the NaOH traps by scintillation counting using a Kontron Betamatic V liquid scintillation counter (Kontron Ins., St. Quentin en Yvelines, France) and Packard Ultima Gold XR scintillation cocktail.

For each crop sequence (CS and SM) a split-plot design with three replicates was used. The main plot factor was WWC management (CF, CCC, RT) and the subplot factor was depth (0–5 and 5–10 cm). The dependent variable was the cumulative atrazine mineralization (23 days) evaluated as % of 14C-CO2 of the 14C initially applied. The software used was Infostat (2003).

2.3. Data and statistical analysis

Kinetics of 14C-ring labelled atrazine mineralization are shown in Fig. 1 (a and b). Cumulative atrazine mineralization at 23 days of incubation is shown in Table 2, together with the mineralization rate (kmin). At the end of incubation, atrazine mineralization was 14.9– 67.5% of 14C applied in the three WWC at both depths for the SM rotation. Atrazine mineralization was only

Atrazine mineralization rate (kmin) was calculated using the first order equation Ct = C0(1ekt), where Ct is the concentration of atrazine at time t; C0, the initial atrazine concentration; k the mineralization rate and t, the time of measurement.

3. Results and discussion 3.1.

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C-atrazine mineralization

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Fig. 1. Release of 14C-CO2 during incubation of 14C-atrazine in an Entic Haplustoll soil, in two crop sequences: (a) rotation soybean–maize (SM) and (b) continuous soybean (CS), under three winter weed control (WWC) management schemes including Chemical Fallow (CF); Cereal Cover Crop (CCC) and Reduced Tillage (RT), each at two soil depth categories (0–5 and 5–10 cm). The standard deviations (error bars) are shown.

6% under CS, which had no previous exposure to atrazine but lower than 6% in all treatments of the CS system. The relation between the yields of atrazine mineralization in SM rotation and the corresponding in CS showed that the atrazine mineralization at the end of the incubation period was 7–79 times higher in soils from SM rotation which are treated annually with atrazine. Atrazine efficacy may have been affected by accelerated mineralization in winter fallow when N fertilizers were not applied, since applications of N fertilizers may decrease atrazine mineralization (Abdelhafid et al., 2000). The CF management had the highest atrazine mineralization in both crop sequences. The CF was the WWC management that received more atrazine during the year. The great number of atrazine degraders (Sparling et al., 1998) or their increased efficacy due to

successive years of atrazine use (Yassir et al., 1999) could explain the rapid atrazine mineralization observed in CF. In decreasing order, the mineralization rate (kmin) in the three WWC managements was CF > CCC > RT. This order was the same in both crop sequences at 0–5 cm depth. The same management order was found in the 5–10 cm depth under CS, but under SM rotation the order of atrazine mineralization rate was RT = CF > CCC. Fliebbach et al. (2000) considered that the soil microbial biomass and their activity were significantly increased by crop rotation compared to monoculture. In our study, only the RT management system was under strict monoculture. The CF and the CCC management systems had some plant diversification due to winter weeds and cover crops, respectively. Crop diversification

Table 2 Cumulative atrazine mineralization and mineralization rate (kmin) in two crop sequence and three weed control management kmin (days1)

R2

5.7 a a 3.1 b 1.2 c

0.0023 (0.00018)b 0.0012 (0.0001) 0.00043 (0.00004)

0.925 0.918 0.890

Chemical Fallow Cereal Cover Crop Reduce Tillage

2.6 b 1.2 c 0.54 c

0.0010 (0.00008) 0.00044 (0.00004) 0.00021 (0.00002)

0.916 0.910 0.850

0–5

Chemical Fallow Cereal Cover Crop Reduce Tillage

67.5 A 58.8 B 55.2 B

0.047 (0.004) 0.036 (0.003) 0.028 (0.003)

0.943 0.936 0.896

5–10

Chemical Fallow Cereal Cover Crop Reduce Tillage

37.2 C 14.9 D 42.4 C

0.017 (0.002) 0.0060 (0.0006) 0.019 (0.002)

0.905 0.902 0.902

Crop sequence

Depth (cm)

Winter weed control

Continuous soybean

0–5

Chemical Fallow Cereal Cover Crop Reduce Tillage

5–10

Soybean–maize

a b

Cumulative atrazine mineralization (% of 14C-initially applied)

Means followed by the same letter are not significantly different at the 1% level of probability. Values in parenthesis are standard deviations of the coefficients.

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could explain, in part, the higher atrazine mineralization found in the CCC and CF management systems in relation to the RT management. 3.2. Soil organic carbon stratification Soil mass distribution among the three soil size fractions and their corresponding concentration of organic C are shown in Fig. 2. With RT management, mass distribution and organic C concentration were similar between depths. This homogeneous depth distribution was caused by tillage implements that mixed soil. Under no-tillage systems (CF and CCC), there was a high concentration of organic C in the superficial depth, as a consequence of the accumulation of organic matter. In CF and CCC systems, independent of rotation, there were similar levels of organic C,

indicating that vegetation during winter (oat or weeds) contributed to the organic matter equally. Tebru¨gge and Du¨ring (1999) considered that crop rotation, intermediate and cover crops, and conservation tillage should be taken into account when comparisons between tillage systems are made. Highest organic C concentration was in the coarsest soil size fractions (200–2000 mm). In this soil size fraction significant differences were detected only between RT and CCC management systems ( p < 0.05). This points out that there were differences in SOM composition among the WWC managements, which were not detected in total soil organic C alone. Yakovchenko et al. (1998) suggested that a soil quality indicator could be related to the organic C of the fractions rather than to the total soil organic C. The stratification ratio for a soil property is defined as the

Fig. 2. Distribution of (a) soil mass and (b) C organic content in the three soil size fractions 2000–200, 200–50, and <50 mm, in two crop sequences: Continuous Soybean (CS) and Soybean–Maize (SM); under three winter weed control (WWC) management schemes including Chemical Fallow (CF), Cereal Cover Crop (CCC) and Reduced Tillage (RT) each at two soil depth categories (0–5 and 5–10 cm). The standard deviations (error bars) are shown.

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239

Fig. 3. Organic carbon stratification ratio (OC0–5 cm to OC5–10 cm) in whole soil and three soil size fractions (2000–200; 200–50; and <50 mm) in an Entic Haplustoll soil, in two crop sequences: Continuous Soybean (CS) and Soybean–Maize (SM); under three winter weed control (WWC) management schemes including Chemical Fallow (CF), Cereal Cover Crop (CCC) and Reduced Tillage (RT). The standard deviations (error bars) are shown. Mean analysis differences were done between the three WWC for the SOC and OC from each soil size fraction. Different letters indicate significant differences at 5% of probability.

ratio between property values in the soil surface and in a lower depth (Franzluebbers, 2002). Stratification ratios for whole soil organic C and organic C in the different soil size fractions were calculated using data from the two depths, 0–5 and 5–10 cm (Fig. 3). The stratification ratio of the humified organic C, contained in the fine fractions <50 mm, showed little variation between the different WWC management systems. Previous studies have indicated that this fraction contained the most stable SOM (Christensen, 2001). On the other hand, the total soil organic C and the organic C in coarsest fractions (200–2000 and 50– 200 mm) reflected the effects of the WWC management and of the crop sequence. While the stratification Table 3 Summary of ANOVA for

14

ratio for RT management was near 1, the stratification ratios for CF and CCC management were higher. The values of these stratification ratios remained always lower than those obtained by Franzluebbers (2002). This apparent disparity may be due to the different depths considered. 3.3. Effect of soil depth on atrazine mineralization In the three WWC management systems and in both crop sequences, atrazine mineralization was greater in the upper depth than in the lower depth. ANOVA for each crop sequence was performed separately to prevent the effect of atrazine application from masking other

C-atrazine mineralized at the end of incubations (14C-Atmin23d) SOC, and organic carbon in each soil size fractions

Fvalue Atrazine mineralization rate (% of initial 14C-atrazine)

Total soil organic C (g kg1 soil)

OCf-2000–200

mm

OCf-200-50 mm (g kg1 fraction)

OCf-<50 mm (g kg1 fraction)

Continuous soybean (soil without atrazine applications) WWC 158*** 50089*** Depth 146*** 82369*** WWC  depth 20*** 15409***

1608*** 53*** 534***

3168*** 1246*** 713***

1788*** 156*** 540***

Soybean–maize rotation (soil with atrazine applications) WWC 90*** ns Depth 866*** ns WWC  Depth 84*** ns

2003*** 9438*** 2112***

3538*** 3397*** 3344***

665*** 1036*** 1159***

***

p 0.001; ns: not significant; WWC, winter weed control; OCf-2000–200 mm, organic carbon in soil size fraction between 2000 and 200 mm; OCf-200– organic carbon in soil size fraction between 200 and 50 mm, OCf-<50 mm organic carbon in soil size fraction <50 mm.

50 mm,

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Fig. 4. Relation between OC coarsest (fraction 200–2000 mm) stratification ratio (OC0–5 cm to OC5–10 cm) and the stratification ratio of cumulative atrazine mineralization in an Entic Haplustoll soil, in two crop sequences: Continuous Soybean (CS) and Soybean–Maize (SM); under three winter weed control (WWC) management schemes including Chemical Fallow (CF), Cereal Cover Crop (CCC) and Reduced Tillage (RT).

differences due to WWC management or other soil properties (Table 3). The stratification ratio for atrazine mineralization rates was calculated in the same way as the organic C stratification. In the three WWC management systems, stratification ratio of atrazine mineralization was close to 2.4 in soil with low atrazine mineralization (CS system). On the other hand, stratification ratio varied with WWC management in soil with high atrazine mineralization capacity (SM rotation). Values were 1.3, 1.9 and 4.0 for the RT, CF and the CCC managements, respectively. The lowest ratio for the RT management was probably due to homogenization of the soil from mechanical tillage. The Table 4 Statistic parameters of correlation matrix between Crop Sequence

pH Clay Total organic C OC f-200-2000 mm b OC f –50-200 mm OC f-<50 mm f-200-2000 mmc f-50-200 mm d f < 50 mm e

14

differences in atrazine mineralization rates between depths, indicate that no-tillage systems not only caused stratification in various soil properties (Franzluebbers, 2002), they also caused stratification in atrazine mineralization capacity. In previous work, differences by tillage system on atrazine mineralization in 0–20 cm soil depths were not observed (Hang et al., 2003). Therefore, we hypothesized that due to the strong stratification of SOM under no-tillage systems, soil sampling should be done and atrazine behaviour studied according to the stratification pattern of SOM. Stratification ratio of atrazine mineralization was correlated with stratification ratio of organic C in the coarsest fraction (200–2000 mm) (Fig. 4). The coarsest fraction had a high capacity to form non-extractable atrazine residues (Barriuso and Koskinen, 1996; Hang et al., 2003). The results of this work indicate that SOM of the coarsest fraction would play a dual role on atrazine retention and on mineralization of the available atrazine. The differences in atrazine mineralization capacity observed in the lower depth in both, CF and CCC, could be related to the type of vegetation developed in each of them. While under CF management there was greater plant diversification due to the presence of weeds, under the CCC management only one species (Avena sativa) developed. Anderson and Coats (1995) considered that enhanced atrazine degradation could result from direct atrazine metabolism in plant roots or in the rhizosphere as mediated by plant enzymes or a constant supply of root exudates. On the other hand, Fang et al. (2001)

C-atrazine mineralized at the end of incubations (14C-Atmin

Continuous soybean (CS) (soils without atrazine)

23d)

and some soil properties

Soybean–maize rotation (SM) (soils with atrazine)

0–5 cma

5–10 cm a

0–5 cm a

5–10 cma

ns ns ns ns 0.95*** ns ns 0.96*** ns

0.87*** 0.97*** ns 0.80*** 0.72*** 0.84*** 0.91*** ns ns

0.77* 0.90*** ns 0.84 *** ns ns ns ns 0.84***

ns ns 0.97*** 0.92*** ns ns ns 0.99*** 0.98***

*** p 0.001; * p 0.05; ns: not significant. a Depth. b OCf organic carbon in three soil size fractions (200–2000; 50–200; <50 mm). c f-2000–200 mm organic carbon in soil size fraction between 2000 and 200 mm. d f-200–50 mm organic carbon in soil size fraction between 200 and 50 mm. e f-<50 mm organic carbon in soil size fraction <50 mm.

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suggested that exudates of grasses did not contain compounds structurally analogous to atrazine, which could induce the increase of atrazine mineralization. Wardle et al. (1999) found that microbial biomass was positively correlated with weed biomass and negatively correlated with crop plant biomass, and considered this effect due to the high decomposability of weed residues relative to those from crop plants. Atrazine mineralization was correlated with various soil properties (Table 4). The pH, clay, total soil organic C, and C concentration of the three soil size fractions were correlated with cumulative atrazine mineralization, although there were different correlations in the different crop sequences and depths. Difficulties to establish relations between soil properties and atrazine mineralization capacity have already been pointed out (Tyess et al., 1995). The interactions between edaphic properties and management of the soil would seem to be more relevant to atrazine behaviour than isolated edaphic properties. 4. Conclusions This study has elucidated not only the stimulating effect that repeated use of atrazine has on the development of a microbial atrazine-degrading community, but also the atrazine mineralization capacity as modified by weed control management and tillage system. At a depth of 0–5 cm, atrazine mineralization in no-tillage systems was 1.9–4 times higher than in 5–10 cm depth. These results suggest the importance to adapt soil sampling to the tillage system employed. Differences observed in atrazine mineralization capacity between the two WWC management systems under no-tillage system (CCC and CF) suggest there could be interactions between different types of plants and the activity of atrazine-degraders. Interactions between edaphic properties and soil management appeared to have greater influence on atrazine behaviour than isolated soil properties. Atrazine persistence in soils is strongly affected by soil use and management, which can lead to safe atrazine use through selection of appropriate agricultural practices. Acknowledgments This work was granted by the ‘‘Program of Cooperation Franco-Argentino ECOS SUD-SETCYP, A00U01’’. S. Hang thanks the Manfredi-INTA Experimental Station, (Co´rdoba, Argentina) by permitting the use of the trial plots.

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