Changes in the atrazine extractable residues

in no-tilled Mollisols

S. Hang a,*, M. Nassetta b, A.I. Canas b, E.A. Rampoldi a,M.V. Fernandez-Canigia c, M. Dıaz-Zorita d

a Dpto. Rec. Nat., Facultad de Ciencias Agropecuarias UNC CC509-5000 Cordoba, Argentinab Ceprocor ACC SE 5164 Santa Marıa de Punilla Cordoba, Argentinac Actividad privada, M.T. Alvear 1665 148 B, Buenos Aires, Argentina

d CONICET-Facultad de Agronomıa (UBA)-Nitragin Argentina S.A., Calle 10 y 11,

Parque Industrial Pilar, 1629 Pilar, Buenos Aires, Argentina

Received 5 March 2006; received in revised form 1 June 2007; accepted 6 June 2007

www.elsevier.com/locate/still

Soil & Tillage Research 96 (2007) 243–249

Abstract

The effect of application dose and soil organic matter (SOM) stratification on changes in atrazine (6-chloro-N2-ethyl-N4-

isopropyl-1,3,5-triazine-2,4-diamine) extractable residues (ER) were investigated. Two soils [Entic Haplustoll (EH) and Typic

Hapludoll (TH)] with contrasting SOM content and form and without previous atrazine exposure were selected. Sampling was

carried out at two depths: 0–2 and 2–5 cm. Atrazine ER were measured at 0, 3, 7, 14, 28, and 56 days in laboratory incubation.

Atrazine concentration recovered 1 h after of its application (Ct0) was used as an index of the soil capacity to reduce the atrazine

extractable fraction. SOM stratification was studied by means of physical fractionation. In both soils, the higher OC concentration

was found in the 200–2000 mm fraction (OCf 200–2000). Soils differed in terms of the OCf 50–200/OCf 200–2000 ratio. This ratio

increased with depth in EH soil: 0.23 (0–2 cm) and 2.00 (2–5 cm). In TH soil, the ratio was 0.80 (0–2 cm) and 0.50 (2–5 cm). The t1/

2 values ranged from 9 to 19 days, depending on soil type and atrazine application dose. The upper layer Ct0 and k were higher for

higher atrazine doses. Implementation of a split application dose of atrazine may be an effective alternative to extend its half-life in

soil solution, as well as involving a lower potential risk of soil accumulation or vertical movement in the soil profile towards deep

soil layers and groundwater.

# 2007 Elsevier B.V. All rights reserved.

Keywords: Herbicides; No-tillage system; Atrazine efficay; Soil organic matter; Soil size fractiontion; Environmental pollution

1. Introduction

Intensification of agricultural practices has led to a

higher use of agrochemicals, thus requiring the develop-

ment of herbicide management strategies that imp-

rove efficiency while minimizing environmental risks.

Currently, atrazine (6-cloro-N2-etyl-N4-isopropyl-1,3,

* Corresponding author. Tel.: +54 351 4334116;

fax: +54 351 4334118.

E-mail address: shang@agro.uncor.edu (S. Hang).

0167-1987/$ – see front matter # 2007 Elsevier B.V. All rights reserved.

doi:10.1016/j.still.2007.06.003

5-triazina-2,4-diamina) is one of the most widely

used herbicides in maize (Zea mays L.) and sorghum

(Sorghum bicolor) crops.

Many worldwide studies have assessed the effects of

the use of atrazine from both an agricultural perspective

as well as from an environmental point of view. The

agricultural approach has been mostly focused on

efficacy of the herbicide and on persistence of active

forms in the soil in terms of susceptible crops and weed

control. The environmental point of view aims to assess

the potential environmental risk of herbicide residues

S. Hang et al. / Soil & Tillage Research 96 (2007) 243–249244

and their accumulation in water or in soils. To develop

suitable herbicide management strategies that make

both agricultural and environmental requirements

compatible, both types of research are needed.

Retention processes can reduce atrazine bioavail-

ability (extractable residues) (Johnson et al., 1999; Nam

and Alexander, 2001). However, results with regard to

medium-term stability of these compounds have not

been conclusive and in general, studies have shown that

atrazine residues became more recalcitrant with time

(Koskinen and Clay, 1997).

Soil organic matter (SOM) content and composition

can affect atrazine retention in soil (Bollag et al., 1992;

Houot et al., 1997). The coarse SOM fraction (>50 mm)

has greater capability for binding atrazine residues than

finer fractions (Barriuso et al., 1994). Changes in the

relative concentration of SOM fractions depend mainly

on soil management practices (Skjemstad et al., 1997;

Yakovchenko et al., 1998). Reduced tillage and no-

tillage systems modify SOM vertical distribution,

leading to SOM stratification with differences in both

the total content and in its composition within a very

short distance (Dıaz-Zorita and Grove, 2002).

Shelton et al. (1998) demonstrated that in the first 15

days after application atrazine concentration in the soil

solution of the surface 5 cm of soil no-tillage was two to

four times lower than in ploughed soil without plant

residue cover. Abdelhafid (1998) determined that the

extractable fraction was a function of dose and that the

ER decreased when atrazine dose increased. Moreover,

Kalita et al. (2006) showed that higher atrazine

application resulted in greater losses to subsurface water.

A feasible alternative to increase atrazine bioavail-

ability is the use of split applications (two applications

of half a dose each). This strategy could satisfy the

requirements of sustainable development in terms of

both agriculture as well as the environment.

We determined changes in concentration of soil

extractable residues of atrazine depending on applica-

tion dose in no-tilled soils with contrasting SOM

content and composition.

Table 1

Mean soil properties

Soil type Sampling

depth (cm)

pHa T

(

Entic Haplustoll (EH) 0–2 6.7 3

2–5 6.4 1

Typic Hapludoll (TH) 0–2 5.9 2

2–5 5.7 1

a pHsoil/water.b TOC: total organic carbon.

2. Materials and methods

2.1. Soils

The study was carried out in plots of a long-term trial

located in Manfredi (Entic Haplustoll (EH), Cordoba,

Argentina) and General Villegas (Typic Hapludoll

(TH), Buenos Aires, Argentina) (Soil Survey Staff,

1998). Soil sampling was carried out at two depths: 0–2

and 2–5 cm, because they are the depths where

germination of most weeds occurs and where herbicide

reaction with soils is most relevant. At both sampling

sites, 20 subsamples from each depth were taken at

random from 6 plots of 10 m2. After sampling, soils

were air-dried and sieved through a 2 mm sieve. Particle

size distribution by wet sieving and sedimentation, soil

water content at field capacity (Klute, 1986), soil water

pH (soil:water 1:2) and total organic carbon (TOC)

content by wet combustion were determined (Table 1).

Soil size fractionation was done after dispersion of

soils in water (soil:water, 50 g:100 mL) by shaking for

24 h in 250 mL centrifuge bottles with 20 glass balls

(0.5 cm of diameter). The fractions 200–2000, 50–200

and<50 mm were recovered by sieving and subsequently

dried at 50 8C. Soil weight and TOC concentration in

each fraction (OCf) was quantified (Table 1).

2.2. Evaluation of atrazine extractable residues

(ER)

Atrazine ER were assayed by triplicate in laboratory

incubations during 56 days at 28 � 1 8C in the dark. One

or 2 mL of atrazine solution (analytical grade, 99.9%

purity, 134.6 mmol L�1) was added to 20 g of air-dried

soil and distilled water was added to achieve 80% of field

capacity. At 0, 3, 7, 14, 28 and 56 days of incubation,

atrazine was extracted from the soil with 20 mL of

acetonitrile:water (90:10). Centrifuge tubes were agi-

tated for 2 h at room temperature (20 � 2 8C approxi-

mately). Soil extracts were purified using 3 mL of

bencelsulfonic acid cartridges under vacuum (Visiprep

OCb

g kg�1)

Clay

(g kg�1)

Silt

(g kg�1)

Sand

(g kg�1)

0 100 560 340

2 140 630 230

8 140 340 520

6 160 540 300

S. Hang et al. / Soil & Tillage Research 96 (2007) 243–249 245

Solid Phase Extraction Vacuum Manifold, Supelco,

USA). Three millilitres of aqueous solution of 1% acetic

acid was added to each cartridge and aspirated before

drying. Five millilitres of each soil extract was mixed

with 25 mL of aqueous solution of 1% acetic acid and

then aspirated through the cartridges at 5 mL min�1. The

cartridges were successively washed with 1 mL acet-

onitrile, 3 mL milliQ water and finally with 1 mL 0.1 M

K2HPO4. An atrazine elution was performed with 5 mL

of a mixture of acetonitrile:0.1 M K2HPO4 (1:1). The

eluate was recovered in 5 mL volumetric flasks and filled

to 5 mL with acetonitrile:0.1 M K2HPO4 solution. The

atrazine quantification was performed by HPLC (SHI-

MADZU, Japan) using an LC 10AS pump, a UV–Vis

SPD 10 VP detector, a SIL 10 ADVP automatic injector,

an SCL 10 A controller and Millenium 03 software.

Samples were injected using acetonitrile:0.05 M

K2HPO4 (40:60) as mobile phase and measurements

were performed at 254 nm. Based on previous assays, it

was observed that the percentage of recovery and

detection limit of this procedure ranged between 95 and

105% and 0.04 mg mL�1, respectively (data not shown).

2.3. Calculations and statistical analysis

The extractable atrazine half-life time (t1/2) was

calculated using a first-order negative exponential

model (Eq. (1)):

Ct ¼ C0 e�kt (1)

where Ct is the atrazine concentration at time t (days),

C0 the initial atrazine concentration and k is the daily

dissipation rate (days�1). The atrazine concentration

(Ct0) recovered 1 h after (t0) its application was used as

Table 2

Distribution of soil mass and organic C content in three soil size fractions

Sampling depth (cm) Soil size fractions (mm)

200–2000

EHa THa

0–2

Mass fraction (g kg�1 soil) 47 � 1.4 48 � 0.1

OCfb (g kg�1 fraction) 130 � 5.6 101 � 4.3

OCc (g kg�1 soil) 6.1 � 0.3 5.0 � 0.2

2–5

Mass fraction (g kg�1 soil) 11 � 0.3 29 � 0.4

OCf (g kg�1 fraction) 122 � 8.0 59 � 0.5

OC (g kg�1 soil) 1 � 0.1 2 � 0.01

a EH: Entic Haplustoll; TH: Typic Hapludoll.b OCf: organic carbon concentration in each fraction.c OC: organic carbon relative to the whole soil.

an index of the soil capacity to reduce the atrazine

extractable fraction.

ANOVA procedures were performed using two main

factors: (i) soil type (location and sampled depth) and

(ii) atrazine application dose. Multiple regression

analysis was also performed according to the ‘‘back-

wards’’ procedure (Analytical Software, 2003). In

addition to the measured soil properties, several indexes

relating soil organic C contents in each of the soil

fractions and soil texture were considered in the

regression analysis. These indexes were classified as

(i) soil properties, (ii) variables that characterize the

TOC according to its concentration in each soil size

fraction and (iii) variables that describe soil texture

[TOC clay�1 and TOC (clay + lime)�1] and the relative

TOC content in relation to each of the soil size fractions

[OC200 (OC50)�1 and OCf 200–2000 (OCf 50–200)�1].

3. Results and discussion

3.1. Soil characterization and carbon distribution

among soil size fractions

Soil properties are shown in Table 1. Soils had

different textures and pH values, which are two properties

that partly regulate atrazine retention and degradation

(Koskinen and Clay, 1997). Water pH values were close

to neutral in the EH soil and moderately acid in the TH

soil. The EH soil had greater silt and lower sand

concentration than the TH soil. Similar results were

observed in mass distribution after soil size fractionation

directly without OC elimination (Table 2). TOC values

were similar between assayed soils, but with highest

concentration in the superficial depth. However, there

were differences in OC concentrations between soils and

in two Mollisols depending on sampling depth

50–200 <50

EH TH EH HT

180 � 5.2 380 � 0.5 752 � 22 571 � 0.8

8.0 � 1.1 9 � 1.5 26 � 0.0 33 � 1.3

1.4 � 0.2 4 � 0.5 20 � 0.00 19 � 0.7

142 � 4.1 381 � 5.7 827 � 24 580 � 9.0

11 � 1.6 3.0 � 0.3 16 � 0.1 25 � 0.8

2.0 � 0.2 1.0 � 0.1 13 � 0.06 15 � 0.5

S. Hang et al. / Soil & Tillage Research 96 (2007) 243–249246

fractions (Table 2). The mean OC concentration was

higher for EH soil than for TH soil, and the highest OCf

concentration was found in the 200–2000 mm fraction

(Table 2). Sampling depth did not modify OCf

distribution pattern, but did change the OCf concentration

and OCf 50–200/OCf 200–2000 ratio. This ratio increased

with depth in EH soil, from 0.23 in the 0–2 cm depth to

2.00 in the 2–5 cm depth. In the TH soil, the ratio was

0.80 in the 0–2 cm depth and 0.50 in the 2–5 cm depth

(Table 2). Yakovchenko et al. (1998) reported that the

OCf of the coarser size fraction (200–2000 mm) was

approximately 25% that of the 50–200 mm size fraction

in the 0–20 cm depth of a soil typic.

Benoit and Preston (2000) reported that straw

amendment did not significantly modify carbon content

of the <50 mm fraction, whereas significant differences

between treatments were detected in the coarsest

fraction. Another helpful index to characterize SOM

quality is the enrichment index proposed by Christensen

(1992). This index estimates the distribution of the OC

in each soil size fraction by means of the ratio between

OCf and TOC. Values higher than 1 suggest a

preferential accumulation of OC in each of the soil

fractions.

In both soils, the largest enrichments were observed

in the coarsest fraction (200–2000 mm) and the

smallest enrichment in the 50–200 mm fraction

(Fig. 1). The relative proportion of the coarser

fractions (200–2000 mm and 50–200 mm) in the

whole soil was low; however, the greater yearly

changes in TOC occur in the non-humified fractions

(Kuzyakov, 1997; Hadas et al., 2004). Elliott et al.

(1994) considered the coarse fraction of SOM more

sensitive to changes in soil management practices. Our

results also showed that in Mollisols under continuous

no-tillage practices, there are relevant changes in TOC

Fig. 1. Relative enrichment of organic carbon in relation to fraction

size (relative enrichment is defined as the ratio between the organic

content in fraction and the organic content in the whole soil).

content and its vertical distribution. This suggests the

occurrence of SOM stratification, not only based on

changes in its concentration, but also in composition

and activity.

3.2. Atrazine extractable residues

Atrazine ER decreased during incubation and

differences were detected between soil sampling depths

and atrazine application doses (Fig. 2). The parameters

to characterize atrazine degradation were estimated

based on Eq. (1) and are shown in Table 3. The t1/2

values ranged from 9 to 19 days and the Ct0, the k and

the t1/2 were different, depending on soil type and

atrazine application dose. A rapid decline in atrazine ER

occurred in both soils. Differences in the percentage of

atrazine recovered in the first extraction (Ct0) varied

among soil sampling depths and herbicide application

dose Ct0 was higher in the TH soil than in the EH soil for

all doses. In the upper soil layer of both soils, there was

a higher degradation rate and shorter half-life of the

herbicide. In the TH, atrazine ER was not different

between atrazine doses and soil layers, which could

have been related to the higher proportion of humified

SOM and the lower OCf concentration in the coarse

fraction (200–2000 mm) observed in this soil. In the two

sampled depths, the EH soil had the greatest Ct0 and also

had the highest OC content in the 200–2000 mm size

fraction along with the highest enrichment and pH

values. Several studies performed with 14C-atrazine

showed a strong and positive correlation between 14C-

atrazine bound residue formation and OCf (Hang et al.,

2003). This can be partially explained because the

atrazine stabilization process reaches its maximum in

the coarse SOM fractions faster than in humified SOM

forms (Barriuso and Koskinen, 1996) and the Ct0 is

considered to be related to atrazine retention in the

coarser organic fraction (200–2000 mm). Although the

coarsest fraction comprised less than half of the TOC in

terms of the total soil mass, its high atrazine retention

capacity not only reduced the effectiveness of the

herbicide, but also represented a potential environ-

mental risk. Since this coarse fraction has greater

turnover rate (Kuzyakov, 1997), it does not ensure final

removal of the herbicide or elimination of associated

risks for the environment.

Atrazine degradation was not significantly related

with other variables that were used to describe the

absolute and relative TOC content at soils (Table 4).

Regression analysis showed that the indexes used to

determine atrazine degradation (k, Ct0, t1/2) were related

exclusively to soil water pH (Table 4). A negative

S. Hang et al. / Soil & Tillage Research 96 (2007) 243–249 247

Fig. 2. Atrazine degradation in two Mollisols depending on sampling depth and application dose. EH: Entic Haplustoll; TH: Typic Hapludoll. The

standard deviations (error bars) are shown when larger than the symbol size.

relationship was observed between soil pH and Ct0

(Table 4). Houot et al. (2000) determined that soil pH

was the most significant factor in terms of atrazine

mineralization; e.g. when soil pH was between 6.0 and

6.5, atrazine mineralization increased with increasing

soil pH.

From the independent analysis by soil type, Ct0 and k

were higher for higher atrazine doses while the half-life

was lower in the 0–2 cm depth of the EH soil (Table 3).

In this soil layer, the coarse soil size particles (200–

2000 mm) were enriched in OC, suggesting that

Table 3

Mean atrazine recovery at the application [Ct0 (%)], constant of degradati

depending on application doses

Soil type Sampling

depth (cm)

Ct0 (%) of atrazine (of initial applied)

LDa HDa

EHb 0–2 77.5 cc Ad 61.2 c B

2–5 75.6 c 78.4 b

THb 0–2 90.2 b 86.1 b

2–5 100.2 a 98.1 a

a LD: low doses (6.7 mmol kg�1); HD: high doses (13.5 mmol kg�1).b EH: Entic Haplustoll; TH: Typic Hapludoll.c Small letter cases show significant differences among soil types and sad Uppercase letters show significant differences between application ra

significance at P = 0.05).

increasing atrazine dose could reduce atrazine bioavail-

ability. Our results were in line with the conclusions

reached by Abdelhafid (1998), in which an increase in

atrazine dose decreased ER, increased mineralization,

and led to greater bound residue formation. Thus, lower

effectiveness of atrazine for weed control in no-tillage

systems cannot only be attributed to plant residue

interception (Sadeghi and Isensee, 1996; Locke and

Bryson, 1997; Shelton et al., 1998) but also to retention

of atrazine compounds within coarse organic fractions

of the upper soil layers. Benoit and Preston (2000)

on rate [k (days�1)] and half-life time [t1/2 (days)] in two Mollisols

k (days�1) t1/2 (days)

LDa HDa LDa HDa

0.070 a B 0.079 a A 10 b A 9 c B

0.045 b 0.058 b 15 a 12 b

0.045 b 0.037 c 16 a 19 a

0.043 b 0.039 c 16 a 18 a

mpling depths (probability level of significance at P = 0.05).

tes within each soil type and sampling depth (probability level of

S. Hang et al. / Soil & Tillage Research 96 (2007) 243–249248

Table 4

Summary of the multiple regression analyses between soil properties

and atrazine degradation parameters in two Mollisols, mean atrazine

recovery at the application time (Ct0), constant of degradation rate (k)

and half-life time (t1/2)

Variables in

the model

Coefficient Adjusted

R2 j

Probability

Ct0 pH �28.08 0.85 0.0012

f200 0.82 0.82 0.0019

TOC:clay 0.47 0.011

f200/f50 0.87 0.87 0.005

k pH 0.03 0.75 0.0039

t1/2 pH �8.15 0.801 0.0026

observed that after adding straw to soil, atrazine-bound

residues changed. Atrazine comprised 85% at the

fraction <50 mm, but after adding straw, bound residue

percentage was reduced to �70% due to accumulation

in coarser fractions.

4. Conclusions

Our results showed that enhanced TOC concentra-

tion in the upper soil layer (0–5 cm) of Mollisols, with

no-tillage coincided with the enriched zone of weed

seeds and herbicides placement. The disappearance of

the extractable fraction of atrazine was related to soil

acidity, to the concentration of organic compounds in

the coarser soil size fractions, and to atrazine doses. In

soils with high capacity to decrease atrazine concentra-

tion in soil solution, split application rate of this

herbicide may be an effective alternative to extend the

atrazine half-life in soil solution, along with lowering

the potential risk of soil accumulation or vertical

movement in the soil profile towards deep soil layers

and groundwater.

Acknowledgment

This work was granted by the Agencia Cordoba

Ciencia-SE, Cordoba, Argentina.

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