Evaluation of Mobility and Dissipation of Mefenoxamand Pendimethalin by Application of CSTR Model and FieldExperiments Using Bare and Tobacco Tilled Soil Columns

Vassilios Triantafyllidis & Dimitra Hela &

Maria Papadaki & Dimitrios Bilalis &

Ioannis Konstantinou

Received: 29 May 2011 /Accepted: 21 September 2011 /Published online: 7 October 2011# Springer Science+Business Media B.V. 2011

Abstract The soil mobility and dissipation of twopesticides with different physicochemical properties,namely mefenoxam, a systemic fungicide, and pendi-methalin a selective herbicide, were determined inbare and tobacco tilled soil columns, which wereinstalled in field conditions for over 125 days. Soilsamples were collected at specific time intervals for a125-day period and the rate of pesticide dissipationand leaching through the soil column was studied.The dissipation half-lives of mefenoxam from the topsoil layer in tilled and bare soil columns wereestimated at 10.3 and 13.1 days, respectively, whilethe corresponding half-lives for pendimethalin were26.7 and 27.5 days, respectively. The dissipation ofmefenoxam and pendimethalin from the top soil intobacco cultivation was faster in comparison withbare soil; however, 120 days after their application,

both pesticide residues were detected in the soil.Maximum concentrations of mefenoxam and pendi-methalin were observed on the 15th and 33rd day,respectively, in the soil layer of 5–10 cm depth and onthe 30th day and 63rd day, respectively, in the soillayer of 10–15 cm depth. Higher concentrations wereobserved in bare soil columns. The leaching of bothpesticides was simulated with the continuous stirredtank reactor (CSTR) in series model. The simulatedpeak concentration and peak time for both pesticidesfitted reasonably well to the experimental values.

Keywords Mefenoxam . Pendimethalin . Leaching .

Half-life . Dissipation . CSTRmodel

1 Introduction

The use of agrochemicals inevitably raises questionsabout the fate of the active substances and theirdegradation products in the environment, as well astheir effects on ecologically sensitive areas. In recentyears, mefenoxam and pendimethalin has been exten-sively used in various crops such as tobacco, citrusfruits, potato, sunflower, grapes, and apples.

Mefenoxam [methyl N-(2,6-dimethylphenyl)-N-(methoxyacethyl)-D-alanilate], the R-enantiomer ofmetalaxyl, is applied at half application rates thanmetalaxyl providing similar efficiency, and it maycontribute to the reduction of environmental riskscompared to metalaxyl formulations (Nuninger et al.

Water Air Soil Pollut (2012) 223:1625–1637DOI 10.1007/s11270-011-0970-y

V. Triantafyllidis :D. HelaDepartment of Business Administration of Food andAgricultural Products Enterprises, University of Ioannina,Seferi 2, 30100 Agrinio, Greece

M. Papadaki : I. Konstantinou (*)Department of Environmental and Natural ResourcesManagement, University of Ioannina,Seferi 2, 30100 Agrinio, Greecee-mail: iokonst@cc.uoi.gr

D. BilalisDepartment of Crop Production,Agricultural University of Athens,Iera Odos 75,11855 Athens, Greece

1996). Mefenoxam tends to replace technical metal-axyl in countries where the registration of metalaxylhas not been renewed. Quantitative studies on the fateof this specific isomer are needed, including appro-priate analytical methods. Mefenoxam has higherwater solubility 26 g l−1 than metalaxyl, and it is alsocharacterized by very low volatility with Henryconstant value calculated at 3.50×10−5 (Pa m3 mol−1

at 25°C), low adsorptivity to soil (Koc=660 ml g−1;range 20–1,299 ml g−1) and moderate persistence inthe field (soil aerobic degradation DT50=39 days)(Tomlin 1997; FOOTPRINT 2006). Many papershave been published concerning metalaxyl (e.g., Horstet al. 1996; Sukul and Spiteller 2000) but a limitednumber of investigations concerning the environmen-tal fate of mefenoxam have been performed (Gardnerand Branham 2001; Monkiedje and Spiteller 2002;Liu et al. 2010; Baker et al. 2010). Horst et al. (1996)showed that the dissipation half-life for mefenoxam insoil was 5–8 days, which is in agreement with theresults of Gardner and Branham (2001), claiming thatthe half-life of mefenoxam in turfgrass was 6±1 daysunder high irrigation and 5±1 days under lowirrigation and that in bare soil was 8±4 days underhigh irrigation and 7±2 days under low irrigation.

Pendimethalin [N-(1-ethylpropyl)-3,4-dimethyl-2,6-dinitrobenzenamine] is a pre-emergence dinitroanili-line herbicide. It is used for selective control of weedsin crops such as tobacco (Nicotiana tobacum), corn(Zea mays), soybeans (Glycine max L.Merr.), peas(Pisum satirum L.) winter wheat (Triticum aestivumL.) and several vegetable crops (Royal Society ofChemistry 1987). Pendimethalin is characterized bylow water solubility (0.3 mg l−1 at 20°C), moderate tohigh vapour pressure depending on temperature (4.0mPa at 25°C), and strong adsorption to soil (Koc=7,011 ml g−1 in loamy sandy soil of 0.87% OC)(Tomlin 1997; U.S. Environmental Protection Agency1997). Degradation of pendimethalin in soil proceedsmore rapidly under flooded, anaerobic conditions thanunder aerobic conditions (Zimdahl et al. 1984), abehaviour observed to be similar to other dinitroani-lines (Zimdahl et al. 1984; Gingerich and Zimdahl1976). European terrestrial field dissipation studieshave shown moderate persistence of this compound insoil with half-lives ranging from 27 to 155 days (Riceet al. 2003; Triantafyllidis et al. 2009).

The fate of these and other pesticides in soils is ofextreme importance owing to their residue migration

in surface and underground water supplies. Thepersistence of pesticides in soils depends on thespecific compound, environmental conditions,physico-chemical interactions, and the biologicaldegradation (Triantafyllidis et al. 2010).

It is a rather common practice for the assessment ofpesticide dissipation and transport in soil to be carriedout by analysing samples of soil taken from subse-quent layers of a soil column (Albanis et al. 1991).The transport of a tracer compound (i.e., pesticide)across the soil can be simulated as a series ofcontinuous-flow stirred tank reactors (CSTR model)following appropriate assumptions (Albanis et al.1988). The successive soil layers can be consideredas a cascade of continuously stirred tanks/reactorsconnected in series and the vertical transport ofpesticides can be treated according to the standardtheory of chemical reactor design. The soil column isdivided into a number of soil layers to accommodatespatial variability in physical and chemical properties(heterogeneities) across the soil. Within each layer,physical and chemical properties of soil–water systemare treated as spatially averaged, effective constants(Albanis et al. 1988, 1991; Basagaoglu et al. 2002).The CSTR in series model has been previouslyapplied in order to describe the transport of pesticidesin soil horizons (Basagaoglu et al. 2002), the transportof methyl-parathion, lindane, and atrazine pesticidesin successive soil layers (Albanis et al. 1991), tosimulate the total flux of volatile organic compoundsfrom the unsaturated zone (Tillman and Smith 2004)and finally, the transport of metalaxyl in Gerberaplants grown in a closed-loop hydroponic system(Karras et al. 2007).

This study deals with the dissipation and verticaltransport of two pesticides having contrasting physi-cochemical properties, mefenoxam and pendimetha-lin, in bare and tobacco tilled soil columns. Thedissipation of the pesticides was determined in fieldconditions and the experimental data were simulatedusing the equal size CSTR model.

2 Experimental

2.1 Experimental Design and Sampling

All experimental conditions were selected to simulateagricultural practices employed in the Mediterranean

1626 Water Air Soil Pollut (2012) 223:1625–1637

region. Twenty-two soil columns, 33 cm in diameterand 20 cm deep, were used for the whole-fieldexperiments, employing soil typically encountered inAgrinio (Western Greece) area. The soil was charac-terized as L-type (loam) with the following properties:loam 46%, sand 34.2%, clay 19.8% (Bouyoucosmethod), organic matter 1.7% (Walkley–Blackmethod), pH 6.3 (measured in a 1:2.5 soil/watersuspension), while trace amounts of CaCO3 weredetected (Calcimeter Bernard Method) (Soil and PlantAnalysis Council, Inc. 1999). The experiments weredivided in two groups of ten columns each. Allcolumns were filled with soil (density=1.45 g cm−3;24.5 kg per column). Ten columns were tilled withtobacco while the other ten were untilled. Twoadditional columns, one of which was tilled withtobacco and the other not, served as control. Allcolumns were placed outdoor in a field and a dose of13.6 mg pendimethalin (Stomp* 330E dissolved in100 ml of water and sprayed with a small pressuresprayer; 1.25 l). After that, the same amount of waterwas added for two more times to flush the sprayer andapplied in all columns 1 day before planting. The totalvolume of water added to the soil column was 300 mlafter pesticide treatment. The first soil sampling wasperformed after pendimethalin application. The col-umns were irrigated using a Tichelmann irrigationsystem. The outlet of the system situates on theopposite side of the inlet. Thus, the route of the waterthrough the system is equal for all supply lines andthe length of the routes in the system is equal for allsupply lines, the distribution of the water is nearlyuniform. One tobacco plant was planted in eachcolumn and irrigated with 150 ml water (all columns)and after 2 days, 30 mg of mefenoxam per columnwere applied (Ridomil gold MZ 68 WP dissolved in100 ml of water and the same amount of water wasadded for two more times to flush the sprayer). Thesecond soil sampling occurred after the mefenoxamapplication (3 days after pendimethalin application).Three soil samples of 20 g each were collected fromthree random cores in each one of the three depths(0–5, 5–10 and 10–15 cm). Soil cores were sampledusing a 2.5-cm-diameter stainless steel core sampler.The soil samples were mixed and kept in freeze (−20°C)prior to analysis. All samples were analyzed within 3days from collection. The moisture content of the soilsamples was determined by oven drying at 105°C. Ninesamplings for mefenoxam and ten for pendimethalin

residues were performed within the cultivation period(June–October). The climatological data of the areaduring the experimental period and the volume ofirrigation in soil columns are given in Table 1.

2.2 Chemicals

Pendimethalin herbicide was obtained as a commer-cial emulsifiable concentrate formulation, Stomp*330E, (33% w/v) active ingredient—BASF (Hellas),and the fungicide mefenoxam was obtained as acommercial wettable powder, Ridomil gold MZ 68WP, (4% w/w) active ingredient (Novartis). Allsolvents were HPLC-grade from Merck (Darmstadt,Germany), pendimethalin and mefenoxam standardswere obtained from Riedel-de Haen (Germany).

2.3 Sample Extraction and Analysis

2.3.1 Soil Extraction

Soil samples were homogenized and passed through a2-mm sieve. Five grams were transferred in a glass tube,and then 20 ml of acetonitrile was added and mixed in avortex for 1 min. The samples were put in a sonicationbath for 10min and the solvent phase was collected. Theextraction was repeated twice with 15 ml acetonitrile.The extracts were centrifuged, passed through funnelsfilled with sodium sulfate, rotary-evaporated to 5 ml andconcentrated to 0.5 ml in nitrogen stream.

2.3.2 Analytical Determination

An HPLC–DAD system (Shimadzu) equipped with aC18 column (25 cm × 4.6 mm × 5 μm; Supelco) anda C18 pre-column (2 cm × 4.6 mm × 5 μm; Supelco)was used for the analysis of the samples at thefollowing conditions: mobile phase gradient of aceto-nitrile–water (AcN:0–2 min 25%, 7 min 50%, 15 min90%, 25 min 25%). The injection volume was 20 μland the mobile flow rate 1 ml min−1. UV detectorresponses were obtained from pendimethalin andmefenoxam at 240 and 210 nm, respectively. Quan-tification was performed using a calibration curveconstructed by analysis of triplicate spiked soilextracts ranging from 0.1 to 10 mg l−1. All chromato-graphic runs were performed in duplicate and thereproducibility of retention times was ±0.5% or better.The recovery of mefenoxam and pendimethalin from

Water Air Soil Pollut (2012) 223:1625–1637 1627

spiked blank soil samples was 86% and 84% (at 50μg kg−1), respectively, while relative standard devia-tion was below 10% for both pesticides. The methodlimits of detection for mefenoxam and pendimethalinwere 2 and 10 μg kg−1, respectively.

2.3.3 Modeling and Treatment of Data

The dissipation of the two pesticides in the soil andtheir movement through the soil profile is certainly acomplex process. The model applied for describingthe vertical movement of pesticides is formulated bydrawing an analogy between the vertical soil layersand a series of non-ideal continuous-flow equal sizestirred tank/reactors model as shown in Fig. 1. Thesoil column is divided into three vertical sequences ofsoil layers considered perfectly mixed to accommo-date spatial variability in physical and chemicalproperties (heterogeneities) across the profile. Withineach layer, physical and chemical properties aretreated as spatially averaged, effective constants. Flowand transport properties are assumed to remainconstant in all specific layers. Details of the non idealCSTR in series model and equations can be found inall standard chemical engineering books on reactordesign (e.g., Levenspiel 1972); thus, it is only brieflydescribed here for the present application. Theprofiles of the exit concentration Cn of a tracer(pesticide) introduced into the first vessel (n=1) (first

soil layer) in a pulse mode (δ function) is given foreach vessel (soil layer), n, by Eq. 1:

Cn ¼ C0ktð Þn�1

n� 1ð Þ ! e�kt or Cn ¼ C0

n� 1ð Þ!t

t

h in�1

exp � t

t

� �

ð1Þwhere C0 is the initial concentration of the tracer(pesticide); k is the apparent rate constant of theprocess, which will be discussed later; t is the meanresidence time of the aqueous solution containing thepesticide in each individual vessel, equal to V/Q,where V is the volume of the vessel and Q is thesolution volumetric feed rate.

Then, considering that the first vessel correspondsto the soil layer 0–5 cm (n=1), the second vessel tothe soil layer 5–10 cm (n=2) and the third vessel tothe soil layer 10–15 cm (n=3) the correspondingequations describing concentration in each layer areEqs. 2, 3 and 4, respectively:

C1 ¼ C01e�k1t ð2Þ

C2 ¼ C02 k2tð Þe�k2t ð3Þ

C3 ¼ C03k3tÞð 2

2e�k3t ð4Þ

The first order apparent rate constant, ki (i=1−3),is controlled by the inverse mean residence time 1=t

Table 1 Climatological data ofthe Agrinio area and irrigationof the columns

June July August September October

Climatic parameters

Temperature min (°C) 18.8 19.7 19.7 15.3 13.0

Temperature max (°C) 36.1 36.7 36.8 29.7 25.1

Temperature mean (°C) 27.0 28.1 27.6 22.0 18.6

Relative humidity min (%) 36.7 35.6 36.0 46.6 58.6

Relative humidity max (%) 100 99.3 99.9 100 99.1

Relative humidity mean (%) 72.8 68.9 72.7 80.3 84.5

Precipitation (mm) 14.2 2.6 77.6 63.8 58.2

Mefenoxam

Irrigation (mm) 4.7 105.84 73.50

Irrigation days 1 11 7

Total water volume (precipitation+irrigation)=400.44 mm

Pendimethalin

Irrigation (mm) 10 105.84 73.50

Irrigation days 2 11 7

Total water volume (precipitation+irrigation)=405.74 mm

1628 Water Air Soil Pollut (2012) 223:1625–1637

in physical processes, while in chemical processes itis controlled by a reaction constant kr. The variation inthe values of the apparent rate constant from vessel tovessel could be attributed to chemical processes orvariation of the volumetric feed rate Q, since thevolume of the vessel V is constant for each soil layer.Apart from the physical transfer by the rainfall orirrigation water, the chemical, biological and photo-chemical decomposition processes should be takeninto account while studying the migration behavior ofsoil applied pesticides. Assuming that those threeparallel removal paths are of first, or pseudo-first,order reactions, relative to pesticide concentration, thetotal pesticide removal rate, R, could be given byEq. 5

R ¼ krC ¼ ðkchem þ kbiochem þ kphotochem þ � � �ÞC ð5Þwhere C is the concentration of the tracer and kr is thefirst-order effective rate constant kr (time−1) ofremoval, which is a sum of chemical, biological andphotochemical processes. The following assumptionsare made relative to those three processes. First, thesum of rate constants of the processes is invariantrelative to soil depth. Second, the decomposition

products do not interfere with the processes or, if theydo, they interfere in a constant way throughout thesoil. It should be also noticed that the effective rateconstant kr in Eq. 5 has the same dimension (time−1)as the quantity (Q/V), which corresponds to theinverse mean residence time of the pesticide mole-cules in each vessel tank. Then, it can be easily shown(Albanis et al. 1991) that the experimental apparentrate constant kapp is given by Eq. 6, which consists oftwo terms, one of chemical–biochemical origin (kr)and a second of physical origin 1=t. The latterexpresses the rate of displacement of the tracer(pesticide) by the flowing solution:

kapp ¼ 1

tþ kr ¼ 1

VQ

þ kr ð6Þ

Therefore, the variation of kapp can be attributednot only to variation of the volumetric flow rate, Q,but also to differences of the total rate constant kr ofthe removal of the pesticide in each soil layer.

The peak effluent/tank tracer concentration Cn, peak

in any but the first tank and the peak time, tmax, aregiven by Eqs. 7 and 8, respectively:

Cn;max ¼ C0

ðn� 1Þ ! ðn� 1Þðn�1Þeð1�nÞ ð7Þ

tmax ¼ ðn� 1Þ t ð8ÞIf reaction processes contribute to the pesticide

removal, tmax is given by Eq. 9; in the absence of areaction, tmax is given by Eq. 8.

tmax ¼ n� 1ð Þt1þ kt

ð9Þ

It would probably be useful to analyze Eq. 9. Ifkt � 1, then tmax ¼ t; in other words, only the rainfalland/or irrigation volume defines tmax. On the otherhand, if kt � 1, then tmax ¼ n� 1ð Þ=k, i.e., tmax isdefined by the combined chemical–biological–photo-chemical processes.

In the first soil layer, the herbicide degradation wasdescribed using first-order kinetics (Eq. 2) then theherbicide degradation half-life t1/2 was calculated withthe following equation:

T1=2ðdaysÞ ¼ lnð2Þ=k ð8Þ

C0

C1

C2

C3

Con

cent

ratio

nC

Time, tTmax,1 Tmax,2

tkeCC 1011

tketkCC 2!1/)( 2022

tketkCC 3!2/)( 3033

Fig. 1 The equal size continuous stirred tank reactor model(CSTR). The concentration Cn at the exit of each vessel n isgiven by Eqs. 2–4; the rate constants kapp in time−1 correspondto the inverse mean residence time; tmax is given by Eqs. 8 and9

Water Air Soil Pollut (2012) 223:1625–1637 1629

The Origin Software for Windows (version Pro8,Microcal Software) was used for regression analysis.The quality of fit was characterized by the relativecorrelation coefficient. The software Statsoft-Statistica (1996) was used for calculating analysis ofvariance and means comparisons. Concentrationswere compared by ANOVA test and mean differenceswere determined using Duncan’s test (p<0.05).

3 Results and Discussion

The concentrations of pesticide residues in thedifferent soil layers in bare and tobacco tilled soilcolumns are summarized in Tables 2 and 3 formefenoxam and pendimethalin, respectively. Theinitial measured pendimethalin concentrations werein the range 2.769±0.410 to 2.783±0.311 mg kg−1,whereas 125 days after application (DAA) the con-centrations in soil samples had dropped in the rangeof 0.135±0.030 to 0.104±0.039 mg kg−1. On theother hand, the initial mefenoxam concentrations werein the range of 6.214±0.804 to 6.285±0.466 mg kg−1,while 120 DAA the concentrations were down to0.005±0.001 to 0.017±0.005 mg kg−1. The dissipa-tion of pesticide residues in the upper 0–5 cm layerand the appearance in the 5–10 and 10–15 cm soillayers are shown in Figs. 2 and 3 for mefenoxam inbare and tobacco tilled soil columns, respectively, andin Figs. 4 and 5 for pendimethalin. The apparent rateconstants kapp as well as the values of C0 estimated ineach case are shown in Table 4.

In all figures, we observe that the concentration ofpesticide residues decreased exponentially in the firstlayer (0–5 cm). Specifically, the simulation of themodel for the dissipation in the first layer wasperformed using the equations (C=6.234e−0.0528t andC=6.453e−0.067t) for mefenoxam (bare and tobaccotilled soil columns, respectively) and equations(C=2.728e−0.0252t and C=2.735e−0.0259t) for pendime-thalin (bare and tobacco tilled soil columns, respec-tively). The half-lives of mefenoxam dissipation intilled and bare soils were 10.3 and 13.1 days,respectively, while the half-lives for pendimethalindissipation were 26.7 to 27.5 days, respectively.

In the second soil layer (5–0 cm), migratedresidues peak maxima (1.402±0.006 and 1.733±0.072 mg kg−1 for tilled and bare soil, respectively)appeared after about 15 DAA for mefenoxam and 33

DAA for tilled (0.523±0.053 mg kg−1) and bare soil(0.718±0.040 mg kg−1) for pendimethalin, respec-tively. The simulation of the CSTR model for thesecond layer was performed using equations C=4.937(0.0683t)e−0.0683t and C=4.033(0.0705t)e−0.0705t formefenoxam (bare and tilled soil, respectively) andequations C=1.85(0.0245t)e−0.0245t and C=1.37(0.0236t)e−0.0236t for pendimethalin (bare and tilledsoil, respectively). The maximum concentrationsdetermined using Eq. 7, closely matched the experi-mental peak concentrations (e.g., 1.4837 vs. 1.402 mgkg−1 for mefenoxam in bare soil and 0.68 vs. 0.718for pendimethalin in bare soil). The determined timefor the appearance of mefenoxam peak using theprevious equations was 14.24 DAA for tilled and baresoil.

In the third soil layer (10–15 cm), migratedresidues peak maxima (0.648±0.039 and 0.734±0.031 mg kg−1) appeared after about 30 DAA formefenoxam in tilled and bare soil, respectively. Forpendimethalin, maximum concentrations appearedabout 63 DAA in tilled and bare soil (0.386±0.151and 0.511±0.098 mg kg−1, respectively). Specifically,the simulation of the CSTRmodel for the third layer wasperformed using equations C=2.777(0.0629t)e−0.0629t

and C=2.414(0.0654t)e−0.0654t for mefenoxam (bareand tilled soil, respectively) and equations C=1.732(0.0313t)e−0.0313t and C=1.234(0.0295t)e−0.0295t forpendimethalin (bare and tilled with tobacco soil,respectively). Again, the maximum concentrationsdetermined using Eq. 7 closely matched the experi-mental peak concentrations (e.g., 0.75 vs. 0.734 mgkg−1 for mefenoxam in bare soil and 0.47 vs. 0.511for pendimethalin in bare soil). The determined timesfor the appearance of pendimethalin peaks using theprevious equations were 67.8 and 63.6 DAA for tilledand bare soil, respectively.

The applied model can be used to identify whetherthe pesticide dissipation is exclusively controlled via achemical, (kr), or physical, (1=t), process. Whentmax ¼ t, the rainfall and/or irrigation volume controlsthe pesticide removal; whereas if tmax ¼ 1=k, it iscontrolled by the chemical–biological–photochemicalprocesses. In all experiments, the quantity t wascalculated via the ratio of the volume of each soillayer (V, cm3) to the volumetric flow rate (cm3/day),i.e., the product of the total amount of rainfall for theexperimental period (405.74 and 400.44 mm forpendimethalin and mefenoxam, respectively) and the

1630 Water Air Soil Pollut (2012) 223:1625–1637

soil column cross-sectional area, divided by thenumber of days of the field experiment period. t wasfound to be equal to 15.40 and 14.98 days forpendimethalin and mefenoxam, respectively.

It can be observed that in the case of mefenoxamthe measured kapp is very close to the 1=t value; thus,the kr constant could be considered negligible, and so1þ krt � 1 and then tmax ¼ t. Indeed, the determinedexperimental time for the appearance of the

mefenoxam peak (tmax) in the second soil layer wasobserved at 14.24 DAA for tilled and bare soilcolumns, a value very similar to the calculated t at14.98 days (Figs. 2 and 3). Additionally, the tmax

determined from the experimental results (30.51 DAAin tilled and 32.54 DAA in bare soil columns) for thethird soil layer is very close to the calculated value(2t ¼ 29:96 days) (Figs. 2 and 3). Thus, we canconclude that the water flow rate controls mainly the

Table 3 Comparison of pendimethalin residues (mg kg−1) in tobacco and on bare soil in the experimental fields

Days after application(DAA) of mefenoxam

Concentration of pendimethalin in soil (mg kg−1)

Soil depth 0–5 cm Soil depth 5–10 cm Soil depth 10–15 cm

Tobacco Bare soil Tobacco Bare soil Tobacco Bare soil

0 2.769±0.410 2.783±0.311 – – – –

3 2.505±0.262 2.519±0.188 – – – –

10 2.014±0.309 1.964±0.311 0.255±0.048 0.360±0.062 0.043±0.014 0.065±0.011

18 1.758±0.186 1.733±0.154 0.321±0.135* 0.461±0.138* 0.103±0.006* 0.156±0.012*

33 1.320±0.070 1.332±0.061 0.523±0.053* 0.718±0.040* 0.181±0.079* 0.302±0.105*

48 0.759±0.051 0.828±0.114 0.503±0.208 0.668±0.194 0.285±0.179 0.431±0.138

63 0.424±0.110* 0.523±0.057* 0.463±0.136* 0.630±0.106* 0.386±0.151* 0.511±0.098*

78 0.359±0.060 0.308±0.081 0.433±0.054* 0.480±0.014* 0.332±0.140 0.443±0.115

95 0.281±0.078 0.322±0.062 0.325±0.152* 0.493±0.134* 0.292±0.085* 0.379±0.047*

125 0.104±0.039 0.135±0.030 0.162±0.030* 0.192±0.027* 0.185±0.048* 0.260±0.009*

*Differences are statistically significant at p<0.05 among treatments in the same soil depth

Each point is the mean ± SE of ten measurements; – not detected.

Table 2 Comparison of mefenoxam residues (mg kg−1) in tobacco and bare soil in the experimental fields

Days after application(DAA) of mefenoxam

Concentration of mefenoxam in soil (mg kg−1)

Soil depth 0–5 cm Soil depth 5–10 cm Soil depth 10–15 cm

Tobacco Bare soil Tobacco Bare soil Tobacco Bare soil

0 6.285±0.466 6.214±0.804 – – – –

7 4.294±0.288* 3.931±0.219* 1.207±0.025* 1.464±0.040* 0.164±0.006* 0.176±0.011*

15 2.569±0.451* 3.642±0.469* 1.402±0.006* 1.733±0.072* 0.435±0.005* 0.479±0.010*

30 0.426±0.068* 1.126±0.156* 1.216±0.051* 1.474±0.075* 0.648±0.039* 0.734±0.031*

45 0.117±0.130* 0.075±0.008* 0.461±0.020* 0.693±0.073* 0.523±0.074* 0.637±0.026*

60 0.039±0.004* 0.034±0.003* 0.145±0.008* 0.185±0.014* 0.451±0.034* 0.546±0.083*

75 0.033±0.003* 0.031±0.002* 0.044±0.010* 0.068±0.009* 0.205±0.023* 0.269±0.021*

90 0.036±0.005* 0.068±0.008* 0.021±0.002* 0.051±0.008* 0.037±0.004* 0.058±0.006*

120 0.005±0.001* 0.017±0.005* 0.005±0.002* 0.010±0.002* 0.015±0.001* 0.020±0.001*

*Differences are statistically significant at p<0.05 among treatments in the same soil depth

Each point is the mean±SE of ten measurements; – not detected

Water Air Soil Pollut (2012) 223:1625–1637 1631

vertical transport of mefenoxam. On the other hand,in the case of pendimethalin the experimental kappwas one-half to two-thirds lower than the 1=t valuethus, the apparent residence time should be higher.Pendimethalin maximum concentrations in the secondsoil layer were determined at 42.37 DAA for tilledand 40.25 DAA for bare soil columns (Figs. 4 and 5),which is 26.97 and 24.85 days, respectively, later thanthe calculated t value (15.40 DAA). Similarly, for thethird soil layer the maximum concentrations forpendimethalin were determined at 67.8 DAA in tilled

and 63.6 DAA in bare soil columns (Figs. 4 and 5),which is 36.99 and 32.79 days, respectively, later thanthe calculated t value (30.81 DAA). A period of 1,300h (21.25 days) and 3,500 h (145.83 days) for theleaching of pendimethalin at 10-cm soil depth wasfound elsewhere for loam and sandy/silty loam soils(Sakaliene et al. 2007).

The above deviation could be attributed to aretardation factor due to the strong adsorption ofpendimethalin into the soil matrix. This herbicide isknown to be strongly adsorbed onto soil and organic

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

Con

cent

ratio

n of

mef

onox

am in

soi

l (m

gkg-1

)

Time (days)

Fig. 2 Change inconcentration ofmefenoxam in three layersof bare soil. The points areexperimental measurements,while the drawn linesrepresent the best curvesaccording to Eqs. 2–4; filledsquare first layer experi-ment, dotted line first layertheory, filled trianglesecond layer experiment,dashed line second layertheory, asterisk third layerexperiment, continuous linethird layer theory

0 20 40 60 80 100 1200

1

2

3

4

5

6

7

Con

cent

ratio

n of

mef

onox

am in

soi

l (m

gkg-1

)

Time (days)

Fig. 3 Change in concen-tration of mefenoxam inthree layers of tobacco tilledsoil columns. The points areexperimental measurements,while the drawn lines rep-resent the best curvesaccording to Eqs. 2–4; filledsquare first layer experi-ment, dotted line first layertheory, filled trianglesecond layer experiment,dashed line second layertheory, asterisk third layerexperiment, continuous linethird layer theory

1632 Water Air Soil Pollut (2012) 223:1625–1637

matter, possibly due to its high potential for hydrogenbonding with soil components resulting in a decreaseof bioavailability (Weber 1990; Zheng et al. 1993).The lower kapp observed could be the result ofadsorption processes that alters the residence time ofthe pesticide in each soil layer.

Two important factors—solubility and adsorption—can be used to explain the behavior of mefenoxam andpendimenthalin herbicides. Comparing the two herbi-cides, mefenoxam presents a high water solubility(26 g l−1 at 20°C) and low adsorptivity (mean Koc=660 ml g−1) in contrast with pendimethalin, which

presents very low water solubility (0.33 mg l−1 at 20°C) and high adsorptivity (mean Koc=18,050 ml g−1).The high water solubility of mefenoxam results in aneasier transfer of the pesticide through the soil masspreferentially following the water flow; therefore, themaxima in the second and third soil layers shouldappear earlier than those of pendimethalin. Its rate ofdissipation should be governed by physical factors.Monkiedje and Spiteller (2002) showed that themobility of mefenoxam depends on the pH, on theorganic carbon and on the clay content of the soil. Inour experiment pH was 6.3, the organic carbon was

0 20 40 60 80 100 120 1400,0

0,5

1,0

1,5

2,0

2,5

3,0

Con

cent

ratio

n of

pen

dim

etha

lin in

soi

l (m

gkg-1

)

Time (days)

Fig. 4 Change in concen-tration of pendimethalin inthree layers of bare soil. Thepoints are experimentalmeasurements, while thedrawn lines represent thebest curves according toEqs. 2–4; filled square firstlayer experiment, dotted linefirst layer theory, filledtriangle second layerexperiment, dashed linesecond layer theory, asteriskthird layer experiment,continuous line third layertheory

0 20 40 60 80 100 120 1400,0

0,5

1,0

1,5

2,0

2,5

3,0

Con

cent

ratio

n of

pen

dim

etha

lin in

soi

l (m

gkg-1

)

Time (days)

Fig. 5 Change in concen-tration of pendimethalin inthree layers of tobacco tilledsoil columns. The points areexperimental measurements,while the drawn lines rep-resent the best curvesaccording to Eqs. 2–4; filledsquare first layer experi-ment, dotted line first layertheory, filled triangle sec-ond layer experiment,dashed line second layertheory, asterisk third layerexperiment, continuous linethird layer theory

Water Air Soil Pollut (2012) 223:1625–1637 1633

1.7% and the clay was 19.8%. According to Monkiedjeand Spiteller (2002), the adsorption of mefenoxam onsandy loam soil was almost completely reversiblewith percentage desorption rates of more than 91.0%.This reduced retention of the adsorbed mefenoxam isalso consistent with the observed control of thedownward movement by the volumetric flow rate.

On the contrary, the high adsorption (high Koc

values) and limited solubility of pendimethalinresulted in a delay of maxima compared to the timedetermined by the water volumetric flow. A retarda-tion factor can be considered, based on the ratio ofadsorptive potential (measured log Koc values) ofmefenoxam (log Koc=1.78) and pendimethalin (logKoc=4.12) in the column soil that is equal to 0.43.This factor multiplied by Q corresponds to theapparent or useful flow of water. Using the correctedQ values, the results show that the maximum forpendimethalin should be 34.84 and 69.67 DAA forthe second and third layers, respectively, which areclose to the experimental values. The term “usefulflow of water” is used to express the resultingpendimethalin concentration in the absence of sorp-tion processes. Moreover, the concentration reachingthe next soil layer is equivalent to a smaller C0

introduced to the first soil layer due to the adsorptionof pendimethalin on the soil. Thus, the simulations forthe second and third soil layers employed a lower C0

as a parameter.A close examination of the calculated C0 and kapp

values in Table 4 leads to several interesting obser-vations. First, the apparent concentration used in thefeeding of the second and third layers of the soil issubstantially decreased as compared to the initialfound in the first layer, reflecting the adsorptionextent and the relative stability of the herbicides. Thedecrease in C0 for mefenoxam reaches 20.8% and

55.5% for the second and third soil layers of the baresoil, respectively; for tilled soil, this was observed tobe 37.5% and 62.6% for the second and third soillayers, respectively, in the tilled soil. Another obser-vation for mefenoxam is that the apparent rateconstants are higher in the second layer in both bareand tilled soil, while the apparent rate constants intilled soil were higher than those in bare soil, for allsoil layers. Additionally, statistical differences wereobserved in all soil layers between the concentrationsmeasured in tilled with tobacco and bare soils for alldays after the application of the pesticide (on day 0)(Table 2). These observations are consistent with theenvironmental data on the fate of mefenoxam(FOOTPRINT 2006; Gardner and Branham 2001)that the primary means of mefenoxam dissipation insurface soil are the aerobic microbial degradation andplant uptake. Hydrolysis, photolysis (breakdown bysunlight) and volatilization (processes that take placein a larger extent in the first layer) are not significantways of breakdown. Our experimental results showedthe effect of tobacco cultivation on mefenoxamresidue in soil. Metalaxyl has been shown to movesystemically in plants via localized translocation by bulkflow with water in xylem tissue, so mefenoxam can beassumed that shares similar properties (Kennelly et al.2007). In addition, the rhizosphere supports consortiaof microorganisms capable of degrading pesticides(Anderson et al. 1995). More information on rhizo-sphere effects on contaminant degradation can befound in literature (Davis et al. 2002; Karthikeyan andKulakow 2003). It was found elsewhere (Baker et al.2010) that soil physicochemical characteristics have agreat influence on the microbial degradation rate ofmefenoxam and lower degradation could be observedas a result from a greater adsorption of mefenoxam.Monkiedje and Spiteller (2002) proposed high

Table 4 Calculated values ofC0 and kapp for the dissipationcurves drawn in Figs. 2, 3, 4and 5

R2 ranged from 0.945 to 0.991

Parameter Mefenoxam Pendimethalin

Bare soil Tilled Bare soil Tilled

C0 first layer 6.234±0.365 6.453±0.217 2.728±0.068 2.735±0.056

C0 secondlayer 4.937±0.272 4.033±0.272 1.851±0.083 1.370±0.053

C0 third layer 2.777±0.190 2.414±0.175 1.732±0.043 1.234±0.059

kapp first layer 0.0528±0.0067 0.0670±0.0050 0.0252±0.0014 0.0259±0.0012

kapp second layer 0.0683±0.0038 0.0705±0.0048 0.0245±0.0015 0.0236±0.0013

kapp third layer 0.0629±0.0036 0.0654±0.0039 0.0313±0.001 0.0295±0.0019

1634 Water Air Soil Pollut (2012) 223:1625–1637

adsorption of metalaxyl due to both high soil organicmatter and high clay content. The organic matter(1.75) and clay (19.8%) content of the soil in ourstudy suggest that microbial degradation of mefe-noxam can take place. Finally, significant lowermefenoxam residues were measured in soil with turfgrass than in bare soil indicating more rapid microbialmetabolism (Gardner and Branham 2001).

The decrease in Co for pendimethalin reaches32.2% and 36.5% for the second and third soil layersof the bare soil, respectively; meanwhile, 49.9% and54.9% were found for the second and third soil layersin the tilled soil, respectively. Apparent rate constantswere lower in the second layer probably due toreduced contribution of volatilization and photode-gradation that are primary dissipation ways ofpendimethalin in the surface soil layer, while the rateconstants increased in the third layer probably due tomicrobial degradation. Kulshrestra and Singh (1992)observed that 11–14% of pendimethalin degradationin a sandy loam soil could be attributed to microbialtransformation. Tobacco cultivation did not have asignificant effect on the concentrations of pendime-thalin in the first soil layer (except for the date 63DAA) as determined by Duncan test (p<0.05)(Table 3). On the other hand, statistical differencesbetween tilled and bare soil were observed for the 5–10 and 10–15 cm soil layers, after 18 and 33 DAA,respectively, except for the 48 DAA in soil depth 5–10 cm and 48 and 78 DAA in soil depth 10–15.

The observed half-life time in the top soil layerwas in the range of those reported in previous fieldstudies: 27–155 days for different European soils(Rice et al. 2003), 23–30 days in turfgrass (Lee et al.2000), and 43–62 days in cotton fields of CentralGreece (Tsiropoulos and Lolas 2004), but highercompared to values reported in other studies, i.e.,14–21 days in subtropical soil (Lin et al. 2007), 13–17 days in tropical and Mediterranean plain fields(Cooper et al. 1994), 10.5–31.5 in Chile vineyards(Allister et al. 2009). In a previous work by Albanisand Manos (1995), pendimethalin showed higherpersistence in the soil of tobacco cultivation (84–97days), but in that study clay content of soil was higherand pendimethalin application was performed early inspring when both temperature and solar irradiationwere lower. The rate of pendimethalin degradation onsoil was shown to be enhanced under the influence ofsunlight and as much as 17% of the applied

pendimethalin was lost by photodecomposition within7 days (Parochetti and Dec 1978). Pendimethalin isknown to be decomposed readily when irradiated atwavelengths λ>250 nm by the mechanisms involvingoxidative dealkylation, nitroreduction, and cyclization(Dureja and Walia 1989). Moreover, volatilization isanother factor for pendimethalin dissipation especiallythe first days after application up to 3.7% of theapplied amount (Garcia-Valcarcel and Tadeo 2003),while in sandy soil the volatilization percentage wasup to 2.78% within 28 days (Scroll et al. 1999). Bothphotodegradation and volatilization must have affect-ed the disappearance of the pesticide under the fieldconditions of this experiment that was conducted inan area of high solar radiation and temperaturesduring the summer period (see Table 1). Triantafyllidiset al. (2009) showed a lower persistence of pendime-thalin in soil cultivated with tobacco (22.3 to 27.2DAA), bearing similar physicochemical character-istics and under similar climatological conditions tothose of our experimental field.

4 Conclusions

The dissipation of mefenoxam and pendimethalin insurface soil and their vertical transport were studiedusing bare and tobacco tilled soil columns under fieldconditions. Half-lives of mefenoxam dissipation inthe surface layer of soil tilled with tobacco and baresoil were 10.3 and 13.1 days, respectively, while half-lives for pendimethalin dissipation were 26.7 and 27.5days, respectively. The effect of tobacco plants wasstatistically significant for the dissipation and move-ment of mefenoxam for all sampling days and all soildepths. In the case of pendimethalin, the measuredconcentrations were higher in bare soil than in tilledsoil, and significant differences were encountered inmost sampling days only for the second and third soildepths. The use of the continuously stirred tankreactor in a series model provided a good represen-tation of the transport and dissipation of pesticidesthrough successive soil layers especially for pesticideswith low adsorptivity and higher leaching potential,such as mefenoxam. The calculated apparent rateconstants represent a sum of different parallel ways ofpesticide dissipation, but can be useful tools for apreliminary estimate of the residue of such pesticidesin fields.

Water Air Soil Pollut (2012) 223:1625–1637 1635

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