Impact of tillage on solute transport in a loamy soil from leaching experiments
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Impact of tillage on solute transport in a
Soil & Tillage Research 112 (2011) 4757
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Soil & Tillage
journal homepage: www.e1. Introduction
Soil is a key component of terrestrial ecosystems where waterrunoff, inltration, drainage and storage interact with chemicalmovement. Flow and transport in soil is complex, space and timescale dependent and subject to human inuence. Soil tillage, forinstance, has a major control on ow and transport. Indeed, themechanical perturbation, aiming at developing desirable soilconditions for a seedbed and establishing specic surface congu-ration for planting, irrigation, drainage or harvesting operations, canhave a considerable impact on the soil structure, and hence, on soilhydraulic functioning (Kepner et al., 1978). Tillage may consist of awide range of practices, ranging fromminimumand reduced tillage,say harrowing, and no-till practices for conservation systems inwhich a substantial part (at least 30%) of the soil remains covered by
previous crop residues (Holland, 2004), tomouldboard ploughing asin traditional (or conventional) systems.
Different tillage practices can explain the differences observed insoil structure and, consequently, in water ow and chemicaltransport in the soil. Several authors underline the benets ofreduced tillage on hydraulic functioning. Plant water uptake, soilwater storage, soil water inltration and transmission are improvedin many conservation systems, mainly as a consequence of themodication in soil physicalandhydraulicproperties (vanDorenandAllmaras, 1978; Hateld et al., 2001; Turner, 2004; De Vita et al.,2007;Moret and Arruea, 2007; Casa and Lo Cascio, 2008; Strudley etal., 2008).Tracer studiesalsoshowthatconservation tillagegenerallypromotes more rapid leaching of non-reactive solutes (Gish et al.,1991; Shipitalo and Edwards, 1993; Shipitalo et al., 2000) andpesticides (Alletto et al., 2010 for an extensive review). Indeed,macroporeowdominatesgenerallysolutetransport inconservativetilled soil as compared to conventionally tilled soil (Petersen et al.,2001; Vervoort et al., 2001; Kulli et al., 2003; Vogeler et al., 2006).Conventional tillage generally reduces solute transport by cuttingfunctional macropores (Jarvis, 2007). As suggested by the studies ofVanclooster et al. (2005) and Javaux et al. (2006), cutting functionalmacroporesby intensecultivationwill alsoaffectowpathways, andgeneral solute mixing regime at the scale of the soil prole.
Yet, the relationship between tillage and ow and transport isnot unambiguous. Whereas the abovementioned studies andadditional numerical studies suggest greater solute leaching under
tillage and compaction. The dominant transport was identied as being a stochastic-convective process
in both lysimeters. The similarity of the mixing regime for the two soil columns can be explained by
preferential solute trajectories activated within structural macropores as a result of the high ow rates
applied.We show that the relationship between tillage practices and transport is complex, not only scale
and time dependent but also inuenced by the boundary conditions and tillage practices.
2010 Elsevier B.V. All rights reserved.
Abbreviations: CT, conventional tillage; RT, reduced tillage; TDR, Time Domain
Reectometry; T1, T2, T3, TDR transects; zm, mean soil depth; Jw1, steady state ow
rate equal to 5 cm per day (cm d1); Jw2, steady state ow rate equal to 20 cm perday (cm d1); DL, longitudinal dispersion coefcient; v, mean solute velocity; lL,longitudinal hydrodynamic dispersivity; Crt*, time-normalized resident concentra-
tion; superscript l, local parameters; superscript m, depth averaged parameters;
R, retardation factor; BTCs, breakthrough curves.
* Corresponding author at: Soil Science Unit, INRA 2163 Avenue de la Pomme de
Pin, CS 40001, Ardon, 45075 Orleans Cedex 2, France. Tel.: +33 02 38 41 78 00;
fax: +33 02 38 41 78 79.
E-mail address: Arlene.Besson@orleans.inra.fr (A. Besson).
0167-1987/$ see front matter 2010 Elsevier B.V. All rights reserved.doi:10.1016/j.still.2010.11.001A. Besson a,b,*, M. Javaux a, C.L. Bielders a, M. Vancla Earth and Life Institute, Environmental Sciences, Universite Catholique de Louvain, Crb Soil Science Unit, INRA 2163 Avenue de la Pomme de Pin, CS 40001, Ardon, 45075 O
A R T I C L E I N F O
Received 9 June 2010
Received in revised form 31 October 2010
Accepted 8 November 2010
Available online 8 December 2010
TDR breakthrough curves
A B S T R A C T
Soil tillage practices can a
time. However, the relatio
understood. Within this p
mouldboard ploughing (C
loamy soil. Solute breakt
lysimeter collected in a CT
function theory. Importan
treatments. The CT treatm
as compared to the RT treloamy soil from leaching experiments
du Sud 2, BP 2, B-1348 Louvain-la-Neuve, Belgium
s Cedex 2, France
t water ow and solute transport processes dynamically in space and in
ip between tillage practices and ow and transport in soils is not yet well
r, we analyze the short term impact of the conversion from conventional
o reduced disc harrowing (RT) on the solute transport process within a
ugh experiments at two ow rates were performed on 2 undisturbed
d RT eld plot. Solute transport parameters were estimated using transfer
ifferences in solute transport were observed between the RT and the CT
exhibited a rapid, more homogeneous and less dispersive solute transport
ent. These results are explained by the changes in soil structure due to
l sev ier .com/ locate /s t i l l
conservation tillage (Masse et al., 1996; Isensee and Sadeghi,1997), other studies show signicantly opposite ndings with noeffect of tillage and even increased ow and transport inconventionally tilled soils (Granovsky et al., 1993; Levanonet al., 1993; Clay et al., 1998).
Differences in climate, in soil type, in physico-chemical soilproperties, in soil historicalmanagement, in crop type residues, inexperimental design and in spacetime variation of initial soilconditions, can overwhelm tillage effects (Logsdon et al., 1993;Logsdon and Jaynes, 1996; Alletto et al., 2010). In addition theimpact of tillage depends on intrinsic soil properties, on tillagecharacteristics such as tillage type, depth and speed and the levelof exerted mechanical stress. In most experimental studies, theimpact of two extreme tillage systems, i.e. conventional versusconservative, on ow and transport has been analyzed. Fewstudies analyze the impact of the mechanical stress applied tosoils, i.e. for instance primary tillage (mouldboard ploughing)versus secondary tillage (harrowing), on ow and transport,notwithstanding tillage implements, and then the tillage intensi-ty, have been reported to have an important impact on ow andtransport (Jarvis, 2007).
Given the above mentioned debate, more detailed studies onthe impact of tillage on ow and transport, in particular themechanisms responsible for the control of tillage on ow and
2. Materials and methods
2.1. Column sampling and soil characteristics
The experimental study was performed in the laboratory ontwo, 0.5 m3 undisturbed lysimeters (0.8 m diameter 1 m height)sampled in a Eutric Luvisol (FAO, 2006) in Louvain-la-Neuve,Belgium. The soil sampling technique was described by Vancloos-ter et al. (1995). The soil prole encompasses (1) a loamy Ahorizon, susceptible to structure deterioration when tilled, giventhe high silt content (about 66%) and (2) an argic Bt horizon with ahigher clay content (about 25%). Under eld conditions, the soil is awell drained loamy soil. Soil texture is summarized in Table 1.
The two soil columns were sampled in April 2007 on bare soilfrom the same eld, which had been previously cultivated with asilage maize crop under conventional tillage. One day prior to
Table 1Soil texture of the A and Bt horizons of Eutric Luvisol.
Depth (m) Clay (gkg1) Silt (g kg1) Sand (g kg1)
A horizon 00.3 156 657 187
Bt horizon 0.31 250 489 261
d from in situ visual descriptions.
A. Besson et al. / Soil & Tillage Research 112 (2011) 475748transport, are needed. Such studies will allow elucidate thebenets of conversion tillage on soil functioning as compared toconventional tillage.
The objective of this study was to compare the impact ofconventional mouldboard tillage (CT), and reduced tillage (RT),consisting of harrowing without prior ploughing, on the solutetransport process within a loamy soil. Our study focused on theshort-termeffects of tillage on the solute transport process, i.e. thecomparison of the effect of changes in level of mechanical stress(CT and RT) on solute transport after one single reduced tillageoperation (RT) following a long term CT. From laboratorycontrolled transport experiments on undisturbed soil lysimeterscollected in eld plots subject to a long term CT treatment,effective solute transport parameters were estimated. Thevariability of transport properties in terms of tillage practicewas analyzed at local and depth averaged scale for two differentow rates.
Fig. 1. Soil structure as schematizesampling, part of the eld was subjected to conventional tillage(CT), i.e. conventional mouldboard ploughing (030 cm) followedby disc harrowing (010 cm). Maize stalks were incorporated intothe topsoil during tillage. The other part of the eld was subjectedto supercial and reduced tillage (RT) consisting only in discharrowing (010 cm). One column (CT) was sampled from theconventionally tilled part and another column (RT) from thesupercially tilled part.
A visual description of soil structure was realized on two soilpits dug in the CT and RT parts, as shown schematically in Fig. 1.The top 10 cm of the soil had a ne, porous and rather uniformstructure for CT soil but encompassed a mix of porous zones anddense clods for the RT topsoil. In the lower part of the loamy Ahorizon (1530 cm depth), undecomposed organic residues,many earth-worm holes and roots, dense clods (0.130 dm3) oraggregates tightly packed were encountered for both tillagesystems with a 5-cm thick plough pan identied at about35 cmdepth. However, the no-tilled horizon of RT soil presented lessorganic residue andwasmore strongly consolidated. To complete
the physical description of soils, bulk densitymeasurementswereperformed bymeans of undisturbed soil cores (100 cm3) sampledin situ at several soil depths and close to the soil lysimeters, i.e. inthe ploughed (CT) plot (n = 39) and in the unploughed (RT) plot(n = 30). Bulk densities variability is used to comfort solutetransport results as described in Section 4.
In the laboratory, each lysimeter was equipped to monitorelectrical impedance, soil permittivity, soil temperature and outletand inlet ow (Fig. 2).
Electrical impedance and permittivity were monitored every15 min at 6 soil depths using twelve Time Domain Reectometryprobes (TDR). TDR probes consisted of three stainless steel rods(42.5 cm long and 0.5 cmdiameter) with 2.5 cm rod spacing. Theywere all horizontally inserted into the soil along three verticaltransects spaced 1208 apart (Fig. 2). The rst transect (T1)consisted of six probes placed at 15, 30, 45, 60, 75 and90 cm depth from the soil surface, the second (T2) consisted ofthree probes placed at 13, 43 and 73 cm depth and the third(T3) consisted of three probes placed at 17, 47 and 77 cmdepth. This specic spatial conguration initially developed byJavaux (2004b) aimed at increasing the horizontal spatialresolution of measurements at the soil prole scale. In thisway, each of the three soil depths (zm) of about 15 cm, 45 cmand 75 cm were monitored by three probes.
TDR measurements were performed using a TDR 100 TimeDomain Reectometer connected to a SDMX50multiplexer system
addition to TDR probes, four temperature NTC probes (NegativeTemperature Coefcient thermistors) were horizontally insertedinto each column at 15, 45, 75 and 90 cm depth (Fig. 2).Temperature measurements were recorded every 15 min. Meantemperatures at 30 and 60 cm depth were calculated frommeasurements performed respectively at 15, 45 cm depth andat 45, 75 cm depth. The electrical impedance dataset was thencorrected for the temperature effect by the well known Keller andFrischknecht equation (Keller and Frischknecht, 1966) at 25 8C,with a temperature coefcient equal to 0.025.
2.3. Upper and bottom boundary conditions
The incoming ow rate was controlled during the inltrationexperiments by a water applicator on each column. The waterapplicator was made of a square reservoir (80 cm 80 cm 1 cm)at the bottomofwhich 280 needles (0.5 mmdiameter)were placedon a regular grid (5 cm 5 cm) (Fig. 2). The applicators werecontinuously fed with water (780 mS/cm) by means of a pressurepump and were placed at 30 cm height from the soil surface. Onecentimeter of ne gravel was spread over the soil surface in orderto avoid surface sealing by the falling raindrops. The lysimeterbottom consisted of a plate with an outlet gate embedded at itscentre from which a drainage tube was connected to a tippingbucket device (GME, Type PR12). The number of tips was recordedevery 30 min. The outow time series were then obtained bytransforming the tipping frequency into an instantaneous ow rateand used to verify whether steady state had been achieved. Thebottom boundary water condition was a seepage face.
A. Besson et al. / Soil & Tillage Research 112 (2011) 4757 49(Campbell Scientic Inc., UK)whichwas controlled by a data loggerand PC computer. From the TDR signals, soil permittivity andimpedances were recorded every 15 min. Soil permittivity valueswere converted into soil volumetric water content using Toppscalibration model (Topp et al., 1980).
Knowing that the variability of soil temperature, for instanceinduced by water application, can impact electrical impedance, in[(Fig._2)TD$FIG]
Fig. 2. Experimental set up for transpo2.4. Transport experiments
After sampling, the soil columns were stored for one year in thelaboratory (1520 8C) without any disturbance and irrigation.During storing, soil columns were closed to restrict soilevaporation. Initial soil water content, i.e. before any inltration
periments in undisturbed lysimeters.
experiments, was relatively wet, close to the water eld capacity.Unsaturated ow experiments were then performed on thelysimeters. A rst constant ow rate Jw1, equal to 5 cm per day(cm d1), was imposed at the soil surface until a steady state owrate was obtained. Slightly mineralized water (780 mS cm1) wasused. Secondly, while maintaining the steady state ow rate Jw1, aDirac like pulse of CaCl2 solution (0.5 mol l
1) was applieduniformly at the soil surface. The application duration was equalto 10 h. Subsequently, the slightly mineralized water(780 mS cm1) was applied with the same ow rate Jw1.
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