Soil & Tillage Research 124 (2012) 24–31

Long-term impact of irrigation with olive mill wastewater on aggregate propertiesin the top soil

Mustafa Mahmoud a, Manon Janssen a, Stephan Peth b, Rainer Horn b, Bernd Lennartz a,*a Soil Physics and Resources Conservation, Faculty of Agricultural and Environmental Sciences, Rostock University, Justus-von-Liebig-Weg 6, 18059 Rostock, Germanyb Department of Plant Nutrition and Soil Science, Christian-Albrechts-Univ. Kiel, Hermann-Rodewald-Str. 2, 24118 Kiel, Germany

A R T I C L E I N F O

Article history:

Received 9 August 2011

Received in revised form 29 March 2012

Accepted 4 April 2012

Keywords:

Olive mill wastewater

Soil aggregate stability

Effective diffusion coefficient

Micro-tomography

Structure of soil aggregate

Solute transport

A B S T R A C T

Olive mill wastewater (OMW) is the main waste product generated by the olive oil extraction process.

The uncontrolled disposal of OMW is becoming a serious environmental problem. The objective of this

study was to investigate the long-term effects of OMW irrigation on soil aggregate stability, on solute

diffusion into aggregates and on aggregate structure formation. The soil aggregates were sampled from

three sites: non-irrigated with OMW (T0) and regularly irrigated with untreated OMW for 5 (T5) and 15

(T15) years. The results showed that the regular application of OMW for 5 and 15 years increased the soil

aggregate stability, as a result of a rising organic matter content of OMW sites. OMW application

furthermore reduced the effective diffusion coefficient into aggregates, because the organic matter of

OMW forms a coating on the aggregates and blocks the pore mouths. OMW is characterized by its

adhesive behaviour that affects the aggregation and structure of the topsoil by binding micro-aggregates

together to form macro-aggregates and larger pore spaces between micro-aggregates. Consequently, the

use of OMW for irrigation over long time periods alters the surface layer of the soil and makes it

fragmented, which may increase the risk for preferential solute transport.

� 2012 Elsevier B.V. All rights reserved.

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1. Introduction

Olive (Olea europaea) oil is a typical and valuable agro-industrialproduct in Syria, which is ranked fifth in the world in production ofolive oil. The average production of olive fruits and olive oil reaches880 and 175 thousand tons, respectively (Al-Ashkar, 2007). Thereare 920 olive mills which are scattered all over the country, and thenumber is expected to rise in the future due to the rapid increase ofolive production (UN, 2009). The main waste generated by the oliveoil extraction process is olive mill wastewater (OMW). The totalwastewater from Syria’s olive oil production amounts to 900,000 m3

per year during the olive mill season from early October to lateDecember (Directorate of Olive Bureau, Ministry of Agriculture andAgrarian Reform, Arab Republic of Syria, personal communication).In the olive growing countries of the Mediterranean, more than 30million m3 OMW are produced per year (D’Annibale et al., 2004).

The OMW characteristics depend on the olive variety andripeness, climate and soil conditions and the oil extraction method.Olive oil production involves one of the following extractionprocesses: (i) press olive oil extraction, (ii) three-phase centrifugalolive oil extraction and (iii) two-phase centrifugal olive oilextraction. The volume of OMW produced in traditional presses

* Corresponding author. Tel.: +49 381 4983180; fax: +49 381 4983122.

E-mail address: Bernd.Lennartz@uni-rostock.de (B. Lennartz).

0167-1987/$ – see front matter � 2012 Elsevier B.V. All rights reserved.

http://dx.doi.org/10.1016/j.still.2012.04.002

and in three-phase extraction systems amounts to about 600 and1000 l per ton of processed olives, respectively, while it is muchlower in the two-phase process (Azbar et al., 2004). In general, OMWhas an extremely high biological and chemical oxygen demand, ahigh organic matter content (polysaccharides, sugars, polyalcohols,proteins organic acids and oil), and contains large amounts ofsuspended solids and mineral elements (Niaounakis and Halvadakis,2004). Moreover, OMW contains high concentrations of phenoliccompounds that are phytotoxic and difficult to biodegrade(Nikolopoulou and Kalogerakis, 2007). Mekki et al. (2007) detectedphenolic compounds at a depth of 1.2 m four months after the lastapplication of OMW, and a moderate phytotoxic residual phenolicfraction was extracted from the topsoil layer one year after OMWapplication. OMW has been shown to inhibit the arbuscularmycorrhizal fungal root colonization, which reduces the nutrientuptake of the olive trees (Mechria et al., 2011).

The need for disposal of OMW on the one hand, and waterscarcity and low soil fertility in olive producing countries on theother hand, has brought about that large amounts of OMW areused for irrigation and fertility purposes in Syria and otherMediterranean countries. The law No. 190/T issued in 2007 by theMinistry of Agriculture in Syria allows the spreading of up to50 m3 ha�1 OMW from traditional presses and up to 80 m3 ha�1

OMW from modern centrifugal mills; the very same applicationrates are also permitted, e.g., in Italy (Giuffrida, 2010). OMWapplication, however, has been found to significantly affect the

Table 1Selected physicochemical properties of the olive mill wastewater.

Parameter Value

pH 5.07

Dry matter (g l�1) 33.25

Total organic carbon (g l�1) 30.57

Mineral matter (g l�1) 2.69

Soluble Cl� (mg l�1) 763.8

Soluble Na+ (mg l�1) 128.8

Soluble K+ (mg l�1) 1050.9

Soluble Ca2+ (mg l�1) 137.5

Soluble Mg2+ (mg l�1) 168.3

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–31 25

soil’s biological (Saadi et al., 2007; Mechri et al., 2008), chemical(Jarboui et al., 2008; Lopez-Pineiro et al., 2008; Kavvadias et al.,2010) and physical properties. The long-term application ofuntreated OMW decreases the saturated hydraulic conductivity,and increases the soil’s disposition to water repellency (Mahmoudet al., 2010). The enhanced organic matter content of OMWfurthermore increases sorption and degradation processes, andOMW application may therefore retard the mobility of pesticides(Cox et al., 1997).

The land application of olive oil solid waste has been shown toimprove the soil aggregate stability (Kavdır and Killi, 2008),because it increases the organic matter content and soil fertility(Yaakoubi et al., 2010). Aggregate stability is important inevaluating erosion resistance, soil permeability, steady stateinfiltration rate, and seedling emergence and in predicting thecapacity of soils to sustain long-term crop production (Letey,1985). However, the effect of liquid OMW application on aggregatestability has not been investigated so far.

OMW organic matter and contained residues of oil and greaseform a coating on soil aggregates and pore walls (Bisdom et al.,1993), which may reduce anion diffusion into soil aggregates. Kohneet al. (2002) found that films or skins on the surface of aggregatesdecrease the diffusive mass transfer of anions between interag-gregate regions and the soil matrix. The rate of diffusion furthermoredecreases with decreased porosity (Hanson and Nex, 1953). Thus, itis expected that the OMW will affect the rate of diffusion into soilaggregates, since the long-term application of OMW decreases thesoil drainable porosity (F < 30 mm; Mahmoud et al., 2010), andchanges the pore size distribution by a decrease of macropores andan increase of micropores (Cox et al., 1997).

The objective of this study was to investigate the long-termimpact of OMW application on soil aggregate stability, onmorphological features of aggregates, and on anion diffusion.

2. Materials and methods

2.1. Experimental sites and sampling of soil aggregates

All field sites are located in Saida village, south-western Syria(328640N, 368180E), at an altitude of 435 m a.s.l. The sites have aMediterranean climate with an average annual precipitation of200 mm and an average temperature of 35 8C in summer and 12 8Cin winter. In January 2009, soil aggregates were sampled at threeexperimental field sites: non-irrigated with OMW (T0) andregularly irrigated with untreated OMW for 5 (T5) and 15 (T15)years. The fields were 350 m � 70 m (T0, T15) and 250 m � 70 m(T5) in size. Samples of soil aggregates were taken at 0–5 cm depth,they were randomly collected within each field. The olive orchardsat all selected sites were irrigated by drop irrigation during thegrowth period. During the time period of olive mill operation fromearly October to late December, however, the experimental plotsT5 and T15 were irrigated by furrow irrigation with untreatedOMW. The volume of OMW varied from year to year depending onthe amount of olive oil production per year. The amount of OMWapplied on experimental field sites T5 and T15 was not quantified.

The field sites T5 and T15 were plowed once in early spring andtwice in early summer and one to three times in late summerand autumn. The frequency of plowing depended on the amountand number of OMW applications and on weed density. The site T0was plowed once in early summer and two times in late summerand autumn.

2.2. Soil and OMW characteristics

The soil at the three sites has been classified as a Cambisolaccording to FAO classification (2006). Soil texture is silt loam, and

soil color is 7.5 YR 4/3 in all investigated profiles (down to 90 cmdepth). The soils are poor in organic matter. The typical claymineral is Montmorillonite, and the soils develop deep shrinkagecracks upon desiccation.

The main physicochemical characteristics of the OMW aregiven in Table 1. Additional details of soil and OMW properties arereported in Mahmoud et al. (2010).

2.3. Laboratory measurements

Total carbon and nitrogen contents were determined with theElementar Vario EL analyzer (Elementar Analysensysteme GmbH,Hanau, Germany) according to DIN ISO 10 694 (1996) and DIN ISO13 878 (1998), respectively, while carbonate content wasdetermined separately with the Scheibler equipment accordingto DIN ISO 10 693 (1995; measurement of the volume of degassingCO2 after addition of hydrochloric acid). Nine replications weremeasured for each treatment, three each for the aggregate stabilitytests: slow wetting (SW), fast wetting (FW) and wet stirring (WS).

Bulk density rb was determined by coating the single, air-driedaggregates with paraffin and measuring their volume in distilledwater (Urbanek and Horn, 2006). Four replications were analyzedfor each treatment. The particle density rs (g cm�3) was calculatedaccording to the following empirical equation (Schmidt, 1992):

rs ¼ 2:65 � ð0:022aÞ (1)

where a is the percentage of organic matter (%).The aggregate stability was determined using the Le Bissonnais

(1996) method, which allows distinguishing the different destruc-tion mechanisms causing aggregate breakdown (Rohoskova andValla, 2004). The experiment was repeated six times for each testand treatment. The soil aggregates were air-dried and gentlysieved to separate the macro-aggregates (3–5 mm in size) from themicro-aggregates. The macro-aggregates were then dried at 40 8Cfor 24 h before the stability analyses. The Le Bissonnais methodconsists of three tests with different wetting conditions andenergies: (i) Slow wetting (SW; wetting by capillarity): Thistreatment tests the behaviour of dry soils with a low moisturecontent to moderate rainfall. 5 g of aggregates were placed on afilter paper on a tension table at a matric potential of �30 hPa for60 min. (ii) Fast wetting (FW; wetting by immersion): Thistreatment tests the behaviour of dry soils to fast wetting suchas irrigation and heavy rainfall. 5 g of aggregates were gentlyimmersed in 50 cm3 of deionized water for 10 min, then the excesswater was sucked off with a pipette. (iii) Mechanical breakdownafter stirring of pre-wetted aggregates in ethanol (WS): Thistreatment tests the behaviour of dry soils to mechanical impacts.5 g of aggregates were gently immersed in 50 cm3 of ethanol for10 min, then the excess ethanol was removed with a pipette andthe soil material was transferred to an Erlenmeyer flask filled with200 cm3 of deionized water. The Erlenmeyer flask was shaken endover end 20 times, then the excess water was sucked off with a

Table 2Selected properties of the diffusion experiment for aggregates with 0 (T0), 5 (T5) and 15 (T15) years of olive mill wastewater application.

Parameter T0 T5 T15

Average thickness of porous layer, l (cm) 0.635 0.716 0.675

Average volume ratio of aggregate to free solution, k (cm cm�1) 0.110 0.125 0.128

Free solution depth, v (cm) 3.52 3.49 3.21

Initial concentration of Br�, Co (mg l�1) 22.1 21.5 21.4

Different letters within each row means significances at p < 0.05.

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–3126

pipette. Following these three tests, the aggregate samples weresuspended in ethanol, passed through a 63 mm sieve, and thesmaller particles and aggregates were discarded. Ethanol was usedfor wet sieving because slaking is greatly reduced when dryaggregates are immersed in ethanol, and particles do notreaggregate during drying and wetting (Merzouk and Blake,1991). The >63 mm fraction was oven-dried and gently dry-sievedby hand on a column of six sieves: 2000, 1000, 500, 200, 100 and63 mm. The mean weight diameter (MWD) was calculated as thesum of the mass fraction that remained in each sieve, multiplied bythe mean aperture of the adjoining sieves. In order to establish arelationship between the slow and fast wetting tests, the slakingresistance index (SRI) was calculated according to Eq. (2) (Ojedaet al., 2008):

SRI ¼ MWDFW

MWDSW(2)

where MWDFW and MWDSW are the mean weight diameters of thesoil aggregates after undergoing fast and slow wetting tests,respectively.

Anion diffusion experiments were carried out at a constantroom temperature of 20 8C with bromide as the tracer. Fivespherical, air-dried soil aggregates were used for each treatment,and each aggregate was enveloped in a fine nylon mesh to preserveits structure (Haws et al., 2004). The enveloped aggregates wereplaced in a tray filled with water to a depth just below theaggregates and allowed to soak for at least 10 h. Thereafter, theaggregates were slowly immersed in a potassium bromide solutionwith an initial bromide concentration of 35 mg l�1. The solutionwas gently stirred with a magnetic stirrer to create a completelymixed system (Kohne et al., 2002). The concentration of thebromide solution was measured by taking 1 ml of solution with apipette at intervals of increasing length over a 12 h period. At theend of each diffusion experiment, the wet and dry weights weremeasured so that the saturated water content us could becalculated using Eq. (3) (Black, 1965). The bromide concentrationwas determined by using an ion-chromatography system (Com-pact IC 761, Metrohm GmbH, Germany)

us ¼weight of wet soil ðgÞ � weight of dry soil ðgÞ

weight of dry soil ðgÞ (3)

The effective diffusion coefficient De (cm2 d�1) was calculatedaccording to Eq. (4):

De ¼ D0F (4)

where D0 = 1.797 cm2 d�1 is the diffusion coefficient in freesolution for bromide (Atkins, 1990), and the impedance factor F

is the ratio of molecular to effective diffusion coefficient. F wasestimated with a nonlinear optimization scheme minimizing thesum of squared differences between Eq. (5) and the normalizedmeasured bromide mass in free solution (C/C0; Kohne et al., 2002).

Mt ¼ ðM0 � MsampÞ �M0 � Msamp

end

1=kend þ 1

� 1 � 2 þ 2

k

� �X1i¼1

expð�D0Fa2i t=l2Þ

1 þ k þ a21=k

" #(5)

where Mt is the mass of solute in the well-stirred solution at time t,M0 is the initial mass of solute at time t = 0, Msamp is the cumulativemass at time t > 0, k is the ratio of the aggregates’ liquid volume tofree solution volume at time t, D0 = 1.797 cm2 d�1 is the diffusioncoefficient in free solution for bromide (Atkins, 1990), ai is thenonzero positive solution of tan(ai) = �aik

�1, l is the averagethickness of the porous layer (cm) [l ¼ kvu�1

s , v is the free solutiondepth (cm), us is the aggregate’s saturated water content (g g�1)],and the subscript (end) is the quantity at the end of the experiment.

Selected properties of the diffusion experiments are given inTable 2.

2.4. Microtomography measurements and analysis

Three soil aggregates were selected for each treatment on thebasis of similarities in form and size, and analyzed using a tube-based X-ray microtomography system (MF-mCT). Samples werescanned with a phoenix nanotom (GE Sensing & InspectionTechnologies GmbH, Wunstorf, Germany) at the University of Kielwith an X-ray energy of 90 keV and a current of 310 mA. For eachaggregate 1440 projections were acquired and reconstructed to a3D volume using the software datosjx (GE Sensing & InspectionTechnologies GmbH, Wunstorf, Germany). Voxel edge length afterreconstruction was 41.1 mm for all samples. The reconstructedvolumes were processed with various algorithms to transform andquantify the image datasets. To render and visualize the 3D porestructures we used Visual Studio Max 2.0 (Volume Graphics GmbH,Heidelberg, Germany). A 300 � 300 � 300 voxel sized cube wasextracted from the original microtomograms from near theaggregate center for further data analysis. The size of the extractedcube was restricted by the aggregate boundary normal to theimage slice.

The statistical analyses of the data, especially the test onsignificance of differences between samples means (Student’s t-test), were performed using the software package SPSS 15.

3. Results and discussion

3.1. Aggregate characteristics

As a result of long-term irrigation with OMW, the organic Ccontent in the soil aggregates increased from 0.79% in the control to3.20 and 3.28% after 5 and 15 years of OMW application, respectively(Table 3). The corresponding values for total N were 0.13, 0.41 and0.34%. The C/N ratio thus increased from 6.08 to 7.80 and 9.65 after 5and 15 years of OMW application, as a result of the large C/N ratio ofOMW (Mekki et al., 2007). Various other studies have shown anincrease of the organic C content in the bulk soil following OMWirrigation (e.g., Yaakoubi et al., 2010; Chartzoulakis et al., 2010).

The bulk density and particle density both gradually decreasedfrom T0 to T15. The reason is the higher organic C content in the soilfollowing OMW irrigation. The organic materials are less dense thanthe mineral fraction of soils (Pagliai et al., 1981), and in consequence,their presence reduces particle density and soil bulk density.Furthermore, organic compounds help binding individual soilparticles together to form aggregates, which leads to an increasein soil porosity, as the pore size between aggregates is larger than

Table 3Selected properties of soil aggregates with 0 (T0), 5 (T5) and 15 (T15) years of olive mill wastewater application.

Parameters T0 T5 T15

Corg (%) 0.79 � 0.08 a 3.20 � 0.54 b 3.28 � 0.17 b

N (%) 0.13 � 0.01 a 0.41 � 0.02 b 0.34 � 0.03 b

C/N 6.08 a 7.80 b 9. 65 c

Bulk density, rb (g cm�3) 1.33 � 0.02 a 1.15 � 0.01 b 0.99 � 0.02 c

Particle density, rp (g cm�3) 2.63 � 0.01 a 2.57 � 0.02 b 2.54 � 0.06 b

Saturated water content, us (g g�1) 0. 49 a 0. 55 b 0. 61 c

CaCO3 (%) 16.43 � 0.52 a 20.68 � 1.35 b 22.30 � 0.75 c

Different letters within each row means significances at p < 0.05.

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–31 27

that between single soil particles. The bulk density of the aggregatesdiffered significantly between T5 and T15, demonstrating that theimpact of OMW depends on application duration.

An increasing carbonate content in the aggregates wasobserved from T0 to T15. This finding can be explained by differingcarbonate contents in the parent material; an increasing carbonatedepletion was observed in the topsoil compared with the subsoil(Mahmoud et al., 2010). Sierra et al. (2001) found that the acidity ofthe untreated OMW was compensated by the soil carbonatealkalinity, and that the soil carbonate was redistributed in the soil.

3.2. Effect of OMW on aggregate stability

The size distributions of aggregates for the T5 and T15treatments were characterized by a very high proportion of

0

200

400

600

800

1000

Diame

Fra

ctio

n o

f m

ass

[g k

g-1

]

a

b

c

ab

c

a

b

c

0

200

400

600

800

1000

> 2 2-1 1-0.5

b

c

c

b

a

a ab

a

0

200

400

600

800

1000

a

b

c

a

bc b

a

c

Fig. 1. Mean values of the percentage of aggregates in each size fraction for the three aggr

control; T5 and T15: 5 and 15 years of olive mill wastewater application. Error bars indica

differences at p < 0.05.

>2 mm aggregates for SW, FW and WS tests, while the aggregatesize distribution was more homogeneous in the control treatmentT0 (Fig. 1). The percentage of >2 mm aggregates was greater forT15 than for T5 in all three tests. The reason for the differentaggregate size distribution is probably the increasing organic Ccontent from 0.79% in the control to 3.20 and 3.28% after 5 and 15years of OMW application, respectively (Table 3). Organic matterplays a very important role in bonding soil particles or smalleraggregates together (Tisdall and Oades, 1982; Emerson, 2008).Lopez-Pineiro et al. (2007) reported a positive and highlysignificant relationship between humic and soil organic mattercontents and the aggregate stability.

The soil aggregate stability decreased in the following order:SW > WS > FW. The reason for this result is that the slaking of theaggregates, which plays an important role in aggregate breakdown,

T0T5T15

SW

tric class [mm]

a

b bab b a b b a b c

0.5-0.2 0.2 -0.1 0.1 -0.063 <0.063

WS

aa

b

a

b b

a

b b

a

b c

FW

a

bc

a

b c

ab ac b b

egate stability tests: (SW) slow wetting, (FW) fast wetting and (WS) wet stirring. T0:

te standard deviation. Different letters within each diametric class denote significant

Table 4Mean weight diameter (MWD) and class of stability after 0 (T0), 5 (T5) and 15 (T15)

years of olive mill wastewater application for three aggregate stability tests: slow

wetting (SW), fast wetting (FW) and wet stirring (WS). Mean � standard deviation.

Treatments Corg (%) MWD (mm) Stability classa

SWT0 0.88 � 0.05 1.23 � 0.14 a Medium

T5 3.67 � 0.01 2.72 � 0.16 b Very stable

T15 3.50 � 0.01 3.39 � 0.06 c Very stable

FWT0 0.72 � 0.02 0.62 � 0.14 a Unstable

T5 2.52 � 0.05 1.82 � 0.17 b Stable

T15 3.16 � 0.01 2.67 � 0.05 c Very stable

WST0 0.77 � 0.03 0.91 � 0.25 a Medium

T5 3.42 � 0.03 2.47 � 0.13 b Very stable

T15 3.19 � 0.09 3.05 � 0.06 c Very stable

Different letters within each test means significances at p < 0.05.a According to Le Bissonnais (1996).

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–3128

increases with increasing wetting rate. Aggregate slaking furtherincreases with increasing clay content of the soil (Ben-Hur andLado, 2008). The clay content in 0–5 cm depth was 24.2, 21.9 and18.6% in T0, T5, and T15, respectively (Mahmoud et al., 2010). Fastwetting of aggregates with high clay (such as Montmorillonite)content increases the extent of differential swelling and thevolume of entrapped air in pore space that, in turn, increasesaggregate breakdown (Mbagwu and Auerswald, 1999). Slowwetting of the aggregates, in contrast, prevents slaking, due to areduction in swelling of clays (Reichert et al., 2007). Slaking isgreatly reduced when dry aggregates are immersed in ethanol as inthe WS test (Merzouk and Blake, 1991), and the rate of wetting andthe degree of swelling are reduced when aggregates pretreatedwith ethanol are immersed in water (Emerson and Greenland,

Rel

ati

ve

mass

, M

t/M

0R

elati

ve

mass

, M

t/M

0R

elati

ve

mass

, M

t/M

0

42 0

1.00

0.95

0.85

0.90

1.00

0.95

0.85

0.90

1.00

0.95

0.85

0.90

T

Fig. 2. Relative mass of bromide in the stirred free solution during diffusion into soil aggr

application). The solid line is best-fit to data of the diffusion model (Eq. (4)).

1990). Table 4 shows that the samples used for the SW tests had aslightly higher organic C content than those used for WS and FWtests, which might have further enhanced the aggregate stability.

The MWD, as a measure of the aggregate stability, confirmedthat aggregate stability increased due to the long-term applicationof OMW. The MWD values differed significantly between the threetreatments in all three tests and decreased from T15 to T0 (Table 4),which is in line with the decrease of soil aggregate fractions<2 mm in the OMW treatments. The mean MWD values for the WStest indicate that the cohesion of wet soil improved in the soilstreated with OMW. The SW test displayed the largest MWD valuesfor the OMW treatments This is likely to be because of the higherorganic C content and carbohydrate content induced by theapplication of OMW (Lopez-Pineiro et al., 2007) to the soil. Theorganic matter increases the resistance of aggregates to slaking andto differential swelling of clays by increasing their internalcohesion through the binding of mineral particles by means oforganic polymers or through the physical enmeshment of theparticles by roots or fungi (Tisdall and Oades, 1982; Chenu et al.,2000). Zaher et al. (2005) reported that greater quantities of soilorganic matter decreased pore pressures and swelling duringrewetting and contributed to improve the aggregate stability of thesoil. The regular application of OMW has been shown to increasesoil hydrophobicity (Mahmoud et al., 2010), which reduces theextent of clay swelling and thereby reduces the extent of aggregatedisruption by microfissuration (Chenu et al., 2000). The effect ofthe enhanced ploughing frequency in the OMW treatments, whichin general decreases aggregate stability, was superimposed by theeffect of the increased organic C content.

The carbonate content was larger at sites T5 and T15 ascompared with T0 (Table 3), and the MWD was related to thecarbonate content with a very significant correlation coefficient ofR = 0.97. Al-ani and Dudas (1988) reported that increasing

measuredmodelled

T0

measuredmodelled

T15

1086 12

ime [h]

measured

modelled

T5

egates for the sites T0 (control), T5 and T15 (5 and 15 years of olive mill wastewater

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–31 29

carbonate content increased the MWD. Calcium carbonate can actas a bonding or cementing agent between soil particles (Rimmerand Greenland, 1976), and binding of the soil organic matter to clayminerals increases the stability of soil aggregates (Oades, 1984;Muneer and Oades, 1989).

The SRI was 0.50, 0.64 and 0.79 for T0, T5 and T15, respectively.These values indicate a greater difference in the distribution ofaggregate sizes generated by the fast and by the slow wetting testin the control in comparison with OMW treatments. The higher SRIvalues of T5 and T15 compared with the control are attributed to anincreased hydrophobicity that reduced disaggregation processesdue to slaking (Ojeda et al., 2008).

Fig. 3. Two-dimensional tomography images for three soil aggregates (T0: control; T5 an

two-dimensional representation of the extracted and analyzed cubical sub-volume. Gray

top: X-Y plane; front: X-Z plane.

Increased aggregate stability at the surface improves the soil’sresistance to wind and water erosion (Blanco-Canqui et al., 2008),which is a widespread problem in olive groves (e.g., Barneveldet al., 2009). Increased aggregate stability may reduce runoffresulting from an increase in soil aggregation, because the waterinfiltration rate tends to be higher on well-aggregated soils(Sukartaatmadja et al., 2002).

3.3. Effect of OMW on the diffusion rate into soil aggregates

In the diffusion experiments, the ratio Mt/M0 of the free solutionfor bromide decreased more slowly in the solutions in contact with

d T15: 5 and 15 years of olive mill wastewater application). Black line indicates the

colour represents the soil and black colour represents soil pores. Right: Y-Z plane;

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–3130

T5 and T15 aggregates than in those in contact with the T0aggregates. At the end of the experiment, Mend/M0 was 0.87, 0.92and 0.93 for T0, T5, and T15 treatments, respectively (Fig. 2).Understanding the diffusive transport of ions into saturated soilaggregate samples requires knowledge of the impedance ortortuosity factor. This factor is a property of the porous mediumand accounts primarily for the tortuosity of the soil pore networkinto which the solute diffuses (Kirk et al., 2003). For the aggregatesof the T0 treatment, F was twice as large as for the aggregates of T5and T15 treatments (Table 5). This may be attributed to a decreasein bulk density of the soil aggregates from 1.33 g cm�3 in thecontrol (T0) to 1.15 and 0.99 g cm�3 in T5 and T15, respectively(Table 3). This result is in agreement with Warncke and Barber(1972). They concluded that compaction of the soil causes part ofthe liquid discontinuities to be filled, resulting in a less tortuousdiffusion path and therefore an increase in F.

The effective diffusion coefficient into soil aggregates (D0)decreased from 1.746 cm2 d�1 in the control (T0) to 0.893 cm2 d�1

(T5) and 0.815 cm2 d�1 (T15) as a consequence of 5 and 15 years ofOMW application. This is likely to be because the organic matter ofthe OMW forms a coating on the aggregates (Bisdom et al., 1993),and these films or coatings decrease the diffusive mass transfer ofthe bromide anion between the inter-aggregate region and the soilmatrix (Kohne et al., 2002). Yasyerli et al. (1999) reported that theeffective diffusivity of bromide into aggregates decreased with achange in pore structure, when filling the micropores of soilaggregates with organic matter blocked the pore mouths.Additionally, the diffusion of bromide is reduced when the waterrepellency of the top soil increases (Van Dam et al., 1990). Anincrease of the water repellency as a result of OMW application hasbeen shown for the study sites (Mahmoud et al., 2010). Celis et al.(1996) reported that the association of organic matter to themineral soil components reduces the sorption capacity of themineral and organic soil colloids.

The slow diffusion of solutes into aggregates decreases theamount of adsorbed ions in the pores of the aggregates, and it alsodecreases the amount that is taken up by the surface of pores (Nyeand Staunton, 1994). Thus, it increases the possibility forgroundwater pollution by organic and phenolic compounds ofOMW.

3.4. Effect of OMW on the structure of soil aggregates

Three spherical soil aggregates from each treatment were usedfor illustration purposes and as case studies. The average radius ofthe aggregates was 17.41, 20.76 and 19.58 mm for T0, T5 and T15,respectively. A single two-dimensional slice was taken fromapproximately the middle of each aggregate to visualize the long-term effect of OMW application on soil aggregate structure (Fig. 3).The figure shows marked differences in CT-detected soil pore spacebetween T0 and the two OMW treatments (T5 and T15). Relativelysmaller visual differences were detected between the two OMWtreatments compared with the T0 treatment for the selectedregions within the figure. The aggregates of longer-treated OMWtend to have a larger pore space and thus an increasingly looserstructure, which is in line with the decreasing bulk density (Table3). The increase in larger pores in T5 and T15 can be attributed to

Table 5Parameters obtained by fitting the model (Eq. (5)) to measured data. F, impedance

factor; De, effective diffusion coefficient of Br�; r2, coefficient of determination.

T0 T5 T15

F 0.972 0.497 0.462

De (cm2 d�1) 1.747 0.893 0.815

r2 0.949 0.971 0.956

the more frequent plowing of the soil compared with the controltreatment (T0), because the soil breakdown by tillage forms largeinter-aggregate pore space (Or et al., 2000). In addition, OMWapplication increased the content of organic C, which acts like glueto cement micro-aggregates together to form macro-aggregatesand form larger pore space between micro-aggregates (Chaofu Weiet al., 2006). In the same experimental fields, Mahmoud et al.(2010) observed lower bulk densities and saturated hydraulicconductivities but higher porosities in OMW treatments comparedwith the T0 treatment. Greater porosity of the OMW treatmentscould be due to enhanced root growth. Lipiec and Hatano (2003)reported that soil with macropores offers greater potential for rootgrowth because the roots can bypass the zones of high mechanicalimpedance.

4. Conclusions

The irrigation of OMW for 5 and 15 years has been shown tohave distinct effects on the properties of soil aggregates. Both theporosity of the aggregates and the aggregate stability increasedwith OMW application. This is a consequence of the enhancedorganic C content, resulting from residual oil and grease, whichbinds the soil particles together. By that, on the one hand, largerinterspaces are created, and on the other hand, stability isincreased. CT images confirmed an increasing pore size and porespace with increasing duration of OMW application. Thediffusion rate into the aggregates decreased after OMWapplication, indicating that the organic C was not homogeneous-ly distributed in the soil matrix, but formed films on theaggregate surface. The reduced solute diffusion into soilaggregates could lead to an increased leaching of dissolvedsubstances. The presented results refer to a silt loam soil. For asandy soil, we would expect on the one hand a larger impact ofOMW application on hydrophobicity and diffusion rate. Withdecreasing clay content, the specific surface area decreases, andless hydrophobic compounds are required to coat the particlesurfaces (Huffman et al., 2001). On the other hand, OMW wouldbe transported to deeper depths. As a conclusion, long-term andintensive application of OMW should be avoided in areas with ashallow groundwater table.

References

Al-ani, A.N., Dudas, M.J., 1988. Influence of calcium carbonate on mean weightdiameter of soil. Soil & Tillage Research 11, 19–26.

Al-Ashkar, H., 2007. Syrian olive oil comparative advantage. National AgriculturalPolicy Sector.In: Paper prepared for presentation at the I Mediterranean Con-ference of Agro-Food Social Scientists. 103rd EAAE Seminar ‘Adding Value to theAgro-Food Supply Chain in the Future Euromediterranean Space’, Barcelona,Spain, April 23–25.

Atkins, P.W., 1990. Physikalische Chemie. VCH-Verlagsgesellschaft, Weinheim,Germany (in German).

Azbar, N., Bayram, A., Filibeli, A., Muezzinoglu, A., Sengul, F., Ozer, A., 2004. A reviewof waste management options in olive oil production. Critical Review ofEnvironmental Science Technology 34, 209–247.

Barneveld, R.J., Bruggeman, A., Sterk, G., Turkelbloom, F., 2009. Comparison of twomethods for quantification of tillage erosion rates in olive orchards of northwestSyria. Soil & Tillage Research 103, 105–112.

Ben-Hur, M., Lado, M., 2008. Effect of soil wetting conditions on seal formation,runoff, and soil loss in arid and semiarid soils – a review. Australian Journal ofSoil Research 46, 191–202.

Bisdom, E.B.A., Dekker, L.W., Schoute, J.F.T., 1993. Water repellency of sieve frac-tions from sandy soils and relationships with organic material and soil struc-ture. Geoderma 56, 105–118.

Black, C.A., 1965. Methods of Soil Analysis: Part I. Physical and MineralogicalProperties. American Society of Agronomy, Madison, WI, USA.

Blanco-Canqui, H., Mikhay, M.M., Benjaminz, J.G., Stone, L.R., Schlegelyy Drew, A.J.,Lyonzz, J., Vigilx, M.F., Stahlman, P.W., 2008. Regional study of no-till impacts onnear-surface aggregate properties that influence soil erodibility. Soil ScienceSociety of America Journal 73, 1361–1368.

Celis, R., Cox, L., Hermosln, M.C., Cornejo, J., 1996. Retention of metamitron by modeland natural particulate matter. International Journal of Environmental Analyti-cal Chemistry 65, 245–260.

M. Mahmoud et al. / Soil & Tillage Research 124 (2012) 24–31 31

Chaofu Wei, G.A.O., Shao, M., Xie, J., Pan, D.G., 2006. Soil aggregate and its responseto land management practices. China Particuology 4, 211–219.

Chartzoulakis, K., Psarras, G., Moutsopoulou, M., Stefanoudaki, E., 2010. Applicationof olive mill wastewater to a Cretan olive orchard: effects on soil properties,plant performance and the environment. Agriculture, Ecosystems & Environ-ment 138, 293–298.

Chenu, C., Le Bissonnais, Y., Arrouays, D., 2000. Organic matter influence on claywettability and soil aggregate stability. Soil Science Society of America Journal64, 1479–1486.

Cox, L., Celis, R., Hermosin, M.C., Becker, A., Cornejo, J., 1997. Porosity and herbicideleaching in soils amended with olive mill wastewater. Agriculture, Ecosystems& Environment 65, 151–161.

D’Annibale, A., Ricci, M., Quaratino, D., Federici, F., Fenice, M., 2004. Panus tigrinusefficiently removes phenols, color and organic load from olive-mill wastewater.Research in Microbiology 155, 596–603.

DIN ISO 10 693, 1995. Bodenbeschaffenheit–Bestimmung des CarbonatgehaltesVolumetrisches Verfahren. Beuth Verlag, Berlin.

DIN ISO 10 694, 1996. Bodenbeschaffenheit–Bestimmung von organischem Koh-lenstoff und Gesamtkohlenstoff nach trockener Verbrennung (Elementarana-lyse). Beuth Verlag, Berlin.

DIN ISO 13 878, 1998. Bodenbeschaffenheit–Bestimmung des Gesamt-Stickstoffsdurch trockene Verbrennung (Elementaranalyse). Beuth Verlag, Berlin.

Emerson, W.W., 2008. Aggregate stability to drying and wetting. In: Chesworth, W.(Ed.), Encyclopedia of Soil Science, Part 1, Springer, Dordrecht, Netherlands,ISBN: 978-1-4020-3994-2pp. 28–30.

Emerson, W.W., Greenland, D.J., 1990. Soil aggregates – formation and stability. In:De Boodt, M., Hayes, M., Herbillon, A. (Eds.), Soil Colloids and their Associationsin Aggregates. Plenum Press, New York, pp. 485–511.

Giuffrida, M., 2010. European and Italian laws for the agronomic use of olive-oil millwastewaters. Terrestrial and Aquatic Environmental Toxicology 4, 1–6.

Hanson, W.J., Nex, R.W., 1953. Diffusion of ethylene dibromide in soils. Soil Science76, 209–214.

Haws, N.W., Das, B.S., Rao, P.S.C., 2004. Dual-domain solute transfer and transportprocesses: evaluation in batch and transport experiments. Journal of Contami-nant Hydrology 75, 257–280.

Huffman, E.L., MacDonald, L.H., Stednick, J.D., 2001. Strength and persistence of fire-induced soil hydrophobicity under ponderosa and lodgepole pine, ColoradoFront Range. Hydrological Processes 15, 2877–2892.

Jarboui, R., Sellami, F., Kharroubi, A., Gharsallah, N., Ammar, E., 2008. Olive millwastewater stabilization in open-air ponds: impact on clay-sandy soil. Bior-esource Technology 99, 7699–7708.

Kavdır, Y., Killi, D., 2008. Influence of olive oil solid waste applications on soil pH,electrical conductivity, soil nitrogen transformations, carbon content and ag-gregate stability. Bioresource Technology 99, 2326–2332.

Kavvadias, V., Doula, M.K., Komnitsas, K., Liakopoulou, N., 2010. Disposal of olive oilmill wastes in evaporation ponds: effects on soil properties. Journal of Hazard-ous Materials 182, 144–155.

Kirk, G.J.D., Solivas, J.L., Alberto, M.C., 2003. Effects of flooding and redox conditionson solute diffusion in soil. European Journal of Soil Science 54, 617–624.

Kohne, J.M., Gerke, H.H., Kohne, S., 2002. Effective diffusion coefficients ofsoil aggregates with surface skins. Soil Science Society of America Journal66, 1430–1438.

Le Bissonnais, Y., 1996. Aggregate stability and assessment of soil and erodibility: 1.Theory and methodology. European Journal of Soil Science 47, 425–431.

Letey, J., 1985. Relationship between soil physical properties and crop production.Advances in Soil Sciences 1, 277–294.

Lipiec, J., Hatano, R., 2003. Quantification of compaction effects on soil physicalproperties and crop growth. Geoderma 116, 107–136.

Lopez-Pineiro, A., Fernandez, J., Albarran, A., Rato Nunes, J.M., Barreto, C., 2008.Effects of de-oiled two-phase olive mill waste on Mediterranean soils and thewheat crop. Soil Science Society of America Journal 72, 424–430.

Lopez-Pineiro, A., Murillo, S., Barreto, C., Munoz, A., Rato, J.M., Albarran, A., Garcıa,A., 2007. Changes in organic matter and residual effect of amendment with two-phase olive-mill waste on degraded agricultural soils. Science of the TotalEnvironment 378, 84–89.

Mahmoud, M., Janssen, M., Haboub, N., Nassour, A., Lennartz, B., 2010. The impact ofolive mill wastewater application on flow and transport properties in soils. Soil& Tillage Research 107, 36–41.

Mbagwu, J.S.C., Auerswald, K., 1999. Relationship of percolation stability of soilaggregates to land use, selected properties, structural indices and simulatedrainfall erosion. Soil & Tillage Research 50, 197–206.

Mechri, B., Ben Mariem, F., Baham, M., Ben Elhadj, S., Hammami, M., 2008. Change insoil properties and the soil microbiological community following land spread-ing of olive mill wastewater affects olive trees key physiological parameters and

the abundance of arbuscular mycorrhizal fungi. Soil Biology and Biochemistry40, 152–161.

Mechria, B., Chehebb, H., Boussadiab, O., Attia, b.F., Ben Mariemb, F., Brahamb, M.,Hammamia, M., 2011. Effects of agronomic application of olive mill waste-water in a field of olive trees on carbohydrate profiles, chlorophyll a fluores-cence and mineral nutrient content. Environmental and Experimental Botany71, 184–191.

Mekki, A., Dhouib, A., Sayadi, S., 2007. Polyphenols dynamics and phytotoxicity in asoil amended by olive mill wastewaters. Journal of Environmental Management84, 134–140.

Merzouk, A., Blake, G.R., 1991. Indices for the estimation of interrill erodibility ofMoroccan soils. Catena 18, 537–550.

Muneer, M., Oades, J.M., 1989. The role of Ca-organic interactions in soil aggregatestability. I. Laboratory studies with glucose 14C, CaCO3 and CaSO4�2H2O. Aus-tralian Journal of Soil Research 27, 389–399.

Niaounakis, M., Halvadakis, C.P., 2004. Olive-mill Waste Management: LiteratureReview and Patent Survey. Typothito–George Dardanos, Greece.

Nikolopoulou, M., Kalogerakis, N., 2007. Design of a phytoremediation strategy forolive mill wastewater treatment. In: The 10th International Conference onEnvironmental Science and Technology, Kos Island, Greece, 5–7 September, pp.1029–1036.

Nye, P.H., Staunton, S., 1994. The self-diffusion of strongly adsorbed anions in soil:two path model to simulate restricted access to exchange sites. EuropeanJournal of Soil Science 45, 145–152.

Ojeda, G., Alcaniz, J.M., Le Bissonnais, Y., 2008. Differences in aggregate stability dueto various sewage sludge treatments on a Mediterranean calcareous soil.Agriculture, Ecosystems & Environment 125, 48–56.

Oades, J.M., 1984. Soil organic matter and structural stability: mechanisms andimplications for management. Plant and Soil 76, 319–337.

Or, D., Leij, F.J., Snyder, V., Ghezzehei, T.A., 2000. Stochastic model for posttillage soilpore space evolution. Water Resources Research 36, 1641–1652.

Pagliai, M., Guidi, G., Lamarca, M., Giachetti, M., Lucamante, G., 1981. Effects ofsewage sludges and composts on soil porosity and aggregation. Journal ofEnvironmental Quality 10, 556–561.

Reichert, J.M., Norton, L.D., Favaretto, N., Huang, C.H., Blume, E., 2007. Settlingvelocity, aggregate stability, and interrill erodibility of soils varying in claymineralogy. Soil Science Society of America Journal 73, 1369–1377.

Rimmer, D.L., Greenland, D.J., 1976. Effects of calcium carbonate on the swellingbehavior of a soil clay. Journal of Soil Science 27, 129–139.

Rohoskova, M., Valla, M., 2004. Comparison of two methods for aggregate stabilitymeasurement – a review. Plant, Soil and Environment 50, 379–382.

Saadi, I., Laor, Y., Raviv, M., Medina, S., 2007. Land spreading of olive mill wastewa-ter: effects on soil microbial activity and potential phytotoxicity. Chemosphere66, 75–83.

Schmidt, W., 1992. Untersuchungen zur Bestimmung der Reindichte von Torfenund Mudden. Archiv Acker- und Pflanzenbau und Bodenkunde 36, 259–265.

Sierra, J., Marti, E., Montserrat, G., Cruanas, R., Garau, M.A., 2001. Characterizationand evolution of a soil affected by olive oil mill wastewater disposal. Science ofthe Total Environment 279, 207–214.

Sukartaatmadja, S., Sato, Y., Yamaji, E., Ishikawa, M., 2002. The effect of rainfallintensity on soil erosion and runoff for latosol soil in Indonesia. BuletinKeteknikan Pertanian 16, 69–77.

Tisdall, J.M., Oades, J.M., 1982. Organic matter and water-stable aggregates in soil.Journal of Soil Science 33, 141–163.

United Nation in Syria Stories, 2009. http://www.un.org.sy/forms/stories/viewStories.php?id=34&pageLang=en.

Urbanek, E., Horn, R., 2006. Changes in soil organic matter, bulk density and tensilestrength of aggregates after percolation in conservation and conventional tilledsoils. International Agrophysics 20, 245–255.

Van Dam, J.C., Hendrickx, J.M.H., van Ommen, H.C., Bannink, M.H., van Genuchten,M.T.H., Dekker, L.W., 1990. Water and solute movement in a coarse-texturedwater-repellent field soil. Journal of Hydrology 120, 359–379.

Warncke, D.D., Barber, S.A., 1972. Diffusion of zinc in soil. II. The influence of soilbulk density and its interaction with soil moisture. Soil Science Society ofAmerica Proceedings 36, 42–46.

Yaakoubi, A., Chahlaouia, A., Rahmanib, M., Elyachiouic, M., Nejdib, I., 2010. Effect ofolive mill wastewater spreading on the physicochemical characteristics of soil.Desalination and Water Treatment 16, 194–200.

Yasyerli, N., Dogu, G., Dogu, T., McCoy, B.J., 1999. Pulse-response study for thehumidity effect on sorption of ethyl bromide on clays. AIChE Journal 45,291–298.

Zaher, H., Caron, J., Ouaki, B., 2005. Modeling aggregate internal pressure evolutionfollowing immersion to quantify mechanisms of structural stability. Soil Sci-ence Society of America Journal 69, 1–12.