Vol.3, No.5, 702-713 (2012) Agricultural Sciences
Long term effects of treated wastewater irrigation on
calcisol fertility: A case study of Sfax-Tunisia
Nebil Belaid1,2*, Catherine Neel2,3, Monem Kallel4, Tarek Ayoub5, Abdelmoneim A yadi1,
Michel Baudu2
1National School of Engineers of Sfax, Radio Analyzes and Environment laboratory (LRAE), Sfax, Tunisia;
*Corresponding Author: belaidnebil@yahoo.fr
2University of Limoges, Research Group, Water, Soil and Environment (GRESE), Limoges, France
3CETE Lyon-DLCF, Clermont-Ferrand, France
4National School of Engineers of Sfax, Water, Energy and Environment Laboratory (L.3E), Sfax, Tunisia
5CRDA-Sfax, Sfax, Tunisia
Received 25 April 2012; revised 28 May 2012; accepted 16 June 2012
The us e of trea ted wastewater (TW) for irrigatio n
is increasingly being considered as a technical
solution to minimize soil degradation and to re-
store nutrient content of soils. Indeed, TW usu-
ally contain large amounts of nutrient elements.
The objective of this study is to evaluate the
impact of long-term irrigation by TW on soil fer-
tility under real field conditions. In the vicinity of
the city of Sfax, a semi-arid region, a calcisol
field has been irrigated for more 15 years with
organic sodic TW; soil was modeled at three
different depths (0 - 30, 30 - 60 and 60 - 90 cm)
and along soil pits in the TW irrigated zone and
in a nearby non-irrigated zone (control). Several
p arameters hav e been measur ed: Soil s pH, CE C,
exchangeable cations, nitrate and ammonia, to-
tal contents of nitrogen, phosphorus and other
essential macro and micro nutrients, electrical
conductivity, soil organic carbon and dissolved
organic carbon. C/N ratio and SUVA were calcu-
lated for each soil layer. The calculation of the
isovolumic mass balance on soil profile scale
was used to measure macro and micro nutrients
supply. The TW irrigation has led to important
supply in organic carbon (+100%), phosphorus
(+80%) and in most essential nutrients (N, Mn,
Zn). Due to the high rate of irrigation and low
CEC of the studied soil, the added nutrient
cations and nitrate are removed with leaching
waters compared to the non-irrigated control
soil. Moreover, Sfax’s TW bring about important
amounts of salts and Na. Therefore the benefi-
cial addition of nutrients could quickly be inhibi-
ted by the excessive supply of salts and avail-
able nitrogen. Apart from future crops produc-
tion risk, groundwater degradation quality and
soil fertility will be endangered over the long
Keywords: Arid Region; Wastewater; Irrigation;
Fertility; El Hajeb-Sfax
In arid and semi-arid regions of countries such as
Tunisia which are facing rising serious water shortage
problems, the reuse of urban wastewater for non potable
purposes, such as agriculture [1-3] has became an usual
practice. Indeed, wastewater reuse for irrigation has been
the largest field of application because it usually offers
some attractive environmental and socio-economic bene-
fits, mainly due to the reduction of effluent disposal in
receiving water bodies, to the supply of nutrients as fer-
tilizers, and to the improvements in crops production
during the dry season [4,5]. Benefits apart, planners are
aware of the potential disadvantages of wastewater reuse
for irrigation which are, aside from pathogenic contami-
nation of irrigated crops, mainly related to the specific
chemical composition of wastewater being somewhat
different from most natural waters used in irrigation [6].
Over time, wastewater irrigation affects some soil para-
meters. Increase in soil pH is observed in acidic soils
[7,8], whereas slight decrease of pH is mentioned for
alkaline soils [5,8-11]. In alkaline calcarous soils, the
sligh acidification is due to the leaching of limestone by
the leaching water [10]. Wastewater can also supply
ammonia anion to soil which is another source of soil
acidification [11]. In general, the decrease of pH is also
explained by the low C/N ratio of effluents and the
subsequent enhancement of the organic mineralization
substances [7,10]. Wastewater irrigation thus usually
Copyright © 2012 SciRes. OPEN A CCESS
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713 703
leads to macro and micro nutrients supply [5,11] which
stimulates the microbial activity [12,13] and promotes
the mineralization of the soil organic matter. This can
lead to the decrease of the soil cation exchange capacity
[7,10] and mitigate the soil buffer capacity. By the same
way, wastewater usually contains elevated concentrations
in metal elements such as Mn, Cu and Zn which cons-
titute essential micro nutrients for plants. Over time, the
same elements can accumulate in the organic topsoil
layer in such manner that they reach their critical level
for plant growth.
Hence, the use of wastewater for irrigation of crops
requires assessment of the balance in supply of macro
and micro nutrient over the long-term. Most of previous
mentioned studies recorded impacts on contents in macro
and micro nutrients without any regard for changes in the
total store of these elements in soil. Moreover, specific
studies dedicated to highlight long-term impacts (along
several decades) have involved soils that have been
irrigated by untreated domestic effluents or municipal
wastewaters (Rattan et al. [8]: 10 and 20 yr; Yadav et al.
[5]: 30 yr; Solis et al. [10]: 50 and 100 yr). Thus, the
treatment of wastewater has been generalized after the
heighties on the spur of the F.A.O. guidelines for ap-
plication in agriculture [4]. The treatment of urban
effluents modifies their nutritional value, it is thus of
great concern to assess whether the irrigation with
treated wastewater (TW) still improves the soil quality or
it could cause degradation to its fertility over the long
The objectives of this study are to evaluate the
changes in soil fertility and to balance its essential ele-
ments in response to 15-year-long treated wastewater
irrigation (TW) within the city of Sfax (the second
largest city in Tunisia). In this arid region, there are many
signs of extremely low groundwater levels which were
registered over the last three decades due to the increas-
ing number of wells especially used for irrigation of
crops [14]. Treated wastewater in Sfax has thus been
used for irrigation since 1989. From that period, the
irrigation perimeter has regularly been expanded as to
reach the area of 600 ha. New extension of the area
irrigated by wastewater is planned for twice over its
present surface. This study is a part of a research pro-
gram which aims at evaluating the impact of waste-
water application on both soil and crops properties in the
arid region of Sfax. The overall goals are to aid mana-
gement of crop irrigation by wastewater, to reduce
overexploitation of the local groundwater resources and
to improve the water recharge of groundwater. Belaid et
al. [15] have evidenced negative impact on the soil
salinity and sodicity, especially in the northern part of the
irrigation perimeter covered by a deep permeable fluvisol.
Therefore, soil salinization and sodification are mitigated
by the amount of exchangeable calcium in irrigated cal-
cisol fields [15]. The present study thus focuses on this
type of calcareous soils, which is mostly found in the
Southern part of the irrigation perimeter.
2.1. Study Area
The study area is settled at ten kilometres in the West
next to the sewage treatment plant (Figure 1) near to the
town of Sfax (approximately with one million of habi-
tants) in crop fields, which are currently irrigated with
treated wastewater whose plant receives domestic as well
as industrial effluents from mainly canning factories and
textile production. The region has an arid climate with
monthly air temperature ranging from 11.3˚C to 26.7˚C,
dry summer and annual rainfalls of 200 mm mostly oc-
curring from October to December. The average annual
potential evaporation of 1200 mm, combined with the
low rainfall and high temperatures makes irrigation es-
sential for crop production.
The present survey has been carried out in the area of
the irrigation perimeter that is covered by a calcisol (ac-
cording to the FAO World Reference Base for soil re-
sources [16]). This soil presents an isohumic character
and shows a homogeneous sandy to sandy loam texture.
As shown in Table 1, the selected area produces alter-
nate cycles of crops, in association with permanent har-
vesting of olives, with successive winter and summer
harvest of annual crops (oat, sorghum) sectioned every
10 years by a 3-year-long cropping of alfalfa. This kind
of cropping system requires irrigation by open surface
furrows distributed every 24 m in-between each row of
olive trees. The soil has been submitted to wastewater
irrigation for 15 years. In order to assess the effects of
the wastewater, a nearby field is taken as a control area
which produces only olives and has been preserved from
any source of irrigation (Figure 1).
2.2. Samples of Preparation and Chemical
Treated wastewater were sampled at the outlet of the
Sfax wastewater treatment plant at different times and
conserved at 4˚C before characterization. Effluent sam-
ples were analyzed for pH and electrical conductivity
(ECw) using a pH meter (AFNOR standard method N˚
NF T 90-008 [17]) and a conductimeter (AFNOR N˚ NF
EN 27888 [17]) respectively. Chemical oxygen demand
(COD), suspended solids (SS), biochemical oxygen de-
mand (BOD) and total phosphorus were measured ac-
cording to standard methods (AFNOR N˚ NF T 90-018,
NF EN 872, NF T 90-103, NF EN 1189 [17]). Cations
and anions were measured using chromatography while
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N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713
Copyright © 2012 SciRes.
Figure 1. Map of studied area with location of the Sfax water treatment plant and of the calcisol site (TW irrigation perimeter in grey
and point for the control site).
Table 1. Main characteristics of the studied calcisol site.
Soil taxonomy Light texture isohumic calcimagnesic soil according to the Tunisian pedological map
Soil depth Moderately deep soil laid over a limestone crust approximately 60 cm deep
(the crust was dismantled in large part of the irrigated area)
Soil texture Sandy to sandy-loam with calcareous nodules in subsurface
Soil bulk density (surface layer) 1.4 g/cm3
Total CaCO3 5% to 35%
Irrigated area Control area (not irrigated)
Cultural system Associated cultivars (olives trees and forages crops) Only olives trees
Crop rotation Winter (oat. ray grass) summer (sorghum) annual (alfalfa) -
Field area 270 hectares including 90 hectares used for summer crops. 1.5 hectares
TW irrigation rate 1000 mm/yr -
Irrigation system Surface irrigation by furrows -
Irrigation duration 15 years -
Number of cores sampled 7 1
carbonates and bicarbonates were estimated by titration
with HCl of an aliquot of the effluent samples (AFNOR
N˚ NF EN ISO 9963-2 [17]).
Soil sampling was performed in October 2006 after
the harvest of summer crops and before the seeding of
winter crops. Two soil sampling survey have been carried
out. The first one concerned each horizon of soil that has
been identified along pedological profiles drilled in the
control area and in the irrigated field whereas the second
sampling was done on plots covered by summer crops
only by 30 cm thick soil layers using an Edelman-type
auger. In order to account for spatial variations of soil
texture and depth, 7 replications were performed in the
irrigated field (IWC1 to IWC7). The control area was too
small to allow such replications (NIC). Only 2 soil layers
down to the depth of 60 cm were sampled in the control
field as well as in the replication site IWC5 because of
the occurrence of a concrete calcareous crust at depth of
60 cm. This crust of sedimentary origin is irregular and
has been generally dismantled in the irrigated field in
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713 705
order to help infiltration of irrigation waters, except in
the control area, which has never been irrigated.
After air-drying, the soil samples were sieved at 2 mm.
Soil pHw and pHKCl were measured in a 2.5 soil to wa-
ter/KCl 1 M slurry using a glass electrode. Saturation
paste extracts of soils were prepared to determine the
electrical conductivity of the soil samples (ECs). Soil
samples CEC and contents in exchangeable cations were
determined at actual soil pH by the cobaltihexamine me-
thod [18]. In calcareous soils, Belaid et al., [15] evi-
denced that the cobaltihexamine method provided more
accurate values than the more usual method using 1 M
NH4 acetate solution [19] because of lesser dissolution of
carbonates. Concentrations in Ca and Mg were performed
by Atomic Absorption Spectrometry (AAS) while Na and
K concentrations were determined by Flame Atom Emis-
sion Spectrometry (FAES).
NO3 and NH4 concentrations were measured in water
soluble extracts using ionic chromatography (DIONEX
DX-120) after water extraction using 1:5 soil to water
ratio for 2 h. Total nitrogen was determined by steam
distillation after acid digestion using the Kjeldal proce-
dure. Soil organic carbon (SOC) was determined by the
Walkley and Black dichromate oxidation method. Water
soluble organic C, which is further referred to as dis-
solved organic carbon in this paper (DOC), was extracted
with deionized water using a 2:1 (water to soil) ratio by
shaking at fixed temperature during 3 hours [20]. The
extracts were filtered using 0.22 mm filters and DOC
concentration was measured using UV-persulfate oxida-
tion on a TOC analyzer (TEKMAR DOHRMANN Phoe-
nix 8000). UV absorption at 254 nm was measured using
a Varian CARY 50 Probe UV-visible spectrophotometer.
DOC quality; in terms of aromaticity of organic com-
pounds, was determined as specific ultra violet absorb-
ance (SUVA) which was calculated by dividing the ab-
sorbance at 254 nm by the DOC.
2.3. Statistical Analysis
One-sample T-test was used for comparing mean val-
ues obtained from replicates of measurements at an irri-
gated site to the values measured at the corresponding
non irrigated control one. All measured values corre-
spond to the average composite samples. However, due
to the limited area of the control sites, and since the sam-
pling was limited to the central part of them in order to
avoid any influence of the neighbouring irrigation then in
the absence of sampling replicate in the control area, no
variance can be associated with the control values.
Hence, we have assumed that the values of the control
site represent exact means to be compared with the vari-
ance of the mean values (N = 7) measured at the corre-
sponding irrigated site. Variance is expected to be larger
in the irrigated zone than in the corresponding control
one, so that the following sample T-tests can be consid-
ered as conservative:
mean ofTWE irrigated replicatedvalues-control value
standard deviation ofTWEirrigated replicated values
A unilateral T-test was used to calculate the parameters,
which are clearly increased or decreased after the irriga-
tion by the treated wastewater (ECs, Exchangeable ca-
tions, NO3 and NH4). Whereas the bilateral T-test was
chosen to identify parameters presenting no obvious re-
sponse to the irrigation (pH, CEC, SOC). The global risk
increases with the number of simultaneous tests per-
formed. Therefore, we have also adopted a more severe
individual rule than the usual one to minimize the in-
crease of the global risk; the differences are considered
significant when p < 1% instead of p < 5%. In case of p
ranging between 1% < p < 5%, we conclude that the dif-
ferences have to be confirmed. The T-tests were per-
formed using SYSTAT Software version 12
3.1. Treated Wastewater Characteristics
During the wastewater survey, the applied treated
wastewater (TW) was always remained alkaline with an
average basic pH value of 7.7 (Table 2). It also always
presented a high level of total dissolved solids (TDS) of
3.7 g·L1 and of suspended matter (SS). The level of bio-
chemical oxygen demand (BOD) and chemical oxygen
demand (COD) are ranged respectively between 37 and
220 mg·L1 and 123 and 700 mg·L1. The mean electrical
conductivity (EC) of effluents reaches 5.7 mS/cm. The
sodium absorption ratio (SARw) of the treated wastewa-
ter ranges between 9.7 and 15.6. Theses parameters are
higher than the usual ranges reported for other Tunisian
TW [20] of similar mixed origins (industrial and domes-
tic) than those of the Sfax TW. The Sfax’ TW also con-
tains great amounts of nitrate, phosphate and potassium
which are crucial nutrients for plant growth and soil fer-
3.2. TW Irrigation Impacts on the Soil
In all sampling points in the irrigated field (Table 3)
and in the two soil profiles, the pH (pHw and pHKCl) of
the soil water extracts remains alkaline at all depths (Ta-
ble 4). There are no significant differences in soil pHw
between the wastewater irrigated calcisol and the control
By the same way, there are no significant differences
in the contents of exchangeable Ca2+ between the irri-
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N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713
Copyright © 2012 SciRes.
Table 2. Mean values of chemical properties of the treated wastewate (TW) generated by the wastewater treatment plant of Sfax from
1984 to 2007.
Examples of wastewater used for irrigation
TW of Sfax
Treated Untreated
Mean Ranges
pH 7.7 7.1 - 8.7 6.5 - 8.5 7.8 7.73 7.3 7.4
ECw mS·cm1 5.7 4 - 7.7 7 3.5 1.14 - 1.74
TDS g·L1 3.7 3.56 - 5.13 - - - 0.95 0.9
SS mg·L1 204 29 - 275 30 32.7 46 -
COD mg·L1 350 123 - 700 90 88 34 - 382
BOD5 mg·L1 107 37 - 220 30 18.5 - - 169
Pt mg·L1 7.5 2.9 - 12.5 - - 1.64 15.5 -
NO mg·L1 21 0.35 - 50 - - 1.63 29 -
Cl mg·L1 1662 903 - 2580 2000 688 134 - -
SO mg·L1 1022 508 - 1950 - - 121 - -
HCO mg·L1 630 490 - 732 - - - - -
Na+ mg·L1 1137 780 - 2100 - 112 103 - -
K+ mg·L1 57 17 - 105 - 333 18 33.3 -
Mg2+ mg·L1 151 129 - 209 - 166 20 - -
Ca2+ mg·L1 296 103 - 521 - 258 53 - -
NH mg·L1 67 61 - 73 - - 33 - -
SAR 12.4 9.7 - 15.6 - 7.7 3 4.6 -
Mean minimum and maximum values of samples characterised since 1984. ECw: Electric conductivity; TDS: Total dissolved solids; SS: Suspended matter; COD:
Chemical oxygen demand; BOD: Biochemical oxygen demand; Pt: Total phosphorus; SAR: Sodium absorption ratio: aKlays et al., 2010; bCappola et al., 2004;
cRusan et al., 2007; dYadav et al., 2002.
gated calcisol and the non-irrigated control calcisol (Ta-
ble 3). This confirms the natural inorganic origin of this
element that certainly reflects the presence of Ca-car-
bonates. On the other hand, contents in exchangeable
Mg2+, K+ and Na+ have increased in all examined layers
of the irrigated calcisol (Table 3). These increases are
very significant in the upper soil layer and can be attri-
buted to the relatively high concentrations of the Sfax
TW in these cations (Table 2). In the deepest layer, the
same intensifications may have been enhanced by the
wastewater drainage through the ripping of the carbonate
crust in the irrigated field. The examination of the elec-
trical conductivity of the saturation paste soil extracts
(ECs) approves that 15-year long irrigation period by the
Sfax TW results in significant supply of ion into the cal-
cisol (Table 4), even in the deepest layers. As a conse-
quence, soil salinity was up to 4 mS·cm1 at all depths,
and sometimes exceeded this level.
Analyses of the ammonium and nitrate concentrations
in the soil water extracts and of the soil organic carbon
also reveal the impact of the irrigation by the Sfax TW.
In the irrigated soil, the contents in extractable NH4
+ re-
main low and never exceed 0.1 cmol+·Kg1 in the dry soil
(Table 3). However, as with the other base cations ori-
gin- nating from the applied wastewater, the NH4
+ con-
tents of the soil generally increase with depth and are
significantly higher in the irrigated soil compared to the
non irrigated one (Table 3). Similarly, significant con-
tents in extractable nitrate are found in the deepest 60 -
90 cm layer of the irrigated soil confirming drainage by
the added TW (Table 3). Although much more variable
than ammonia, the nitrate contents of the soil water ex-
tracts are also systematically higher in the irrigated soil
than in the non-irrigated one. Soil organic carbon con-
tents (SOC) follow similar trends with values decreasing
with depth in the irrigated soil (Tables 3 and 4).
According to the isohumic natureof the studied calci-
ol, SOC contents are elevated reaching 0.8%. Contents s
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713 707
Table 3. Effects of TW irrigation on some chemical properties (significant differences between irrigated soil compared to control soil
showing a p < 0.01).
Values for irrigated sites One sample T-test
Depth (cm) Control value
N Mean SEM T P (%)
0 - 30 8.54 7 8.28 0.0764 3.32 1.581
30 - 60 8.85 7 8.65 0.0626 3.17 1.924
0 - 30 1.38 7 4.09 0.4608 5.89 0.053*
ECs mS·cm1
30 - 60 0.87 7 4.35 0.4881 7.14 0.018*
0 - 30 0.02 7 0.07 0.0062 8.29 0.008*
NH cmol+/kg
30 - 60 0.03 7 0.06 0.0058 5.98 0.048*
0 - 30 0.02 7 0.19 0.0276 6.26 0.038*
NO cmol/kg
30 - 60 0.00 7 0.09 0.0191 5.00 0.122*
0 - 30 0.42 7 0.52 0.1388 1.23 26.311
30 - 60 0.42 7 0.30 0.0772 2.68 3.638
0 - 30 17 7 43.92 1.6445 16.36 0.0003*
DOC mg/kg
30 - 60 - 7 - - - -
0 - 30 5.00 7 10.25 1.0788 4.86 0.1403*
K+ cmol+/kg
30 - 60 3.94 7 9.74 1.0028 5.78 0.0583*
0 - 30 0.32 7 2.59 0.2347 9.69 0.0035*
Na+ cmol+/kg
30 - 60 0.65 7 2.44 0.2643 6.78 0.0250*
0 - 30 39.62 7 40.96 2.0748 0.64 54.2487
Ca2+ cmol+/kg
30 - 60 49.50 7 43.7 0.9096 6.32 0.0732*
0 - 30 1.00 7 6.44 0.2512 21.66 0.0000*
Mg2+ cmol+/kg
30 - 60 1.32 7 5.63 0.1172 36.78 0.0000*
N: Number of samples; T: Observed Student statistic (T = Mean-control/SEM); SEM: Mean standard error for measurements in irrigated field; p%: Error risk
*significant at p < 1%. SOC: Soil organic matter; DOC: Dissolved organic carbon; ECs: Electric conductivity of soil.
Table 4. Chemical properties of irrigated and not irrigated soil profile.
Soil depth (cm) pHW pHKCl CEC (cmol+/kg)S (%) Nt (%) SOC (%) C/N
Irrigated soil profile
H1 (0 - 10) 8.54 7.79 10.01 83.29 0.0476 0.80 16.80
H2 (10 - 30) 9.15 8.04 9.97 75.67 0.0049 0.44 88.77
H3 (30 - 50) 8.56 8.06 7.81 110.87 0.0245 0.31 12.65
Not irrigated soil profile
H1 (0 - 20) 8.55 7.9 7.70 79.29 0.0028 0.15 53.57
H2 (20 - 35) 8.85 7.78 10.15 76.55 0.0042 0.10 23.89
H3 (35 - 50) 8.83 7.79 8.12 106.74 0.0245 0.25 10.20
EC: Cation exchange capacity; S: Saturation in exchangeable cations; Nt: Total nitrogen; SOC: Soil organic carbon.
Copyright © 2012 SciRes. OPEN A CCESS
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713
in organic carbon are not significantly higher in the irri-
gated soil than in the control one (Table 3). However, the
same values around 0.4% are found in the two studied
soil layers of the non irrigated calcisol while SOC con-
tents are generally higher in the surface layer than in the
deeper one in the irrigated soil. This indicates the supply
of organic carbon by the irrigation by the treated waste-
The examination of the DOC, C/N ratio and SUVA
(Table 5) highlights the impact of the TW irrigation on
soil organic matter quality (SOM). The C/N ratio ranges
between 20 and 36 in the irrigated calcisol whereas it
remains around 70 in the control site. This indicates an
acceleration of the SOM humification in the irrigated
field compared to the non irrigated one. The DOC con-
tent of the soil water extract, which is related to the frac-
tion of labile carbon, is three times higher in the irrigated
calcisol than in the control. By the same way, the fraction
of aromatic carbon as estimated by the SUVA ratio is
more important in the calcisol that has been submitted to
the wastewater irrigation for 15 years.
3.3. Quantitative Supply in Macro and Micro
Nutrients by the TW irrigation
Isovolumic mass balance was calculated in order to
quantify the supply of micro and macro nutrients by the
15-year-long TW irrigation. Organic carbon, total nitro-
gen and phosphourous, as well as essential cations pools
were expressed in mass per unit of soil area (Table 6)
using thickness and bulk density of each horizon of soil as
employed by Keller and Védy [21]. The percentages of
pool change were calculated between the two surficial
horizons in reference to the equivalent thickness of the
deepest horizon of soil by assuming a conservative ele-
ment. It was thus assumed that the deepest horizon of soil
has not been impacted by the treated wastewater (Table 4).
Similarly, the equivalent deep scope of reference was
found by using Si and Ti as conservative elements: 31 cm
and 40 cm for the irrigated and the control area respec-
tively. A negative pool change reflects a loss of elements
from the H1-H2 horizon to the H3 while a positive value
reflects a gain. However, because of the risk of error ac-
cumulation, a 15% range of uncertainty has to be con-
sidered especially for micro nutrients (Mn, Cu, and Zn).
Results show no variation in the change of K, Fe and Cu
between the control and the irrigated pedons (Table 6).
Conversely, significant differences appear for other es-
sential elements. Compared to the control pedon, the
irrigated pedon shows switch in Ca and Na due to the
supply of sodium by the TW. Mn and Zn are less pre-
served in the irrigated soil profile. Concerning amounts of
SOC, P and N (Table 6), results indicate that the irrigation
by the Sfax TW has mitigated the loss in total nitrogen and
increased the pool in organic carbon and in total phos-
4.1. Impact of Irrigation on the Soil Macro
and Micro Nutrients
The quality of the Sfax treated wastewater (TW) has
varied since 1984. However, values of parameters indi-
cating the salinity (ECw, Cl) and the sodicity (SAR, Na)
remained largely superior to the limits established by the
F.A.O. [4] for the reuse of wastewater in agricuture. Aside
salts and sodium, the Sfax TW concentrations in dis-
solved organic carbon, BDO5 and suspended matter ex-
ceed the Tunisian standards for water reuse in irrigation
(NT 106.03). Compared to other treated urban effluents
from Sardinia, Tunisia and Jordan respectively [6,11,22],
the Sfax TW also provide higher concentrations in macro
nutrients such as ammonia, nitrate, total phosphorus and
Table 5. Organic pattern of the TW and of the surface soil layers (0 - 30 cm) of irrigated and not irrigated soils.
Samples Nt (g/100g) SOC (g/100g) C/N DOC (mg/kg) Abs 254 SUVA
IWC1 0.018 0.53 29.4 43.1 0.80 1.86
IWC2 0.021 0.77 36.6 45.6 0.94 2.07
IWC3 0.017 0.34 20.0 38.8 0.60 1.54
IWC4 0.018 0.35 19.5 40.0 0.62 1.55
IWC5 0.035 0.81 23.0 43.8 0.78 1.80
IWC6 0.019 0.59 31.3 52.1 0.86 1.65
IWC7 0.014 0.28 20.0 44.1 0.71 1.62
NIC 0.006 0.42 70.5 17.0 0.17 1.02
IWC: Irrigated Wastewater Calcisol; NIC = Control: Not Irrigated Calcisol; TN: Total Nitrogen; SOC: Organic Carbon; C/N: Soil C:N Ratios. DOC: Dissolved
Organic Carbon; Abs 254: UV absorbance of soil water extract at 254 nm; SUVA: Specific UV Absorption = Abs 254/DOC.
Copyright © 2012 SciRes. OPEN A CCESS
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713 709
Table 6. Isovolumic masse balance of change between H1-H2 and H3 layers in the irrigated and not irrigated profiles.
Soil depth (cm) Mg
Irrigated soil profile
H1 (0 - 10) 0.7 5.5 0.2 1.2 0.02 1.5 0.68 2.4 1.2 0.07 0.07
H2 (10 - 30) 1.4 10.7 0.4 2.5 0.04 3.1 1.14 4.0 1.2 0.01 0.11
H3 (30 - 50) 1.6 33.6 0.5 2.3 0.03 3.5 1.29 4.3 1.0 0.08 0.07
Change (%) 17 69 23 0 65 17 5 10 49 35 66
Not irrigated soil profile (control pedon)
H1 (0 - 20) 1.0 6.9 0.3 2.1 0.05 2.5 0.96 5.0 0.4 0.01 0.05
H2 (20 - 35) 1.1 9.3 0.3 2.0 0.04 2.9 1.06 3.4 0.2 0.01 0.04
H3 (35 - 50) 0.9 11.1 0.2 1.7 0.03 2.5 0.86 2.7 0.5 0.05 0.04
Change (%) 16 45 6 8 106 17 12 17 50 86 15
potassium (Table 2). Nevertheless, concentrations in
nitrate, phosphorous and potassium are much lower in
the Sfax TW than in untreated wastewater [5].
The quantitative mass balance results obtained in the
irrigated and not irrigated fields (Table 6) shows the ef-
fects of the Sfax’s TW properties. Indeed, the 15 year-
long irrigation period does not lead to change the pools
in Mg, K, Cu and Fe. Conversely, it has increased the
pools of SOC (+100%) and of total P (+80%) in the top-
soil layer compared to the subsoil level. Likewise, the
TW irrigation has led to preservation of 50% of the pool
in total N. Consequently, concentrations of available
mineral nitrogen (3
O and 4
H) have significantly
been increased by the irrigation (Table 3). The nitrate
and ammonium can either be directly brought by the
wastewater or indirectly by the turnover of the organic
matter and subsequent N mineralization.
In the case study, results suggest simultaneous in-
creases in the organic pool (cf. Tables 5 and 6 either for
SOC and total Nitrogen) and in the rate of organic matter
turnover (cf. Table 4 for nitrate and ammonia and Table
5 for DOC) indicating enhancement of the C and N mi-
neralisation. Such impacts have already been noticed in
incubation experiments [23] for loamy Appalachian for-
est soils (Typic Hapludult) that have been irrigated with a
municipal treated wastewater for two years. The used
treated wastewater contained similar nitrate (19.4 mg·L1)
and total P concentration (3.15 mg·L1) than in the Sfax’s
TW but much lower concentration in ammonia (1.73
mg· L 1). However, the enhancement of the N mineraliza-
tion by TW irrigation is not systematic. Ramirez-Fuentes
et al. [13] has not recorded any changes in the N turn-
over during the incubation of various Mexican types of
soils irrigated with untreated wastewater. Magesan et al.
[12] have even noticed a reduction of the amount of
leached nitrate for soil irrigated with TW. As mentioned
by Herpin et al. [7] TW impacts on the nutrients turnover
are mostly influenced by the difference between the C/N
ratio of the soil and of the wastewater effluent.
In the studied case, the enhancement of the organic C
and N mineralization can be explained by the quantita-
tive variations in pools of micro nutrients. Mn and Zn are
thus both known to be associated with soil organic matter.
The rise of the SOM turnover has certainly intensified
the availability of these micro nutrients turnover and
facilitates their uptake by plants. Such kind of depend-
ence between the turnover of soil organic matter and Zn
translocation to the aerial parts has already been ob-
served in other soil types (Andic soil) [24]. As opposed
to the irrigated field, the control area is not used for an-
nual crops production. The cultural differences between
the irrigated and the non-irrigated field thus make it dif-
ficult to interpret data because the sum of essential nu-
trients uptaken by plant is certainly much lower in the
control area than in the irrigated field.
4.2. Impacts of Irrigation on the Soil Organic
Matter Quality
Several parameters have to be considered altogether in
order to understand the impact of the TW irrigation on
the quality of the soil organic matter (Table 5): C/N ratio,
total content of dissolved organic carbon (DOC) and
specific UV absorption ration (SUVA).
In the irrigated topsoil layer (0 - 30 cm), the soil C/N
ratio is low compared to those of non-irrigated soil, im-
plying an enhancement of the SOM biodegradability.
This is confirmed by the amount of DOC extracted from
the irrigated soil samples. The SUVA was originally de-
signed for estimating amounts of aromatic C in DOC
fraction in waters [25,26]. It has been successfully used
Copyright © 2012 SciRes. OPEN A CCESS
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713
to compare these overall quantities in humic substances
of waters as well [27]. The latter aromatic C fraction can
be sought for assessing water contamination by halogen-
organic compounds [28]. In soil, aromatic C fraction is
considered as being more stable than the labile DOC soil
fraction made of proteins or carbohydrates [29]. Thus, a
high SUVA value of the soil DOC fraction reveals a more
important consumption of the labile C fraction by the soil
microbial communities [30]. This not only indicates a
higher degree of degradation of the labile organic matter,
but also a change in the SOC quality. Korshin et al. [31]
stated that the absorbance at 254 nm is even increased by
the presence of polar functions in aromatic compounds,
such as by hydroxyls, carbonyls, carboxylic and ester
functions. An elevated SUVA ratio can therefore also
outlines the SOM reactivity or its ability to form combi-
nations with the soil mineral fraction.
As mentioned in the previous studies, the input of
available micro nutrients and labile fraction of organic C
by TW irrigation can over stimulate the microbial acti-
vity of the soil [12,13]. In adequate soil conditions, this
results in an enhancement of the C mineralization rate,
with subsequent decrease of the total SOC content in less
than 4 years of TW [7]. Yet, inconsistent results are
found over the long term. In Mexican Leptic calcaric soil
[10], the irrigation for 50 years with untreated wastewa-
ter has depleted the SOC content in rate of 53% whereas
in India (various soil types of pHw ranging between 5.1
to 9.9), Rattan et al. [8] recorded an increase of the SOC
content in rate of 59% after 20 years of irrigation with
untreated effluents.
In the case study, the combination of values of C/N ra-
tio, DOC, SUVA and SOC contents clearly reveals an
enhancement of the organic material turnover in the irri-
gated soil compared to the non irrigated one. However,
despite the DOC supply in large amounts by the Sfax’s
TW (Table 2), the irrigation of the studied calcisol has
not systematically implied changes in the total content of
soil organic carbon (SOC), compared to the non irrigated
soil (Table 3). Huge variability of SOC contents are in-
deed noticed in the topsoil layer of the irrigated field, so
that the mean values are not statstically different from
the reference value of the non irrigated field (Table 3).
This result suggests the influence of contradictory pro-
cesses to the organic matter mineralization.
It has to be noticed that the Sfax’s TW are particularly
rich in salt and sodium. Chow et al. [25] have perceived
that the soil salinisation and sodification affects the SOM
structure as to decrease the DOC content in the leaching
soil waters. As shown in Figure 2, a significant negative
correlation is indeed found between the soil ECs and the
DOC in the irrigated soil (R = 0.63, p < 0. 05). Romkens
and Dolfing [32] explained that free Ca2+ cations ex-
changed by the added Na+ flocculate more than 50% of
Figure 2. The correlation between dissolved organic carbon
(DOC) in the water soil extracts and electric conductivity (ECs)
of the surface layers of irrigated soils.
the soil DOC. Rietz and Haynes [33] added that the in-
crease of the soil salinity leads to an inhibition of the soil
microbial activity, and thus to significant decrease of the
SOM mineralization. Likewise, the latter contents stabi-
lization takes place in the studied calcisol corresponding
to its mineralization enhancement by means of TW’ ap-
plication. Subsequently, this procedure tends to explain
the variability of resulting contents of SOC in the irri-
gated field (Table 5). Over the long term, salt-inducing
SOM stabilization can also lead to a decrease of crop
yields [33].
4.3. The Risk of Fertility Loss over the Long
Over the long term, salt-inducing SOM stabilization
can lead to decrease of crop yields [33]. The relationship
between salinity of irrigated soils (ECs) and their content
in labile carbon (DOC) thus gives an insight of further
risks of soil losses of the studied calcisol fertility after-
wards (Figure 2). The relationships between nitrate,
ammonium contents and the evolution of the quality of
the soil organic matter (as seen by the C/N ratio, SUVA
and DOC contents in soil water extracts) suggest long
term risk of degradation of groundwater quality by the
leaching waters.
The irrigation clearly leads to a decrease of the con-
tents in nitrate and ammonium in the water extracts of
soils, not only at the soil surface, but also in the deepest
soil layer (see Table 3 and [15]). DOC of the applied
effluent, as well as the soil DOC fraction only represent
small parts (0.01% up to 0.1%) of the total SOM in soils.
It is also the more mobile and the further used C fraction
for heterotrophic microbial communities involved in the
N turnover. Kim and Burger, [23] showed that nitrogen
supply by treated effluents favored nitrification and
Copyright © 2012 SciRes. OPEN A CCESS
N. Belaid et al. / Agricultural Sciences 3 (2012) 702- 713 711
leaching of nitrate because the amounts of available ni-
trogen, supplied by the TW, largely exceed the plant de-
mand. As seen in Tables 4 and 6, the control calcisol is
naturally poor of nitrogen. The presence of active car-
bonates in this type of soil also usually causes natural
inhibition of microbial activity, which is indicated in this
present case by a C/N ratio above 70. Moreover, the CEC
of the studied calcisol remains low compared to other
equivalent to calcareous soils in which CEC reach values
up to 20 cmol+·Kg1 [15,34]. Due to the low CEC and to
the elevated irrigation rate, ammonia cations supplied by
the Sfax TW have limited chances of being absorbed by
the soil. In such context, nitrate and ammonium contents
occurring in the deepest layer are certainly deriving
straight from the applied wastewater. The two mineral
nitrogen ions are leached down to the root zone and risk
to reduce the upper soil layer fertility. As a matter of fact,
this can also cause degradation of the soil leaching water
and consequently of the free groundwater below roots
and transient zones. Apart from risks of degradation of
groundwater quality, there are also concerns for the con-
dition of the crop harvest. Indeed the excess of available
mineral nitrogen in soil usually lead to the breaking of
forage crops during the maturation stage (oat and so-
The previous studies have also identified losses in es-
sential nutrients (Ca, Mg, K, Na, Cu, Zn, Mn) and soil
acidification as others risks of degradation of the fertility
of alkaline calcareous soils being irrigated by domestic
effluents for several decades [5,8,10,11]. In the studied
case, the 15-year long wastewater irrigation has not af-
fected the pH of the soil. This result can be explained by
an important buffer capacity of the examined calcisol.
Calcium represents the most abundant exchangeable base
cation (Table 3). As seen along the two pedological pits,
horizons of soil are nearly saturated in essential base
cations (Table 4). Belaïd et al. [15] already mentioned
significant linear correlations (p < 0.05) between the
CEC and the contents of exchangeable cations added by
the TW: Na+ (R = 0.94), K+ (R = 0.79) and Mg2+ (R =
0.99). The quantitative analysis of pool changes for Ca
and Na in the irrigated pedon validates the exchanges of
natural Ca2+ by the supplied Na+ (Table 5). Exchange-
able contents in Mg2+, K+ and Na+, have generally in-
creased only in the irrigated soil (Table 3). These cations
developments are corresponding with its relatively high
concentrations in the treated wastewater used for irriga-
tion. Although total amounts in these essential cations
have not significantly changed at the scale of the soil
profile (Table 5), there is a risk over the long term of loss
of these elements due to the high rate of irrigation and
low soil CEC (Table 4). On the other hand, due to the
salinity of the Sfax’s TW, important amounts of the sup-
plied Na are leached down to the deepest horizon of soil
in the irrigated field initiating sodification of groundwa-
The Sfax treated wastewater (TW) is particularly rich
in available organic carbon and mineral nitrogen. Irriga-
tion for 15 years with the Sfax TW has not significantly
changed the pH of the studied calcisol and the SOC con-
tent of the topsoil layer. The results of the present study
also confirm, under field conditions, processes previ-
ously identified in laboratory experiments about the im-
pact of wastewater effluents on the quality of the soil
organic matter. From a quantitative respect, the 15-year-
long irrigation by the Sfax TW has limited the loss of
micro nutrients such as Mn and Zn. It has also supplied
important amounts in most essential macro nutrients (P
and N). The irrigation by the TW has also clearly en-
hanced the C and N turnover in the studied calcisol
which fertility was naturally limited by the elevated con-
tent in active carbonates. If these beneficial effects could
reduce the cost of mineral fertilization and aid the pro-
duction of crops, the continuous use of the TW arises
some questions concerning soil fertility and groundwater
protection over the long term. In the case study, because
of the elevated concentrations in sodium and salts of the
applied TW, the beneficial activation of the microbial
activity and the resulting availability of essential ele-
ments could be quickly inhibited. Moreover, due to the
elevated irrigation rate and the low CEC of the soil, the
TW irrigation has clearly increased the leaching of mi-
neral nutrients such as nitrate and exchangeable K, Mg,
and Na. Further irrigation, even with natural water, could
stand for an imminent threat for the quality of the free
watertable in the Sfax region by increasing concentra-
tions in nitrate, sodium and salt. Apart from this problem,
the excessive supply of salts and available nitrogen al-
ready constitutes a risk for the future crops production.
However, since these speculations only concerned one
calcisol filed, the validity of our conclusions need to be
verified across a wider study area and for the other types
of soils that were irrigated with the Sfax TW.
The authors gratefully acknowledge the staff of the CRDA-Sfax for
their cooperation during site selection and soils sampling.
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