Long-terms effects of irrigation with treated municipal wastewater on soil, yield and olive oil quality

Agricultural Water Management 160 (2015) 14–21

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Long-terms effects of irrigation with treated municipal wastewater on soil, yield and olive oil quality Saida Bedbabis a,b,∗ , Dhouha Trigui a,c , Chedlia Ben Ahmed a,b , Maria Lisa Clodoveo d , Salvatore Camposeo d , Gaetano Alessandro Vivaldi d , Béchir Ben Rouina a a

Olive Tree Institute, P.O. Box 1087, 3000 Sfax, Tunisia Faculty of Sciences of Sfax, P.O. Box 1171, 3018 Sfax, Tunisia c ISA Chott Mariem, Sousse, Tunisia d Department of Agricultural and Environmental Science, University of Bari, Via Amendola 165/a, 70126 Bari, Italy b

a r t i c l e

i n f o

Article history: Received 17 January 2015 Received in revised form 17 June 2015 Accepted 22 June 2015 Keywords: Treated wastewater Soil characteristics Olive tree yields Virgin olive oil composition

a b s t r a c t In Tunisia, water scarcity is one of the major constraints for agricultural activities. The reuse of treated wastewater (TWW) in agriculture can be a sustainable solution to face water scarcity. The research was conducted for a period of ten years in an olive orchard planted on a sandy–silty soil and subjected to two different irrigation treatments: (a) well water (WW) and (b) treated wastewater (TWW). The main aim of the present study was to investigate the influence of irrigation with TWW on soil chemical properties, olive tree yield and on virgin olive oil (VOO) quality during an heavy crop year (“on year”) in “Chemlali” olive orchard. Soil samples were collected at the beginning of the study (before irrigation), after five and ten years for each treatment. pH, electrical conductivity (EC), organic matter, major elements, salts and heavy metals contents in soil were investigated. Standard quality parameters, chlorophyll, ␤-carotene, total phenols (TP), induction time and total tocopherols such as ␣-,␤-,␥-,␦-tocopherol of VOOs were also investigated. Results showed that irrigation with TWW increased soil pH, EC, OM, major elements, salts and heavy metals contents. Data obtained indicated that standard quality indices (free acidity, K232 , and K270 ) of VOO and oil content were not affected significantly by water quality. Instead, chlorophyll, total phenols, induction time and ␦-tocopherol values decreased significantly after ten years of irrigation with TWW. However, both fruit water content, and the concentrations of ␤-carotene and tocopherols (␣, ␤ and ␥) in VOO increased. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Olive tree is drought tolerant (Fernández and Moreno, 2000), able to survive severe water stress, and still produce a crop. However, prolonged drought stress is one of the major limiting factors in the production and yield of the fruit of the olive tree, as this directly affects crop load, oil production per tree, oil quality and alternate bearing. So, there are two main reasons for irrigating the olive orchard. On one hand, the plant has a marked response to additional water supplies, even if only small doses of water are applied. On the other hand, considering that increment in the diffusion of new orchards characterized by a high-density cropping system (Camposeo and Vivaldi, 2011; Camposeo and Godini, 2010;

∗ Corresponding author at: Faculty of Sciences of Sfax, P.O. Box 1171, 3018 Sfax, Tunisia. Fax: +216 241 033. E-mail address: saida [email protected] (S. Bedbabis). http://dx.doi.org/10.1016/j.agwat.2015.06.023 0378-3774/© 2015 Elsevier B.V. All rights reserved.

Godini et al., 2011), the irrigations becomes a necessity. Under Mediterranean climate, where olive tree is the most cultivated crop, the limited water resources and the increased need for good water quality for urban and industrial sector uses, led to the use of nonconventional water sources: agricultural drainage water, brackish or saline water and industrial or municipal wastewater for agricultural irrigation particularly under actual conditions of rainfall scarcity. The reuse of treated wastewater (TWW) in agriculture could be among the management practices to promote olive tree extension and cultivation at least under described experimental conditions in the south of Tunisia as reported by several studies (Bedbabis et al., 2009, 2010a; Ben Rouina et al., 2011). In Tunisia, this practice is actually more and more extended. Indeed, in 2009, wastewater treatment stations were of 106 generating approximately 238 mm3 year−1 of TWW among which 30% were recycled and supplied for irrigation of 9600 ha of agricultural lands (ONAS, 2010). The treated wastewater (TWW) use in agriculture has potential benefits that can be summarized as follows:

S. Bedbabis et al. / Agricultural Water Management 160 (2015) 14–21

• provides a reliable source of water supply to farmers and nutrients source for crop production (Jimenez-Cisneros, 1995); • conserves nutrients, thereby reducing the need for chemical fertilizers (Gil and Ulloa, 1997); • increases crop yields and returns from farming also if applied on olive orchards (Bedbabis et al., 2009, 2010a; Ben Rouina et al., 2011); is a low-cost method for sanitary disposal of municipal wastewater. However, application of TWW can have some risks either for agriculture products or soil properties (Yadav et al., 2002; Tarchouna et al., 2010 ; Vivaldi et al., 2013). The changes of the physical and chemical properties of soil, as a consequence of irrigation with TWW, can affect water movement in the soil thus also altering the soil hydraulic properties. Moreover, TWW could also increase exposure of farmers, consumers and neighboring communities to infectious and lead to groundwater contamination. However, in the Mediterranean countries and other arid and semi-arid regions which are confronting increasing water shortages, the reuse of TWW for purposes such as agricultural and landscape irrigation reduces the amount of water that needs to be extracted from natural water sources as well as reducing discharge of wastewater to the environment. Thus, treated municipal wastewater is a valuable water source for recycling and reuse. Tunisia belongs to the North Africa area, which is considered one of the driest regions in the world (World Bank, 1996). The reuse of TWW in Tunisia could satisfy the increasing water requirements for agriculture and may constitute an opportunity to preserve freshwater resources for human consumption. TWW is typically reclaimed at the secondary level by using biological processes. These processes consist of eliminating biodegradable material by transforming it into microbial residues in the case of olive mill waste water treatment Hachicha et al. (2009) and Jarboui et al. (2010). Agriculture is the major mainstay of the Tunisian economy, and the cultivation of olive trees constitutes one of the principal economical sectors of agriculture. In fact, about 65 million olive trees are spread over 1.6 million hectares (Hannachi et al., 2007). Chemlali is the main olive cultivar grown in northern and central Tunisia, and accounts for 80% of Tunisia’s oil production (Baccouri et al., 2007). The chemical and organoleptic characteristics of this oil depend, besides the fundamental genetic basis, on several factors clustered into four main groups: environmental (soil, climate), agronomic (irrigation, fertilization), cultivation (pruning, ripening, harvesting) and technological factors (fruit storage, extraction procedures) (Aparicio and Luna, 2002; Clodoveo, 2012, 2013; Clodoveo et al., 2013a,b, 2014; Camposeo et al., 2013). Among these factors, irrigation seems to plays a key role for oil quality (Gomez-Rico et al., 2007). Several studies have focused on the effects of irrigation with treated wastewater and the application of olive mill wastewater and moderate saline water on soil chemical properties (Al-Absi et al., 2009; Mechri et al., 2011; Bedbabis et al., 2010a, 2014) and little information is known about the effects of TWW on chemical properties of a cultivated sandy–silty soil. Studies on the effect of TWW on olive’s growth, productivity and oil quality in Tunisia are scarce (Bedbabis et al., 2009, 2010a; Ben Rouina et al., 2011). The aim of this study was to assess the effect of TWW in a “Chemlali” olive orchard on soil characteristics, olive yield and VOO quality. 2. Material and methods 2.1. Study area, plant material and irrigation schedules The olive orchards were located at ‘El Hajeb’ experimental farm in Sfax, (34◦ 43N, 10◦ 41E) in Central–Eastern Tunisia. In this

15

Table 1 Chemical analysis of irrigation waters from both sources. Characteristics

WW

TWW

Tunisian limits

pH EC (dS m−1 ) TDS (g L−1 ) HCO3 − (mg L−1 ) SO4 2− (mg L−1 ) Ntotal (mg L−1 ) N-NO3 − (mg L−1 ) N-NH4 + (mg L−1 ) N-NO2 − (mg L−1 ) Ptotal (mg L−1 ) K+ (mg L−1 ) Na+ (mg L−1 ) Cl− (mg L−1 ) Ca2+ (mg L−1 ) Mg2+ (mg L−1 ) Pb2+ (mg L−1 ) Cd2+ (mg L−1 ) Zn2+ (mg L−1 ) Mn2+ (mg L−1 ) SM (mg L−1 ) COD (mg L−1 ) BOD (mg L−1 )

7.95 ± 0.10 4.70 ± 0.02 1.51 ± 0.02 288.50 ± 0.3 87.50 ± 0.8 – 1.11 ± 0.01 2.24 ± 0.01 0.08 ± 0.02 0.80 ± 0.11 30.00 ± 0.09 355.00 ± 0.01 1580 ± 0.04 184.50 ± 0.01 126.20 ± 0.01 0 0 0.10 ± 0.01 0.19 ± 0.01 4.30 ± 0.02 0 0

7.60 ± 0.11 6.30 ± 0.03 1.82 ± 0.01 370.00 ± 0.20 363.00 ± 1.50 58.80 ± 1.20 15.90 ± 0.05 37.90 ± 0.01 5.00 ± 0.01 10.30 ± 0.01 38.00 ± 0.02 470.00 ± 0.02 1999.00 ± 0.04 95.80 ± 0.03 83.80 ± 0.02 <0.004 <0.004 0.42 ± 0.01 0.50 ± 0.01 13.40 ± 0.03 73.00 ± 0.11 22.00 ± 0.04

6.50 - 8.50 7.00 2.00 600.00 1000 30.00

0.05 50.00 300.00 600.00

0.10 0.005 5.00

90.00 30.00

Data represents mean values ± standard deviation. EC: electrical conductivity; TDS: total dissolved solids; SM: suspended matter; COD: chemical oxygen demand; BOD: biological oxygen demand; WW: well water; TWW: treated wastewater.

geographical area, the average annual rainfall and temperature averages over 52 years were 250 mm and 23 ◦ C, respectively. The study was carried out from 2003 to 2012 in an olive orchard planted in 1987, with ‘Chemlali’ olive trees. Trees were spaced 24.0 × 24.0 m, trained to vase and rain-fed. Fifteen-years ‘Chemlali’ olive trees cultivated at the density of 17 trees ha−1 were selected to be similar in canopy and potential yield. A randomized block design with three blocks and two treatments (TWW and WW irrigation) were used. Both plots contained twenty four olive trees (8 × 3 replications). The water used was either that supplied after biological treatment process (TWW), or the WW from a well situated in the area of the experimental station. The amount of water supplied to olive trees was estimated according to the Penman–Monteith–FAO equation (Doorenbos and Pruitt, 1977) as described by Ben Ahmed et al. (2007). The irrigation was delivered using a drip irrigation system, with four drip nozzles (two per side) set in a line along the rows (at 0.5 m from the trunk). Without taking rainfall into account, the daily water supply per olive tree was 4.5 m3 , for a total water supply of 5000 m3 ha−1 year−1 . Since the beginning of the irrigation design, trees of both WW and TWW plots does not receive any chemical or organic fertilization. The characteristics of TWW and WW were reported in Table 1. 2.2. Soil characteristics and mineral analysis Soil samples were collected quarterly from each plot at both 0–40 cm and 40–80 cm depth twice by year (July and December). They were saved in plastic bags and stored in a portable cooler. The samples were air–dried at room temperature, ground and crushed to pass through a 2 mm sieve, and mixed thoroughly for analysis. Soil texture was performed by pipette method according to the method described by Gee and Or (2002). Soil pH was determined using a pH meter (420A, Orien) in water (pHH2 O) and in saline solution of 0.01 M CaCl2 (pHCaCl2 ). Soil/water ratio of the suspensions was 1:2.5 (w/v). The soil textural classes were determined at the beginning of the trial (2003) according to the USDA soil texture classification. The soil salinity was assessed by determination of electrical conductivity (EC) at 25 ◦ C on a saturated paste using a conduc-

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S. Bedbabis et al. / Agricultural Water Management 160 (2015) 14–21

tivity meter (MC 226). Soil organic carbon was measured with a Shimadzu TOC-5000 Analyzer. Total nitrogen was determined with the Kjeldahl method. Chloride (Cl− ) was determined titrimetrically with AgNO3 (Karaivazoglou et al., 2005), whereas K, Na and Mg contents were determined on ammonium acetate soil extract (Richards, 1954) using a JENWAY flame photometer. P was determined by a vanado-molybdate colorimetric procedure with a JENWAY 6405 UV/vis spectrophotometer (Milan, Italy). Boron (B) was determined colorimetrically (420 nm) by the azomethine-H method (Wolf, 1974). Heavy metals (Zn, Mn, Fe, Cd and Pb) were measured with an atomic absorption spectrophotometer (A Analyst 200, PerkinElmer).

Induction time was evaluated by the Rancimat method (Laubli and Brutel, 1986). Stability was expressed as the oxidation time (hours) with the Rancimat 679 model (Metrohm, Switzerland). 2.4. Statistical analysis All collected data were subjected to the analysis of variance, with the two treatments as the independent variables. Statistical analyses were carried out with SPSS 10 for Windows (SPSS Inc., Chicago, IL, USA). The mean values of all parameters were compared using the LSD test. 3. Results and discussion

2.3. Olive and oil analyses

3.1. Chemical properties of TWW and WW

Samples of approximately 5 g of olive flesh from 50 olives were weighed, then dried for 24 h at 105 ◦ C, cooled for 30 min in a desiccator and reweighed. The oil content was determined by Soxhlet extraction and was expressed as a percentage of dry olive paste weight. Oil extraction was carried out in similar industrial extraction conditions using an Abencor analyzer (MC2 Ingenieriay sistemas, Sevilla, Spain). Olives (1.5–2 kg) were crushed with a hammer mill and were slowly mixed for 30 min at 25 ◦ C. Then, the obtained paste was centrifuged at 3500 rpm over 3 min. The oil was separated by decanting, transferred into dark glass bottles and was stored in the dark area at 4 ◦ C. Free acidity, expressed as percent of oleic acid (% C18:1), peroxide value (PV) and UV absorption characteristics (K232 and K270 ) were determined according to the European Union Commission (EEC, 1991) and subsequent amendments. Total chlorophyll compounds were determined by the method described by Minguez-Mosquera et al. (1991). Chlorophyll pigments were determined by measuring the absorbance at 630, 670 and 710 nm with spectrophotometer (PerkinElmer UV/vis spectrophotometer, Norwalk, CT). The results were expressed in mg Kg−1 and obtained from the following formula (Wolf, 1958). Totalchlorophylls(mg Kg

−1

) = [A670 -(A630 + A710 )/2]/0.10786 × L.

where L is vat thickness (1 cm) and 0.10786 is the variable coefficient according to the spectrophotometer used. Carotenoid contents were calculated from the absorption spectra of each virgin olive oil sample (7.5 g) dissolved in cyclohexane (25 mL) following the method of Minguez-Mosquera et al. (1991). The maximum absorption for carotenoid fraction is 470 nm. Carotenoid(mg Kg

−1

6

) = [A470 × 10 ]/2000 × 100 × d.

where A is the absorbance and d is the spectrophotometer cell thickness (1 cm). Total tocopherols were evaluated following the AOCS Method Ce 8-89 (AOCS, 1989). A solution of oil in hexane was analyzed by HPLC (HP series 1100) on a silica gel Lichrosorb Si-60 column (particle size 5 mm, 250 mm × 4.6 mm, i.d.; Sugerlabor, Madrid, Spain), which was eluted with hexane/2-propanol (98.5:1.5, v/v) at a flow rate of 1 mL/min. A fluorescence detector (Waters 470) with excitation and emission wavelengths set at 290 and 330 nm was used. Total phenols were extracted with water:methanol buffer (60:40) three times, from an oil-in-hexane solution, according to the method described by Vazquez Roncero et al. (1973) and determined colorimetrically using Folin–Ciocalteau reagent (Singleton and Rossi, 1965). The absorbance was measured at 727 nm with a spectrophotometer (PerkinElmer UV/vis Spectrophotometer, Norwalk, CT). Results were expressed as mg of caffeic acid per kilogram of oil.

The results of the analysis of treated wastewater (TWW) as well as the well water (WW) used in this experiment are presented in Table 1. The pH of the TWW and WW were 7.60 and 7.95, respectively, thus falling within the limits for irrigation, which range from 6.00 to 9.00 (Pescod, 1992). The electrical conductivity (EC) was 6.30 dS m−1 for TWW and 4.70 dS m−1 for WW, indicating, respectively, a high and moderate level of salinity (Rhoades et al., 1992; Wiesman et al., 2004). Also Cl concentration was higher than the threshold values, as reported by Chartzoulakis (2005) in the guidelines for olive irrigation. The analysis shows that TWW contains high amount of N, P and K which are considered essential nutrients for plant growth and development. It is low in toxic pollutants such as heavy metals. In general, levels of Ca, Mn, Zn, Pb and Cd were below the recommended maximum concentrations and within the guidelines for irrigation of agricultural crops (Ayers and Westcot, 1985). Both chemical and biological oxygen demands (COD and BOD) were below the Tunisian thresholds for water reuse (90 and 30 mg L−1 , respectively). Na and Cl contents indicated a possible risk of soil salinization. 3.2. Effects of TWW use on soil properties 3.2.1. Soil textural properties Results of physical analysis of soil samples from both experimental plots showed that both soil types were sandy–silty soils according to USDA classification (Table 2) with a percentage of 68.30–67.60% of sand, 22.80–25.00% of silt and 7.40–8.90% of clay in TWW and WW irrigated plots, respectively. 3.2.2. Soil chemical properties For all chemical parameters considered, greater effects were found in the soil layers after 5 years and 10 years of irrigation with TWW as compared to WW. The average pH values at the beginning of the experimental period ranged from 7.70 to 7.80. The slightly alkali values was probably a consequence of the buffering capacity of such Tunisian soils rich in limestone and with an intense degree of ammonification. There was a significant increase of pH values after 10 years of irrigation with TWW. This finding is in contrast with those Table 2 Characterization of soil texture (depth of 0–80 cm). Treatment

TWW plot

Element

Clay

Silt

Sand

CaCO3

Clay

Silt

Sand

CaCO3

Mean (g kg−1 ) S.D.a

8.90 4.80

22.80 0.90

68.30 5.50

12.90 4.90

7.40 2.20

25.00 0.80

67.60 14.50

10.30 4.50

WW plot

Clay F < 2 mm; Silt 2 < F < 20 mm; very fine sand 20 < F < 50 mm; fine sand 50 < F < 200 mm; coarse sand 200 < F < 2000 mm. a Standard deviation.

S. Bedbabis et al. / Agricultural Water Management 160 (2015) 14–21

reported by several studies (Ben Rouina et al., 2011; Bedbabis et al., 2014), who reported that irrigation with TWW decreased the pH when compared with irrigation with freshwater in a sandy soil. Differences can be explained by soil texture which is an essential factor for determining soil acidity. Other studies demonstrated that municipal wastewaters contain high concentrations of bicarbonate (Kumar and Christen, 2009), thus application to soils through irrigation can increase soil pH (Suarez et al., 2006). The TWW addition caused short and sudden increases of soil pH, nevertheless, the pH values remained within the range (7.00–8.50) for olive tree development (Gargouri, 1998). Indeed, no negative effects were observed over time when appropriate doses were utilized. The EC values (0.12–0.15 dS m−1 ) at the beginning of the experiment were low in each plot and similar to values detected in Mediterranean type climates with low seasonal rainfall Shalhevet (1994). The EC values increased significantly after 5 and 10 years of irrigation either with WW or TWW. Nevertheless, in both treatments, EC values remained below the salinity threshold (4.00 dS m−1 ). The significant increase in EC was both due to the higher concentration of salts and TDS in both waters, as previously suggested by several authors (Gil and Ulloa, 1997; Xanthoulis and Kayamanidou, 1998; Massena, 2001). At the beginning of the study, there were no statistically significant difference in the organic matter (0.42% and 0.45%) contents and macronutrients concentrations between TWW and WW plots (Table 3). Throughout the 10 years of the study, there were statistically significant differences in the OM contents between TWW treated and control plots (Table 3). The high content of organic matter reported after 5 and 10 years of irrigation with TWW made this material an excellent organic soil fertilizer. These results are different from other studies who reported a decrease of OM content in a coarse sandy soil irrigated with domestic TWW rich in nutrients (Jueschke et al., 2008; Tarchouna et al., 2010). In this regard, the long-term (10 years) sharp OM increase detected in TWW treatment can be explained by the composition of the water, which presented high values of BOD and COD. Available P and K contents progressively increased in all plots likely due to the chemical fertilization potential. During the whole experimentation, the differences in available P and K amounts between treated and control plots were generally uneven. Nevertheless, the quantities of the two elements recorded at the end of the trial were significantly higher where TWW had been applied. An increase of P and K recorded at the end of experimentation, suggested that a certain fertilizing effect of TWW was possible as a consequence of (i) its high soluble P and K contents and (ii) organic matter adsorption, as already found by previous works in field experiments irrigated with treated wastewater (Emongor and Ramolemana, 2004; Heidarpour et al., 2007). Indeed, TWW may be a source of P and K and can have ecological and economical advantages avoiding or reducing the use of P and K fertilizers (Di Serio et al., 2008). As reported by Segal et al. (2011), Cl concentration served as an indicator to estimate salt load due to its (i) having a strong correlation to ECe under the current experimental conditions, (ii) low relative uptake rate (ratio between uptake and supplied), (iii) being an anion with low adsorption rate and high mobility in the soil and (iv) being the most frequently occurring anion in the wastewater. In this study we observed a statistically increase of Cl concentration in the soil irrigated with TWW after 5 and 10 years compared with plots irrigated with FW. The same behavior was observed also for Na. These salts accumulation could be primarily due to high concentration of Na and Cl for both water resources. The alteration of plant growth, even at slight rate, such salinity levels could not be with a serious problem for Chemlali olive tree, at least under described environmental

17

conditions as reported by Ben Ahmed et al. (2009). The high Na concentration in soil solution can be explained by: (i) the antagonistic activity of either K+ or NH4 + which reduced the adsorption of Na on exchangeable complex and (ii) high calcium supply that enhanced the selectivity for the uptake and transport of K+ over Na+ . A high K/Na ratio was also measured in different olive tree organs (data no shown). This result suggests that measures of mitigation could be necessary, under regime of reuse of municipal wastewater, to avoid negative effects not only for crops but also for the environment. A significant increase of Mn, Zn and Fe were found, after 5 and 10 years of irrigation with TWW. This might be attributed to cumulative addition of these metals to the soil thought irrigation, as reported in previous investigations (Bedbabis et al., 2009; 2010; Ben Rouina et al., 2011). The B, Cd and Pb values were low the detection limit. After 10 years of regularly use of TWW, soil salinity and pollution problem were not severe as no phytotoxicity symptoms were observed. Nevertheless, a particular attention should be made at long term scale as the wastewater may also contain significant quantities of toxic metals and therefore its long-term use may result in toxic accumulation of heavy metals. 3.3. Effects of TWW use on olive yield At the beginning of experiment, the yields on both treatments (TWW and WW) were low and irregular due to the alternate bearing. Olive’s production was significantly higher for TWW with 2918 Kg ha−1 compared with 1535 Kg ha−1 of WW (Table 4). Although it is reported in other studies that saline water might reduce yield (Klein et al., 1994; Wiesman et al., 2004; Ben Ahmed et al., 2007) compared with control conditions, in the present study we did not observe such a negative effect. The higher yield obtained in TWW irrigated plot was probably a consequence of the presence of nutrient elements such as Nt , P and K, and the irrigation treatment worked as fertigation. In a recent 9-year study (Melgar et al., 2009) on irrigation of olive trees with saline waters (0.5, 5, 10 dS m−1 ), no differences were observed in annual and cumulated yield among treatments after irrigation with saline waters obtained by adding chloride salts to good quality water. Based on these results, an efficient fertilization schedule, in particular with N, P and K could constitute a good tool to mitigate biannual alternate bearing phenomenon characterizing the “Chemlali” olive tree. In another study (Segal et al., 2011), thanks to nutrients in TWW it was possible the reduction of applied fertilizer and, at the same time, to obtain the same production with respect to plots conducted with conventional techniques. 3.4. Effects of TWW use on fruit and olive oil quality parameters The effect of irrigation with TWW on ‘Chemlali’ fruit and VOO quality parameters are shown in Table 4. The fruit quality parameters measured were fresh (FW) and dry weight (DW), water content (WC) and oil content (OC). The experimental data show that TWW irrigation of ‘Chemlali’ olive trees after 10 years of treatment increased fruit FW, but had not significant influence on the OC in the mesocarp (on a DW basis) at harvest. The increment in fresh FW is due to the increment in fruit WC. In fact, comparing the mean values of fruit WC between WW and TWW treatments, it is possible observing that when olive trees were irrigated with WW the difference in WC was not significant; on the contrary, the analysis of variance clearly showed that the irrigation treatment with TWW determined a significant increase in WC equal to 4.65%. This aspect presents an interesting technological advantage. In fact, in arid regions when the WC of fruit is low, the drupes tend to be shrivelled and less turgid, making the extraction process more difficult. On the other hand, a too high WC (upper than 60%), mainly if the olives are harvested at a green degree of maturation, reduces

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S. Bedbabis et al. / Agricultural Water Management 160 (2015) 14–21

Table 3 Soil chemical analysis before irrigation, after 5 and 10 years of successive irrigation with well water (WW) and treated wastewater (TWW). Chemical elements

pH EC (dS m−1 ) O. M (%) Ntotal (g kg−1 ) P (mg kg−1 ) K (mg kg−1 ) Na (mg kg−1 ) Cl (mg kg−1 ) Mg (mg kg−1 ) Fe (mg kg−1 ) Zn (mg kg−1 ) Mn (mg kg−1 ) B (mg kg−1 ) Cd (mg kg−1 ) Pb (mg kg−1 )

BI

5 years

10 years

WW

TWW

t-Test

WW

TWW

t-Test

WW

TWW

t-Test

7.80 0.12 0.42 2.60 70.30 143.30 106.00 162.20 53.30 35 25.00 120.20 1.69 <0.004 <0.004

7.70 0.15 0.45 2.60 69.90 142.10 105.30 162.50 49.20 32 28.00 123.00 1.51 <0.004 <0.004

ns ns ns ns ns ns ns ns ns ns ns ns ns ns ns

7.80 0.65 0.81 2.80 65.10 195.20 305 220 60.50 31 25.50 125.10 1.43 <0.004 <0.004

7.60 1.12 1.23 9.20 93.20 615.50 451 384 73.90 39 46.80 163.60 2.53 <0.004 <0.004

ns

7.70 1.15 0.63 3.10 61.10 215 280 312 78.20 42 26.30 105.20 1.57 <0.004 <0.004

8.10 1.79 1.27 12.10 82.30 681 512 816 158.00 51 53.90 168.70 3.61 <0.004 <0.004

**

** * *** * *** ** ** *

ns *** * *

ns ns

*** *** *** ** *** ** *** ***

ns * * **

ns ns

ns, Statistically no significant; BI: before irrigation; WW: well water; TWW: treated wastewater. * Statistically significant at p < 0.05 level of significance, respectively. ** Statistically significant at p < 0.01 level of significance, respectively. *** Statistically significant at p < 0.001 level of significance, respectively. Table 4 Standard quality parameters, total phenols, induction time, pigment contents and tocopherols of oils obtained from both treatments (WW and TWW). Oil characteristics

Olive yields (Kg ha−1 ) FW (%) DW (%) WC (%) OC (% DW) FA PV K270 K232 TP (mg kg−1 ) Chlorophyll (mg kg−1 ) ␤-Carotene (mg kg−1 ) Total tocopherol (mg kg−1 ) ␣-Tocopherol (mg kg−1 ) ␤-Tocopherol (mg kg−1 ) ␥-Tocopherol (mg kg−1 ) ␦-Tocopherol (mg kg−1 ) IT (h)

BI

10 years

WW

TWW

– 18.36 44.50 51.80 44.28 0.28 6.60 0.15 1.87 130.52 1.28 – – – – –

– 16.53 51.71 51.83 43.76 0.28 6.60 0.13 1.87 191.53 0.66 – – – – –

39.20

50.00

t-Test * *

ns ns ns ns ns ns ** **

**

WW

TWW

t-Test

1535.10 21.55 44.75 53.50 44.75 0.26 6.80 0.16 1.81 105.92bf 1.15 6.65 575.21 443.26a 14.60 2. 77 0.42 32.90

2918 24.2 50.70 54.24 44.82 0.24 6.80 0.15 1.90 180.00dh 0.46 7.72 583.47 512.35b 15.40 3.01 0.35 45.30

** * * *

ns ns ns ns *** ** * * ** * * * **

ns, Statistically no significant; BI: before irrigation; WW: well water; TWW: treated wastewater; FW: fresh weight; DW: dry weight; WC: water content; OC: oil content; FA: free acidity; PV: peroxide value; IT: induction time. * Statistically significant at p < 0.05 level of significance, respectively. ** Statistically significant at p < 0.01 level of significance, respectively. *** Statistically significant at p < 0.001 level of significance, respectively.

the extractability of the oil. This is not the case of the TWW irrigation treatment, in which the mean value of fruit WC is equal to 54.24%. The values of free acidity, peroxide value and absorptions characteristic in the UV region at 270 nm and 232 nm for all olive oil samples studied were lower than the maximum limits imposed for the EU legislation (EU 1989/2003 and subsequent modification) for the extra virgin olive oil category. Free acidity (FA) and peroxide (PV) value are not significantly influenced by the quality of water applied. As shown in Table 4, FA ranged from 0.24 to 0.28% oleic acid; while PV ranged from 6.60 to 6.80 meq/kg. These results are coherent with the results obtained by Baccouri et al. (2007) for “Chemlali” variety grown under irrigated conditions. The specific extinction coefficient at 232 nm wavelength, K232 , is related to the primary oxidation of oil and gives indications of conjugation of poly-unsaturated fatty acid. K270 is an indicator of carboxylic compounds (aldehydes and ketones) in olives and

it is related to the secondary oxidation products (Boskou et al., 2006). Ultraviolet (UV) specific extinction determination permits an approximation of the oxidation process in unsaturated oils (Guiterrez-Rosales et al., 1992). The mean values of the specific extinction coefficients ranged from 1.81 to 1.90 and from 0.13 to 0.16, respectively, for K232 and K270 . These values did not exceed the limit of 2.50 and 0.22 established for extra virgin olive oil, respectively, for K232 and K270 (IOOC, 2003). No substantial differences on these analytical parameters were noticed which was related to the irrigation conditions, to the water quality and to the period of irrigation. This was also observed by previous studies (Palese et al., 2006; Bedbabis et al., 2010b) in irrigated olive trees with TWW. In addition, the relatively lower FA and the specific extinction coefficients levels can be attributed to the healthy status of fruits (Bedbabis et al., 2010b). The natural pigment contents (chlorophylls and carotenoids) of the oils are important for the consumers because they correlate with color of the product and play a key role as hedonistic factor and

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positively influence the sensorial acceptability of foods (Allalout et al., 2009). Virgin olive oil has a color from green–yellow to gold, depending on the variety and the stage of maturity (Salvador et al., 2000). VOO color may change during fruit ripening and oil storage and it could be considered as a product freshness indicator (GandulRojas et al., 1999). As shown in Table 4, chlorophylls and ␤-carotene present significant differences between the treatments. In general, both the irrigation treatment affect the chlorophyll content of the resulting oil after 10 years causing a decrease most marked if olive trees were irrigated with TWW. In WW treatment, the chlorophyll content change significantly with a decrease equal to 11.30%. This decrement increases when the irrigation treatment was performed employing TWW, and it was equal to 43.48%. The carotenoid content of the oil after 10 years of irrigation treatments (WW and TWW) showed an opposite behavior respect to the chlorophyll content. The TWW irrigation treatment determines an increase in carotenoid content of VOO. These results are due to a faster maturation of drupes treated with TWW and are in agreement with Wiesman et al. (2004) that observed a quick maturation phenomenon induced by water salinity. In fact, it is well know that the pigment concentration decreases with ripening and chlorophylls disappear faster than carotenoids (Mínguez-Mosquera and Gallardo-Guerrero, 1995). The decrease of chlorophyll contents is probably due to the peroxidase activity stimulation at later fruit maturity stage, which involves chlorophyll degradation. The presence of phenolic compounds in olive oil contributes to healthy and sensory characteristics of VOO (Guiterrez-Rosales et al., 1992). The average total phenols content in VOO olive produced from fruit of the trees grown under rain-fed conditions, before the irrigation treatments, was much higher, about 24%, than that obtained by applying the different irrigation treatments studied (WW and TWW for ten years). The irrigation both with WW and TWW causes a decrement of VOO total phenols (TP). This phenomenon is well documented by several authors (Angerosa and Di Giovacchino, 1996; Tovar et al., 2001; Romero et al., 2002; BenGal et al., 2009). In fact, the total phenol content, which affected the sensory bitterness in the oils, decreased significantly as the amount of supplied water increased. TP contents were significantly higher in TWW irrigated plot than in WW irrigated one. This result is due to the saline stress situation caused by the irrigation with TWW, which induces the production of phenols. Surprising, the oils extracted from more mature olives picked in the TWW irrigated plot showed a higher TP content than the oils from WW plot characterized by a low maturity index of the fruits. These results contrast with what is known on the effects of ripening on VOO TP content. In fact, during the olive ripening, the phenolic compounds decreased (Beltrán et al., 2005). Probably, the high oil phenol content observed after irrigation with TWW as compared to WW can be related to the significant decrease in the olive N and P content. In the case of low nutrient availability in fruits (data not shown), phenylalanine preferentially flows in to phenylpropanoids synthesis via phenylalanine ammonia lyase rather than toward synthesis of protein (Mechri et al., 2009). Oxidative stability expressed as induction time (IT), although not considered a standard parameter of quality, is useful to provide information on the oil’s resistance of the product to begin the oxidation process characterized by free radical reactions. The induction time is the time period until a critical point of oxidation is reached and corresponds to the sensorial degradation of the oil as a consequence of a sudden acceleration of the oxidative process. The irrigation both with WW and TWW causes a decrement of VOO IT after ten years of treatments. A lower value of the induction time was found in oils from the WW irrigated trees than the TWW irrigated ones. The difference was positively correlated (r2 = 0.86) with the total phenol contents (Gomez-Alonso et al., 2002; Andrews

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et al., 2003; Amirante et al., 2006; Caponio et al., 2008), which were higher in oils from the TWW irrigated trees than the WW irrigated ones. Results demonstrated that irrigation with TWW over 10 years decreased total phenol contents and, thereafter, induction time affecting the shelf life of the obtained oils. However, this decrease can be considered of minor importance and it is well balanced against other advantages achieved with the use of TWW as alternative to the use of natural water sources in agriculture in the Mediterranean countries and other arid and semi-arid regions which are confronting increasing water shortages. As suggested by several authors, the tocopherol fraction in virgin olive oils consisted mainly of ␣-tocopherols; these substances exert both vitamin potency and antioxidant action. Total tocopherol contents ranged from 575.21 to 583.47 mg Kg−1 . Levels are lower than these found by Beltran et al. (2005) in Hojiblanca cultivar. As shown in Table 4, ␣-tocopherol amount showed significant differences between treatments. In fact, ␣-tocopherol amounts are ranging from 443.26 mg Kg−1 (WW plot) to 512.35 mg Kg−1 (TWW plot). These results are higher than the values previously found by several authors Ranalli et al. (2000) and Allalout et al. (2009) in other varieties suggesting that tocopherol contents are highly variety-dependent (Deiana et al., 2002). While only low amounts of ␤-, ␥-, and ␦-tocopherol were detected. The mean value of ␤-tocopherol ranged from 14.6 mg kg−1 to 15.4 mg kg−1 and ␥tocopherol varied from 2.77 mg kg−1 to 3.01 mg kg−1 . The mean ␤-, ␥-tocopherol contents are different to the results of Beltran et al. (2005). The total tocopherols, ␣-,␤-, ␥-tocopherol amounts increased in TWW irrigated plot, it may be due fruit maturation induced by high K concentration into the fruits, which has been reported to determine an earlier change of fruit color from green to black (Chartzoulakis, 2005). Results are in contrast with the finding of Beltran et al. (2005). A significant decrease of ␦-tocopherol contents was reported in TWW irrigated plot. This phenomenon might be a result of the TWW can influence the ␦-tocopherol enzyme synthesis. Results could not be compared with other findings for virgin olive oil because this appears to be the first time that it has been described. 4. Concluding remarks TWW reuse in olive orchards in a semi-arid region such as Tunisia has been shown to be an important element in strategies for the sustainable use of limited freshwater resources because of its potential benefits. From the agronomical point of view, TWW irrigation of “Chemlali” olive trees results a significant yield increase when compared to yields from plot using WW. With regard to the chemical properties of the soil, TWW increased the pH, organic matter, major nutrients, salts and heavy metals such as Mn, Zn and Fe, however, these last ones did not exceed the Tunisian limits. The water quality affects both fruit and oil quality parameters. The water content of olives significantly increases while the fruit oil content is not influenced. The salinity level of TWW determined a faster ripening of olives respect to the WW treatment. This phenomenon influenced pigment contents (chlorophyll and ␤-carotene) and the concentration of total tocopherols of resulting VOO. In contrast with the faster ripening induced by the TWW treatment, TP of resulting VOOs were higher than the WW ones. The PT was positive correlated with the IT. The data obtained in the present study clearly show the ability to cultivate the “Chemlali” olive cultivar in a semiarid area with TWW irrigation. References Al-Absi, K.M., Al-Nasir, F.M., Mahadeen, A.Y., 2009. Mineral content of three olive cultivars irrigated with treated industrial wastewater. Agric. Water Manage. 96, 616–626.

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