When saline water is used to irrigate crops in arid environments, appropriate irrigation management should be applied to −1 avoid negatively impacting soil characteristics. In this study, the effects of irrigating date palms with saline water (2.24 g l ) on soil physicochemical characteristics such as the electrical conductivity (EC ), the pH of the saturated soil paste (pH ), e e 2+ 2+ + the concentrations of soluble cations (Ca, Mg, Na ), the sodium adsorption ratio (SAR), the saturated soil hydraulic conductivity (K ), and the volumetric water content of the soil (θ ) were evaluated in a Tunisian Saharan cropland, the s v Dergine Oasis, during a 4-year period (2012–2015). The effects of three different irrigation treatments of date palms on soil properties were investigated: low treatment (90% of the net irrigation requirement (NIR) of date palms was applied); medium treatment (100% of NIR was applied), and high treatment (110% of NIR was applied). The results showed that the applica- tion of saline water for irrigation inevitably has a negative impact on the physicochemical properties of the soil. Irrigation with saline water was observed to have severe negative impacts on the soil characteristics, especially EC, Na , K , and θ . e s v However, among the three irrigation treatments applied, statistical analysis (Duncan’s multiple range test) indicated that the high treatment significantly ( p < 0.05) minimized the degradation of soil characteristics by the saline water; this treatment decreased EC, Na , and SAR and increased the water content, θ , of the studied soil. e v Keywords Irrigation treatment · Soil characteristics · Saline water · Arid environments · Oasis Introduction primarily land-use changes, development, and agricultural activities such as increased crop production resulting from In arid regions, meager rainfall (annual amounts vary the intensive use of agrochemicals (Knapp and Baerenklau between 100 and 300 mm; FAO 1989) coupled with an 2006). During the past few decades, a rapid increase in extreme shortage of fresh groundwater necessitate the use of irrigated lands and the climatic conditions of the Saharan saline groundwater to satisfy the water demands of the agri- environment have together caused major salinization of the cultural sector (Kolsi et al. 2013; Zaidi and Kassem 2012). groundwater resources in this region; consequently, inten- This saline groundwater arises through processes known as sive exploitation of saline groundwater has led to severe primary and secondary salinization. Primary salinization degradation of agricultural soil (King and Thomas David occurs naturally in groundwater where there is naturally 2014). Indeed, soil salinization is considered the main rea- occurring salt (e.g., halite and gypsum) in the parent aqui- son for cropland degradation in these areas. Hamdy (2005) fer material or lateral flow from adjacent saline aquifers. reported that the soil salinization process has negatively Secondary salinization arises through human activities, affected about 30% of the irrigated lands in the Sahara. As demonstrated by numerous researchers (e.g., Amé- zketa 1999; Tedeschi and Dell’Aquila 2005; Al-Zu’bi Y * Zied Haj-Amor 2007; Huang et al. 2011; Askri et al. 2014), frequent irriga- firstname.lastname@example.org tion with saline water can accelerate the soil salinization Water, Energy, and Environment Laboratory, National process, significantly degrading the quality of agricultural Engineering School of Sfax, 3038 Sfax, Tunisia soil in various ways. A high concentration of salt, especially Department of Water Resources Engineering, and Center sodium salt, in the soil can cause physicochemical deterio- for Middle Eastern Studies, Lund University, Box 118, ration of the soil. This can lead to structural damage to the 221 00 Lund, Sweden Vol.:(0123456789) 1 3 22 Page 2 of 12 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 soil due to the dispersion of clay particles, decreased soil Materials and methods hydraulic conductivity, soil instability due to the clogging of soil pores, and the formation of a thin crust at the surface The Dergine Oasis of the soil, which reduces the rate of infiltration of irrigation water into the soil profile. Physicochemical deterioration of Field work was performed during a 4-year period between the soil also leads to a severe reduction in soil productivity, January 2012 and December 2015 in the Dergine Oasis hence degrading soil fertility (Al-Zu’bi 2007). (Fig. 1), a modern Saharan irrigated oasis located in south- The appropriate management of saline water and the soil western Tunisia, where environmental conditions (De Gre- to ensure sustainable agriculture in Saharan croplands by nade 2013) favor the growth of date palms. The climate avoiding or minimizing the adverse effects of saline water of the study area is arid (the average yearly rainfall rarely on agricultural soil is therefore an extremely important issue. exceeds 90 mm) and hot (exceeding 40 °C in the summer Key to achieving sustainable agricultural management is a months). In contrast, the average annual potential evapo- good understanding of the processes behind the degradation transpiration reaches 1800 mm. The average monthly rela- of soil physicochemical characteristics by the saline irriga- tive humidity varies between 62 and 32% in December and tion water (Tedeschi et al. 2007). Particular attention needs August, respectively (El-Fahem 2003). In 2003, the Dergine to be paid to agricultural soils that receive large amounts of Oasis, 88 ha in area, was completely planted with the date water through surface irrigation, considering the complex- palm Phoenix dactylifera to a density of 120 palms per hec- ity involved in maintaining a safe level of salinity within tare. These trees receive surface irrigation with saline water −1 −1 the soil (Huang et al. 2011). In the Saharan croplands, the (3.5 dS m ≈ 2.24 g l ) supplied by a deep artesian well considerable water requirements of date palms Phoenix dac- with a depth of 175 m. Chemical and physical properties of tylifera (Sperling et al. 2014) lead to the application of enor- the applied irrigation water are given in Table 1. The soil mous volumes of saline water during the irrigation process texture is loamy sand and gypsum is present throughout the (Tripler et al. 2011). A number of studies have addressed oasis (depth concentration: 1 m; average gypsum distribu- the effect of saline irrigation water on soil physicochemi- tion: 6%). The extensive application of saline water to irri- cal characteristics (e.g., Al-Zu’bi 2007; Huang et al. 2011; gate the palm trees as well as the unsuitable agricultural soil Singh et al. 2011), but very few studies have studied this (loamy-sand soil with a high infiltration rate) have led to the effect for soil in which date palms are growing. The primary creation of a shallow saline groundwater layer at an average objectives of the work reported in the present paper were depth of less than 2 m in the studied oasis (Askri and Bouh- therefore to assess the effects of four years of irrigation of lila 2010). To prevent the potential for this shallow ground- date palms with saline water on the characteristics of the water layer to cause irrigated soil salinization, the oasis was soil, and to identify the appropriate irrigation treatment (i.e., protected with a subsurface drainage system (buried pipe the appropriate volume of saline water) that will minimize drains) installed at a depth of 1.5 m. The drainage system the degradation of soil physicochemical characteristics. is designed to stop the water table from rising to a critical Fig. 1 Location of the Dergine Oasis in the town of Douz, Kebili, southwestern Tunisia 1 3 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Page 3 of 12 22 Table 1 Chemical and physical properties of the saline irrigation soil depth of 1.5 m (defined based on the agroclimatic and water applied in the Dergine Oasis (data based on the analysis of 12 soil conditions in the oasis; Ben Aissa et al. 2013; Bouarfa samples taken in the year 2012, i.e., one sample per month) et al. 2009). Parameter Minimum Maximum Average Experimental design and monitored plots −1 EC (dS m ) 3.40 3.62 3.51 iw SAR 4.70 6.10 5.80 The main objective of the present work was to investigate pH (−) 7.83 7.92 7.87 the effect of saline water quantity (irrigation treatment) on + −1 Na (meq L ) 12.50 18.50 16.6 soil physicochemical characteristics. To achieve this aim, 2+ −1 Ca (meq L ) 7.20 09.36 8.40 the soil characteristics observed following the application of 2+ −1 Mg (meq L ) 6.62 8.75 7.80 different quantities of saline water were investigated. Three + −1 K (meq L ) 0.25 0.32 0.30 irrigation treatments (Table 2) were applied in three different − −1 Cl (meq L ) 12.17 14.20 13.3 plots (Fig. 2) located in the Dergine Oasis during the studied 2− −1 SO (meq L ) 10.12 11.19 10.8 period (2012–2015). These plots were selected largely on the − −1 HCO (meq L ) 2.10 3.30 2.80 basis of their location within the Dergine Oasis (Fig. 2). The Na (%) 24.0 28.0 27.0 Table 2 Properties of the monitored plots and the irrigation treatments applied to them Plot Surface area Soil texture Irrigation Number of irriga- Palm density Water quantity Irrigation treatment type b b −1 (ha) frequencytion events(palms ha ) (days) Plot 1 0.5 Loamy-sand 15 96 120 90% of NIR Low treatment Plot 2 0.5 Loamy-sand 15 96 120 100% of NIR Medium treatment Plot 3 0.5 Loamy-sand 15 96 120 110% of NIR High treatment Determined using the sedimentation method (Gee and Bauder 1986) Applied during the studied period (2012–2015) NIR is the net irrigation requirement of date palms (NIR = actual evapotranspiration (ET ) + leaching requirement (LR)) Fig. 2 Irrigation system topol- ogy in the Dergine Oasis and locations of the three monitored plots (plots 1, 2, and 3) 1 3 22 Page 4 of 12 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 selected plots (the three in the top left corner in the figure) Na ), and the sodium adsorption ratio (SAR). The cation were the most elevated plots, so they did not receive lateral concentrations and the SAR were measured for a saturated shallow groundwater flow from neighboring irrigated plots. extract, whereas pH and EC were measured in a 1:5-diluted e e In other words, we eliminated the possibility that lateral extract (“1 soil/5 water”). Physical analyses of the collected flow from neighboring plots could influence the properties soil samples included the saturated soil hydraulic conduc- of the soil in each plot of interest. No significant rainfall tivity (K) and the volumetric water content of the soil (θ ). was recorded during the study period, meaning that the soil Each soil property was measured in four replicates to check in the studied plots mainly received moisture from irriga- the consistency of the results. Furthermore, for each plot, the tion. Whereas palm trees were planted in the other plots effect of the local heterogeneity on the soil physicochemical in the Dergine Oasis in 2003, the palms in the monitored property measurements was neglected due to the size of the plots were planted in January 2012 (at the beginning of the investigated plot (0.5 ha) as well as the homogeneity of the studied period) and were surface irrigated using available texture of the soil (loamy sand). saline water (Table 1). The quantity of saline water applied to each plot over the studied period is presented in Table 3. Electrical conductivity of the saturated soil paste (EC ) It should be mentioned that regular and adequate drainage was observed in all monitored plots. This parameter was measured to evaluate the level of soil salinization. A high soil salinization level, especially around Soil sampling the roots of the crop, imposes ion toxicity, osmotic stress, nutrient deficiency, and oxidative stress on crops and limits The soil properties of all monitored plots (plots 1–3) were their uptake of water from the soil, decreasing crop produc- investigated by analyzing the first 90 cm of the soil profile tion (Shrivastava and Kumar 2015). High soil salinity can in each plot (90 cm is the maximum depth of the root zone also negatively affect many soil physicochemical character - for planted date palms). In the soil profile, three soil hori - istics such as soil structure and soil porosity (Huang et al. zons were identified: 0–30, 30–60, and 60–90 cm. The varia- 2011). tions in the properties of the soil within those horizons were Following the method proposed by Rhoades (1996), a determined. Throughout the studied period (2012–2015), 96 filtrate was first extracted from a 1 soil/5 water mixture to irrigation events were recorded for each plot (Table 2). Soil measure the electrical conductivity of the 1:5-diluted extract samples were collected from each plot at the three depth (EC ). The extraction process was performed through a 1/5 intervals 24 h after each irrigation event. A truck-mounted Buchner funnel. A linear regression equation was then hydraulic push probe (Giddings Machine Company, Fort derived for each soil layer (Eqs. 2.1, 2.2, and 2.3 below) to Collins, CO, USA) was used to collect intact soil samples. convert the EC values into electrical conductivity values 1/5 This allowed 288 soil samples to be collected from each plot of the saturated soil paste (EC ). The 864 soil samples col- (96 irrigation events × 3 soil layers), or 864 samples in total lected from the Dergine Oasis (96 samples from each soil (96 irrigation events × 3 soil layers × 3 plots). The collected layer) were used to establish the relationships between EC 1/5 soil samples were subject to careful physical and chemical and EC (regression coefficients R > 0.95). The measured −1 analyses carried out via standard methods (Page et al. 1982). EC was expressed in dS m . Before the analyses, all of the collected soil samples were dried for 24 h, crushed, and sieved using a screen with a grid EC = 1.89 + 1.11 × EC 0 − 30 cm; R = 0.98 (2.1) e 1∕5 size of 2 mm and then stored for further analyses. Chemical analyses of the collected soil samples included EC = 1.63 + 1.16 × EC 30 − 60 cm; R = 0.97 (2.2) the pH of the saturated soil paste (pH ), the electrical con- e 1∕5 2+ 2+ ductivity (EC ), soluble cation concentrations (Ca, Mg , EC = 1.59 + 1.27 × EC 60 − 90 cm; R = 0.95 (2.3) e 1∕5 Table 3 Total amount of water applied (in mm) to each of the moni- tored plots during the studied period (2012–2015) pH of the saturated soil paste (pH ) Years Plot 1 Plot 2 Plot 3 (low treatment) (medium treat- (high treatment) This parameter is considered a key influence on the phys - ment) icochemical condition of the soil and on plant growth (Roy 2012 1245 1386 1464 et al. 2006). The filtrate used for EC measurements was 1/5 2013 1278 1398 1481 also employed to conduct pH measurements with a glass- 2014 1288 1409 1492 electrode pH meter. The obtained values (pH ) were trans- 1/5 2015 1293 1422 1518 formed into the pH of the saturated soil paste (pH ) based on 1 3 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Page 5 of 12 22 established relationships between pH and pH for the three required, the measurement steps to perform, and the calcu- 1/5 e soil layers (Eqs. 2.4, 2.5, and 2.6 below). The soil samples lation method to use. K was calculated using the following −1 collected for each layer (80 samples) in the Dergine Oasis equation and expressed in cm day : were used to establish the relationship between pH and 1/5 ΔV pH for each layer. K = , (2.8) e s ΔT × i × A pH = 0.21 + 1.02 × pH 0 − 30 cm; R = 0.96 where ΔV is the volume of the inflow or outflow, ΔT is the (2.4) e 1∕5 time step, i is the hydraulic gradient, and A is the cross- sectional area of the soil column (for more details, see pH = 0.18 + 1.13 × pH 30 − 60 cm; R = 0.94 (2.5) e 1∕5 Gholizadeh-Sarabi and Sepaskhah 2013). Volumetric water content (θ ) pH = 0.11 + 1.22 × pH 60 − 90 cm; R = 0.97 (2.6) e 1∕5 In each of the three monitored plots, the volumetric soil 2+ 2+ + water content (θ ) in each soil layer (0–30, 30–60, and Soluble cation concentrations (Ca, Mg, and Na ) 60–90 cm) was measured 24 h after each irrigation event (to ensure that the soil conditions had stabilized after irrigation). The sodium cation (N a ) concentration was measured using The θ measurements were performed in situ using a time- a flame photometer (model 410, Corning Inc., Corning, NY, v 2+ 2+ domain reflectometry (TDR) probe (TRASE 6050X1, Soil USA). The calcium (Ca ) and magnesium (Mg ) cation Moisture Corporation Company, Goleta, CA, USA). The concentrations were measured with an atomic absorp- TDR probe was calibrated according to the method reported tion spectrophotometer (model SP3, Pye Unicam, Cam- in Hamouda et al. (2005). As recommended by Cichota et al. bridge, UK). The guideline proposed by Page et al. (1982) 2+ 2+ + (2008), the direct measurement of θ by TDR is an appropri- was followed when measuring the Ca, Mg , and Na ate technique for soil with a high potential salt concentration. concentrations. 3 −3 The values of θ were expressed in cm cm . Sodium adsorption ratio (SAR) Statistical analysis This parameter was analyzed to probe the potential for soil The data collected, i.e., the measurements of soil properties properties, especially the soil structure, to be degraded by in the three monitored plots obtained during 2012–2015, high sodium concentrations (with respect to the calcium were analyzed using a standard statistical approach. Differ - and magnesium concentrations) in the irrigation water. The ences between the soil property values obtained under the 2+ 2+ + measured Ca, Mg , and N a concentrations were used to three irrigation treatments (low, medium, and high; Table 2) calculate the sodium adsorption ratio (SAR) using Eq. 2.7 were detected using analysis of variance (ANOVA). Dun- below (Oster and Sposito 1980): can’s multiple range (DMR) test at a significance level of (Na ) 0.05 (Duncan 1955) was conducted to identify which of the SAR = , values dier ff ed significantly among the three irrigation treat - (2.7) 2+ 2+ (Ca + Mg ) 2 ments. The SPSS software package (Kinnear and Gray 2000) was used to perform this statistical analysis. The DMR test −1 where ionic concentrations are expressed in meq L . is very useful for distinguishing the impacts of various treat- ments on soil properties and for identifying the best amount Soil hydraulic conductivity (K ) of saline irrigation water to apply to minimize the degrada- tion of soil physicochemical characteristics. The saturated hydraulic conductivity (K ) of each soil core sample was determined based on the constant-head method described by Blake and Hartge (1986). For this purpose, the Results and discussion collected intact soil cores (8.2 cm in length and 6.2 cm in diameter) from the studied plots and soil layers were satu- During the studied period, the shallow groundwater did not rated in the laboratory. The saturated hydraulic conductiv- rise to the critical soil depth (2 m) after each irrigation event. ity (K ) values of the samples were then measured using a For this reason, we ignored any contribution of the ground- constant-head permeameter. We strictly followed the guide- water to the deterioration in soil properties. Therefore, the line for K measurements produced by Gholizadeh-Sarabi main focus of the present study was to assess the impact of and Sepaskhah (2013) regarding the laboratory materials the saline irrigation water on the soil characteristics. 1 3 22 Page 6 of 12 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 In the following section, we present the soil properties characteristics in all three soil layers. As presented in recorded for plot 2 before irrigation (i.e., just before the first Table 4, pH decreased from neutral (7.2–6.9) in January irrigation event in the first year) and after irrigation (i.e., 2012 to slightly acidic (6.5–6.1) by the end of the studied just after the last irrigation event in the fourth year). This is period. As reported by Roy et al. (2006), microbial activ- because a normal irrigation treatment (i.e., 100% of the net ity and nutrient efficiency are maximized in neutral to irrigation requirements of date palms) was applied to plot 2 slightly acidic soil, thus facilitating optimal crop growth. during the studied period. We also compare the impacts of Also, the density of palm roots in the deep (60–90 cm) soil the different treatments in the three monitored plots on the layer increases over time (Zaid and Jiménez 2002), which soil physicochemical properties. increases the amount of organic acid in this layer (i.e., the pH decreased with soil depth). This may be another reason Eec ff t of saline irrigation water on soil for the increase in the observed soil acidity following three characteristics years of the saline water irrigation treatment. Regarding the effects of the applied saline irrigation water 2+ 2+ + We now present a comparison of the soil physicochemical on the soluble cation concentrations (Ca, Mg , and Na ) characteristics before (January 2012) and after (December in plot 2, the following findings should be noted. First, the 2+ 2015) the application of saline water for irrigation to plot saline irrigation water decreased the calcium (Ca ) concen- 2 (a medium irrigation treatment was applied in this plot). tration in the studied soil. The acidity level of the irrigated It is apparent from the results presented in Table 4 that the soil may be the main reason for this noticeable decrease −1 application of saline water for irrigation (2.24 g l salinity) (Fig. 3). Good irrigation management by the farmers dur- significantly ( p < 0.05) changed the soil physicochemical ing the studied period (i.e., a strategy to reduce the gypsum Table 4 Comparison of soil properties before (January 2012) and after (December 2015) irrigation treatment in monitored plot 2 Parameter Soil layer: 0–30 cm Soil layer: 30–60 cm Soil layer: 60–90 cm Before irrigation After irrigation Before irrigation After irrigation Before irrigation After irrigation −1 EC (dS m ) 6.20 8.62 5.81 9.84 4.71 11.81 pH (−) 7.20 6.50 7.10 6.40 6.90 6.10 + −1 Na (meq L ) 34.50 44.50 32.50 42.60 30.50 40.30 2+ −1 Ca (meq L ) 36.20 28.36 34.4 26.4 32.4 25.4 2+ −1 Mg (meq L ) 17.62 13.75 14.8 12.8 13.8 11.8 SAR (−) 6.60 9.70 6.50 9.60 6.40 9.50 −1 K (cm day ) 99.32 81.25 91.22 76.31 85.31 72.13 3 −3 θ (cm cm ) 0.064 0.032 0.082 0.041 0.098 0.044 2+ 2+ + Fig. 3 Comparison of the soluble cation (Ca , Mg , and Na ) concentrations observed before and after irrigation 1 3 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Page 7 of 12 22 concentration) could be another reason for this decrease. 2+ Second, a significant decrease in the magnesium (Mg ) concentration was also observed at the end of the studied 2+ period (Fig. 3). A low Mg concentration in the irrigated 2+ soil hinders the growth of planted trees, as Mg deficiency leads to leaf yellowing with brilliant tints (Roy et al. 2006). This was observed in the date palms planted in the Dergine Oasis after applying saline water for irrigation. Third, in 2+ 2+ contrast to the Ca and Mg concentrations, a significant increase in the Na concentration was noted after the irriga- tion period (Fig. 3). This is most likely due to the high Na concentration in the applied irrigation water (the N a level in the irrigation water is presented in Table 1). Generally, the three common cations in the studied soil can be listed as Fig. 5 Measured soil electrical conductivity (EC ) distributions follows in order of how negatively their concentrations were before and after irrigation + 2+ 2+ impacted by the saline irrigation water: Na > Ca > Mg . 2+ 2+ The observed decreases in Ca and Mg concentra- −1 > 1500 mm year ) and the extensive application of saline tions and the increase in N a concentration were employed to calculate the sodium adsorption ratio (SAR) using Eq. 2.7. water (Sperling et al. 2014) are the main drivers of the major salinization observed in the irrigated areas of various Tuni- The result showed that there was a major increase in the SAR over the studied period (Fig. 4). Al-Rasbi (2010) sug- sian Saharan oases (Askri et al. 2014). The changes in EC with soil depth (Fig. 5) showed a gest that a continuous increase in the SAR in irrigated soil could degrade date palm growth characteristics such as plant slight increase in EC in the topsoil layer (0–30 cm), whereas a substantial increase in EC was noted for the bottom layer height, plant girth, and leaf length. Hence, it is very impor- tant to reduce the SAR level in the irrigated soil. (60–90 cm, the high root density zone; Zaid and Jiménez 2002). Therefore, there is a high risk of decreasing crop After four years of irrigation with saline water, a sig- nificant increase in the electrical conductivity (EC ) of the production due to the impact of soil salinization on the root zone (Huang et al. 2011). The variations in EC throughout irrigated soil was observed (Fig. 5). This implied a progres- sive accumulation of salt in the soil profile during the stud- the soil profile revealed that the leaching effect of the irri - gation water (i.e., salt removal from soil) was significantly ied period. As seen in Fig. 5, the EC of the irrigated soil −1 exceeded the desired EC level of the date palm (< 4 dS m ; higher for the topsoil than for the deeper soil layers. This could be the main factor in the large difference between the Maas and Hoffman 1977). A similar salinization trend has been noted for irrigated soil in many other Tunisian Saharan EC of the topsoil and that of the deep soil layer. As reported in many studies (e.g., El-Haddad and Noaman 2001; Mosta- oases (e.g., Askri and Bouhlila 2010; Haj-Amor et al. 2016). It appears that the climatic conditions of Tunisian Saha- fazadeh-Fard et al. 2007), accurate estimation of the leaching −1 requirement is key to reducing soil desalinization. To cal- ran oases (rainfall < 80 mm year and evapotranspiration culate this, it is important to take into account the depth of the soil profile (to prevent soluble salt from being deposited in the deeper soil layers), the salinity threshold of the crop −1 (4 dS m for the date palm), and the salinity level of the applied water. As stated by Mostafazadeh-Fard et al. (2007), the accurate estimation of the leaching requirement, the application of irrigation water of a suitable salinity, and the use of a well-designed drainage system represent an effective overall approach to managing soil salinization in arid lands. Regarding the physical properties of the studied soil, the hydraulic conductivity (K ) of the irrigated soil decreased −1 −1 from 99.3 to 81.2 cm day and from 85.3 to 72.1 cm day in the 0–30 cm and 60–90 cm soil layers, respectively (Fig. 6). The major decrease in K over the studied period can be directly related to the observed increase in soil salini- zation. Increasing the water-soluble salt concentration in the Fig. 4 Comparison of the sodium adsorption ratio (SAR) before and after irrigation soil can suppress soil dispersion, leading to a decrease in 1 3 22 Page 8 of 12 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 management plan that has the potential to minimize the impact of saline water on irrigated soil. Eec ff ts of the irrigation treatments on the soil characteristics In this section, we present the effects of the different irriga- tion treatments applied to the three monitored plots (Table 2) on their soil properties. Duncan’s multiple range (DMR) test at a significance level of 0.05 was utilized to compare the results of the three treatments. Based on the measurements obtained and the results of the statistical test, we were able to identify the irrigation treatment that minimized the nega- tive effects of the saline water treatment on the soil physico - Fig. 6 Measured saturated soil hydraulic conductivity (K ) values chemical characteristics, thus optimizing plant growth and before and after irrigation at different soil depths productivity. Table 5 summarizes the effects of the different irrigation treatments on the soil properties of each layer. Eec ff t on soil electrical conductivity (EC ) The results presented in Table 5 show a significant (p value < 0.05) difference in soil electrical conductivity (EC ) between the three irrigation treatments (T , T , and T ) for 1 2 3 each soil layer. Figure 8 shows the average value of EC recorded during the studied period for each irrigation treat- ment. According to the figure, it is evident that EC built up to levels that exceeded the salt tolerance of date palms, −1 estimated at 4 dS m (Maas and Hoffman 1977). The main causes of this soil salinization are the high salinity of the −1 irrigation water (EC = 3.51 dS m ), the climatic condi- iw tions (especially the aridity and very low rainfall), and the Fig. 7 Comparison between the volumetric water content (θ ) after the first irrigation event (January 2012) and that after the last irriga- soil properties (texture, salinity, etc.). However, a remark- tion event (December 2015) able effect of the irrigation treatment on EC was noted. For example, for the 0–30 cm soil layer, it was observed that hydraulic conductivity (Singh et al. 2011; Adhikari et al. the average EC of the soil under treatment T was 47.2% e 3 2014; Hanson et al. 1999). The observed decrease in K has lower than the average EC of the soil under treatment T . s e 2 a negative effect on the soil water content by decreasing the Comparison of the EC values of the top layer under treat- infiltration rate of the irrigation water into the soil profile. ments T and T revealed that the EC was 52.1% lower 3 1 e This argument is supported by measurements of the volu- under treatment T . Thus, the high irrigation treatment sub- metric soil water content (θ ), as shown in Fig. 7. stantially contributed to reducing soil salinity. As shown in θ was measured after each irrigation event. The results Fig. 8, the observed decrease in EC with the high treatment v e revealed that for all soil layers, and assuming that the same helped to reduce salinization stress by keeping the electrical amount of irrigation water was applied at each event, θ gen- conductivity of the soil close to the tolerance level (salinity −1 erally decreased over the studied period. It appears that the threshold) of the date palm, which is about 4 dS m (Maas decrease in θ was primarily due to the long-term (four-year) and Hoffman 1977). application of saline groundwater to irrigate the palm trees in the studied oasis. Eec ff t on soil water content (θ ) In summary, the results show that the evaluated phys- 2+ 2+ icochemical properties of the soil (EC, pH, Ca, Mg , A significant difference (p value < 0.05, Table 5) in soil e e Na , SAR, K , and θ ) are interdependent; for example, an water content (θ ) was observed between the different irriga - s v v increase in EC could cause a major increase in K . There- tion treatments. As shown in Fig. 9, a positive effect on θ of e s v fore, all of these soil properties should be taken into con- increasing the amount of irrigation water applied was noted. sideration when devising an appropriate soil and water As can be seen in the figure, for the 60–90 cm soil layer 1 3 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Page 9 of 12 22 Table 5 Effects of the irrigation treatments [low (T ), medium (T ), and high (T ) treatments] on soil properties (mean ± S.D. values are shown, 1 2 3 as are significance levels) Soil layer (cm) Soil property Irrigation treatment Significance levels T T T Between T Between T Between 1 2 3 1 1 and T and T T and 2 3 2 −1 0–30 EC (dS m ) 9.2 ± 1.7 7.41 ± 1.1 4.21 ± 1.2 0.039 0.031 0.035 pH (−) 6.5 ± 0.4 6.85 ± 0.6 6.71 ± 0.7 n.s. n.s. n.s. + −1 Na (meq L ) 38.9 ± 0.3 39.5 ± 3.2 32.5 ± 3.1 n.s. 0.029 0.026 2+ −1 Ca (meq L ) 27.2 ± 1.3 32.2 ± 2.3 29.2 ± 2.6 n.s. n.s. n.s. 2+ −1 Mg (meq L ) 12.7 ± 0.8 15.6 ± 1.3 14.8 ± 1.1 n.s. n.s. n.s. SAR (−) 10.4 ± 1.5 8.1 ± 0.6 6.6 ± 0.8 0.036 0.033 0.037 −1 K (cm day ) 92.2 ± 0.6 90.2 ± 0.8 91.2 ± 0.5 n.s. n.s. n.s. 3 −3 θ (cm cm ) 0.034 ± 0.003 0.048 ± 0.002 0.055 ± 0.001 0.022 0.021 0.045 −1 30–60 EC (dS m ) 9.9 ± 0.7 7.8 ± 0.9 4.4 ± 1.3 0.041 0.033 0.037 pH (−) 6.4 ± 0.4 6.7 ± 0.5 6.6 ± 0.2 n.s. n.s. n.s. + −1 Na (meq L ) 37.1 ± 0.3 37.5 ± 1.8 31.1 ± 0.5 n.s. 0.031 0.028 2+ −1 Ca (meq L ) 25.2 ± 1.1 30.4 ± 1.9 28.2 ± 1.2 n.s. n.s. n.s. 2+ −1 Mg (meq L ) 11.5 ± 0.8 13.8 ± 0.9 13.6 ± 0.3 n.s. n.s. n.s. SAR (−) 10.3 ± 1.5 8.1 ± 06 6.4 ± 0.5 0.038 0.035 0.039 −1 K (cm day ) 84.2 ± 0.6 83.7 ± 0.9 84.8 ± 0.4 n.s. n.s. n.s. 3 −3 θ (cm cm ) 0.048 ± 0.002 0.061 ± 0.003 0.068 ± 0.003 0.024 0.023 0.047 −1 60–90 EC (dS m ) 12.4 ± 1.7 8.26 ± 0.6 4.7 ± 0.3 0.040 0.034 0.038 pH (−) 6.1 ± 0.4 6.5 ± 0.4 6.3 ± 0.2 n.s. n.s. n.s. + −1 Na (meq L ) 36.1 ± 0.6 35.4 ± 2.4 30.7 ± 0.1 n.s. 0.033 0.029 2+ −1 Ca (meq L ) 23.2 ± 1.2 28.9 ± 1.2 25.2 ± 1.5 n.s. n.s. n.s. 2+ −1 Mg (meq L ) 11.2 ± 0.9 12.8 ± 1.3 13.1 ± 0.3 n.s. n.s. n.s. SAR (−) 10.1 ± 1.5 7.95 ± 0.8 6.1 ± 0.5 0.039 0.037 0.038 −1 K (cm day ) 79.25 ± 0.6 78.72 ± 0.6 79.81 ± 0.4 n.s. n.s. n.s. 3 −3 θ (cm cm ) 0.055 ± 0.002 0.071 ± 0.001 0.075 ± 0.003 0.026 0.025 0.044 n.s. not significant at the 0.05 level Fig. 8 Average soil electrical conductivities (EC ) in the three soil layers under various irriga- tion treatments (T , T , and T ) 1 2 3 (the high root density zone), the average θ was 24.7 and increases in EC and θ , respectively, were directly related to v e v 14.1% higher under the treatment T than under treatments the application of T during the studied period. As reported 3 3 T and T , respectively (Fig. 9). These large decreases and by many scholars (e.g., Haj-Amor et al. 2016; Askri et al. 1 2 1 3 22 Page 10 of 12 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Fig. 9 Average soil water contents (θ ) of the three soil layers under various irrigation treatments (T , T , and T ) 1 2 3 2014), improved knowledge of the EC and θ values of irri-Eecfft on pH e v gated soil permits more accurate irrigation scheduling, thus reducing the negative effects of saline irrigation water on The results presented in Table 5 suggest that pH and K e s the agricultural soil. were not significantly influenced by the treatment applied during the studied period. The pH measured in each plot 2+ 2+ + Eecffts on the Ca, Mg, and Na concentrations was in good agreement with the results of the study con- and the SAR ducted by Rahil et al. (2013), who showed that the pH of the irrigated soil was not affected by the amount of saline A comparison of the effects of the different irrigation treat- irrigation water applied. This study revealed that the pH is 2+ 2+ + ments on the Ca, Mg , and Na concentrations is pro- a function of the quality (especially the salinity level) of the vided in Table 5. As can be seen in the table, for all three irrigation water, not the quantity of irrigation water used. 2+ investigated soil layers, the concentrations of Ca and 2+ Mg did not vary significantly with the irrigation treatment Eec ff t on K applied. However, the N a concentration was significantly (p < 0.05) lower for treatment T (relative to the Na concen- The hydraulic conductivity (K ) of the soil also did not dif- trations for T and T ). The decrease in Na concentration fer significantly between the three monitored plots (i.e., did 1 2 under the high irrigation treatment could help to enhance not vary with the irrigation treatment applied; see Table 5). date palm growth. As depicted in Fig. 10, for all soil lay- This can be explained by the fact that K is mainly dependent ers, the lowest average value of the sodium adsorption ratio on the quality of the irrigation water (Bardhan et al. 2007). (SAR) was obtained with the high irrigation treatment. This As discussed above, it is evident that when the date palms is directly related to the significant decrease in Na concen- were irrigated frequently (every 15 days over the 4-year −1 tration under T (Fig. 10). period) with saline water (2.24 g l ), the high irrigation Fig. 10 Average sodium adsorp- tion ratios (SAR) for the three soil layers of interest under vari- ous irrigation treatments (T , T , 1 2 and T ) 1 3 Euro-Mediterranean Journal for Environmental Integration (2018) 3:22 Page 11 of 12 22 treatment (i.e., applying 110% of the net irrigation require- Saharan oases that are irrigated with saline water in order ment of date palms) was the best irrigation strategy for main- to optimally manage the salinization process and potentially taining the appropriate properties of the irrigated soil and prevent further soil deterioration in Saharan croplands. thus achieving the highest potential yield of planted trees. Acknowledgements The present work was financially supported by Such an approach requires continuous control of the irriga- the Ministry of Agriculture, Tunisia. The authors thank the Agricul- tion water and soil properties (Bouksila et al. 2013), and ture Development Office of Kebili, Tunisia for technical support. The considerable effort must be made to educate farmers about second author acknowledges the support of the Swedish Research Council for Environment, Agricultural Sciences, and Spatial Planning the tremendous benefits of the high irrigation treatment for (FORMAS, grant: 220-2014-977). The authors would also like to thank soil productivity. Therefore, as stated by Ghazouani et al. Hafedh Rigane (Faculty of Sciences of Sfax, Tunisia) for his thoughtful (2009), special attention should be paid to improving farm- recommendations. ers’ awareness of the effects of irrigation strategy, particu- larly the combined influence of water quantity and quality, Compliance with ethical standards on soil characteristics. Our understanding of the water man- agement strategy currently employed in the studied oasis and Conflict of interest The present paper is an original work and the au- the findings of Ghazouani et al. (2009) both suggest that the thors all declare that they have no conflict of interest. farmers’ perceptions of water conservation and their appli- Open Access This article is distributed under the terms of the Creative cation of a low irrigation treatment have degraded the stud- Commons Attribution 4.0 International License (http://creativecom- ied region’s soil productivity. Therefore, the farmers should mons.org/licenses/by/4.0/), which permits unrestricted use, distribu- be educated about the need to ensure sustainable agricul- tion, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the tural activity in the studied oasis by applying the appropri- Creative Commons license, and indicate if changes were made. ate long-term strategy for irrigating with saline water. On the other hand, a similar study has shown that the farmers have the ability to adapt and reduce the problems caused by soil salinization due to their long-term practical experience References (Omrani and Dieter 2012). Adhikari P, Shukla MK, Mexal JG, Daniel D (2014) Irrigation with treated wastewater: quantification of changes in soil physical and chemical properties. 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Published: May 29, 2018
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