Effects of Recycled Wastewater Applications with Different Irrigation Practices on the Chemical Properties of a Vertisol

Abstract

The soil degradation caused by the use of low-quality waters in agriculture may be restricted with reorganization of irrigation practices. Therefore, a 2-year study was conducted to determine the changes in chemical and biological properties of soil layers of 0–30, 30–60, and 60–90 cm of a tomato field that was irrigated by recycled municipal wastewater with different drip practices (full, DI: deficit irrigation, and PRD: partial root-zone drying irrigation). The study results showed that the soil electrical conductivity (EC) and exchangeable sodium percentage (ESP) values after the experiment were quite low considering the problematic levels (>4 dS/m for EC, and >15% for ESP), and the organic matter content did not change significantly. The highest N, P₂O₅, and K₂O concentrations were determined in 0–30 cm soil layer for fully irrigated with wastewater, whereas the PRD and DI treatments irrigated with 50% water saving resulted the lowest soil fertility. Wastewater treatment also increased concentrations of micro- and trace minerals, but the toxic element concentrations were in safe limits considering the FAO/WHO and national standards. In this 2-year study, it can be mentioned that treated wastewater applications increase soil fertility owing to the increase in macro and micronutrients. It may be suggested that wastewater treated with PRD and DI applications can be used in case of water shortages.

Introduction

Approximately 70% of fresh water across the globe is used in the agriculture. In the past half century, agriculture has expanded and intensified to meet the increasing food requirement due to especially population growth. The water demand in the agriculture is predicted to increase significantly over the coming decades. Therefore, both using the wastewater as the alternate water resource and selecting the irrigation practices that allow irrigation with fewer quantities can be considered to be two important ways for protecting limited freshwater (FW) resources.

Treated and untreated wastewaters are used in irrigation of more than 20 million hectares of arid and semiarid agricultural regions worldwide (Corcoran et al., 2010). Moreover, the use of wastewater for agricultural irrigation is prominent and rapidly spreading especially in low-income countries, and in high-income countries with arid and semiarid climate [Energy Transport and Water Department Water Anchor (ETWWA), 2010].

Municipal wastewaters include organic matter (OM), suspended solids, nutrients (N, P, and K), dissolved minerals, toxic heavy metals, and pathogens. Treated and untreated wastewater, as a rich nutrient source, is extensively used in agriculture to reduce fertilizer costs (Hussain et al., 2002). Many researchers reported increases in soil fertility in the wastewater irrigation conditions (Kiziloglu et al., 2008; Singh and Agrawal, 2012; Alrajhi et al., 2015; Disciglio et al., 2015). However, wastewater irrigation also brings about chemical pollution problems in soils. Especially long-term wastewater application may increase heavy metal concentrations in soils (Al-Omron et al., 2012). Wastewater may also induce salinity and sodicity in the soil (Kallel et al., 2012; Schacht and Marschner, 2015).

The other alternative to protect limited water resources is related to the application of water. Deficit irrigation (DI) and partial root-zone drying irrigation (PRD) practices allow irrigation with less water. The main benefit of DI and PRD practices is the improvement in water productivity (Alrajhi et al., 2015). Wastewater applications with less quantity using different irrigation practices may decrease soil salinity, heavy metal accumulation, and field fertilizer requirement and improve soil properties for better crop production. On the contrary, leaching of salts is limited under DI and thus average salt concentration in the irrigated zone might be higher under DI. However, irrigation with a wetting area ratio of <1% in the drip irrigation method can increase salinity under higher irrigation quantities due to decreased leaching.

Effective use of recycled wastewater (RW) in agricultural irrigation requires improvement on the irrigation quantities and application techniques in different cultivation regions. Appropriate practices may reduce the adverse effects of pollutants in wastewater and protect or even enhance the soil fertility. Therefore, this study was conducted to determine the effects of RW applications with the DI and PRD practices under surface drip irrigation on chemical properties of a Smectitic Vertisol soil in the tomato cultivation in a region with hot-dry growing period. The total area of tomato production and its amount in Turkey are 117,509.5 ha and 8,414,920 t, respectively. Also in the research area, this crop is one of the mostly cultivated crops with a production amount of 8,431 t in area of 350.9 ha [Turkish Statistical Institute (TUIK), 2019].

Materials and Methods

Study area, climate, and soil properties

The study was conducted in an experimental field (38.8839° N, 40.5492° E and 1,030 m above sea level) cultivated with tomato plants in Bingöl, Turkey in the years 2013 and 2014. The region has a continental climate (Dsa; D: cold continental climate, s: dry summer, a: hot summer), with hot and dry summers and cold and snowy winters considering Köppen classification (Kottek et al., 2006). According to the long period (1961–2016) data obtained from the Bingöl Meteorological Station located near the experimental site, the annual mean temperature and total precipitation in the region are 12.1°C and 943.6 mm, respectively. About 80% of precipitation falls during November to April period (761.1 mm). July (5.6 mm total precipitation and 26.8°C mean air temperature) and August (3.2 mm total precipitation and 26.4°C mean air temperature) are the driest and hottest months. Tomato growing seasons for the experiment years were May 20 to October 10 in 2013 and May 31 to October 4 in 2014. The mean temperature and the total precipitation values of the first experiment year's growing season were 23°C and 24.1 mm, respectively. The values of the second experiment year were also 24.2°C and 36.2 mm. The precipitation values were measured by using a pluviometer in the experimental site.

The soil type in the study area is Vertisol according to the USDA soil taxonomy. Demir (2016) confirmed also that the soils of research area is Smectitic Vertisol due to montmorillonite clay type. Physical, chemical, and biological properties of the experimental field soil before planting are given in Table 1. The experiment field had well-drained clay soil. There was no salinity and sodicity problem.

Table 1.

The Physical, Chemical, and Biological Properties Before Planting in Three Soil Layers

Property	0–30 cm	30–60 cm	60–90 cm
Texture	Clay	Clay	Clay
Sand, %	30.2	26.4	27.4
Silt, %	28.6	29.1	27.9
Clay, %	41.2	44.5	44.7
Bulk density, Mg/m³	1.30	1.31	1.36
Field capacity, % of weight	28.5	30.3	30.8
Wilting point, % of weight	17.2	18.1	18.4
AWC, mm	44.1	47.9	50.6
pH	8.01	7.94	7.92
EC, dS/m	0.53	0.51	0.45
Organic matter, %	1.60	1.30	1.10
CaCO₃, %	4.60	3.40	2.10
Total N, %	0.08	0.07	0.05
P₂O₅, kg/decare	8.30	5.70	1.50
K₂O, kg/decare	71.3	66.2	58.8
Ca, cmol/kg	25.1	28.7	29.4
Mg, cmol/kg	5.60	4.63	4.84
Na, cmol/kg	0.50	0.40	0.40
CEC, cmol/kg	32.9	36.5	37.5
ESP, %	1.52	1.10	1.07
B, mg/kg	0.57	0.51	0.54
Fe, mg/kg	14.5	15.7	15.0
Zn, mg/kg	0.60	0.80	0.40
Cu, mg/kg	0.60	0.80	0.80
Mn, mg/kg	13.2	11.3	12.7
Cd, mg/kg	0.20	0.30	0.30
Ni, mg/kg	1.90	1.40	0.90
Pb, mg/kg	0.09	0.09	0.05

AWC, available water content; CEC, cation exchange capacity; EC, electrical conductivity; ESP, exchangeable sodium percentage.

Experimental design and irrigation applications

Plant material was Joker F1tomato hybrid (Lycopersicanesculentum). The plants were drip-irrigated with two different water types, namely RW collected from the Bingöl city wastewater treatment plant and FW obtained from an irrigation network in the region. The characteristics of the irrigation waters as the mean of three sampling periods (June, July, and August) in the intensive irrigation season are presented in Table 2.

Table 2.

The Quality Properties (Mean ± Standard Error of the Mean) of the Waters Used in Irrigation as 2-Year Averages

Property	Unit	RW	FW
pH	—	7.75 ± 0.19	7.86 ± 0.17
EC	dS/m	0.469 ± 0.026	0.202 ± 0.022
TSS	mg/L	19.1 ± 2.41	16.1 ± 2.11
Total N	mg/L	13.2 ± 0.43	—
Total P	mg/L	1.80 ± 0.08	—
Ca	me/L	1.92 ± 0.22	1.30 ± 0.21
Mg	me/L	1.10 ± 0.18	0.72 ± 0.04
Na	me/L	1.32 ± 0.22	0.18 ± 0.01
K	me/L	0.34 ± 0.05	0.08 ± 0.01
CO₃	me/L	0.02 ± 0.02	0.01 ± 0.01
HCO₃	me/L	0.36 ± 0.05	0.27 ± 0.03
SO₄	me/L	2.86 ± 0.29	1.49 ± 0.07
Cl	me/L	1.37 ± 0.26	0.47 ± 0.13
B	mg/L	0.42 ± 0.08	0.22 ± 0.07
Fe	mg/L	0.18 ± 0.06	0.14 ± 0.02
Zn	mg/L	0.04 ± 0.01	0.03 ± 0.01
Cu	mg/L	0.05 ± 0.02	0.03 ± 0.01
Mn	mg/L	0.06 ± 0.01	0.02 ± 0.01
Cd	mg/L	0.07 ± 0.02	0.06 ± 0.01
Ni	mg/L	0.05 ± 0.01	0.03 ± 0.01
Pb	mg/L	0.05 ± 0.01	0.04 ± 0.01
Co	mg/L	0.19 ± 0.00	0.16 ± 0.01
Cr	mg/L	0.37 ± 0.03	0.32 ± 0.03
SAR	—	1.13 ± 0.22	0.18 ± 0.02
FC	cfu/100 mL	677 ± 48.1	—

FC, fecal coliform; FW, freshwater; RW, recycled wastewater; SAR, sodium adsorption ratio; SEM, standard error of the mean; TSS, total suspended solids.

Before planting, diammonium phosphate was applied to the experimental field at the dose of 500 kg/ha. After planting, compound NPK (15:15:15) and potassium nitrate (13:0:46) fertilizers were applied to experimental plots with irrigation water five times in total and with 1 week intervals. Application rates were 50 kg/ha each fertilizer in each of the irrigation.

The experiment was conducted as a randomized complete block design, in a 2 × 5 factorial arrangement, corresponding to two different water resources (RW and FW) and five different irrigation practices as detailed below. Each treatment was replicated 3 times, with a total of 30 plots. Each experimental plot was arranged as five plant rows with 1 m spacing. Nine plants with 0.50-m interval on each row were planted. Two different irrigation water sources were applied based on indicating five practices; (1) FI: Full Irrigation, (2) DI25: Deficit Irrigation, (3) DI50: Deficit Irrigation, (4) PRD25: Partial Root-zone Drying Irrigation, and (5) PRD50: Partial Root-zone Drying Irrigation. The plots irrigated with the DI and PRD practices received lesser water of 25% (in the DI25 and PRD25 practices) and of 50% (in the DI50 and PRD50 practices) compared with the full-irrigated plots. While the irrigation water was applied to both sides of the root zone in FI and DI practices, just one side of the root zone in PRD treatments was irrigated alternately. Therefore, driplines were placed on both sides of plant rows at a distance of 0.5 m away from the plant rows. Drip irrigation system was installed with a pump, control unit, and pipelines. Driplines of 16 mm diameter included in-line emitters with 25 cm spacing. Flow rate of emitters was 4 L/h under 0.1 MPa operation pressure.

The irrigations were done when 40% of available soil water in control plots (fully irrigated with FW) was consumed. Tensiometers (IRROMETER Company, Inc.) calibrated to the experimental field were established to determine approximately irrigation time in control plots. Twenty-three irrigations were applied each year. Seasonal irrigation quantities in FI practice were 640.2 mm in 2013 and 648.1 mm in 2014. While 2 years average irrigation quantities in DI25 and PRD25 practices received 23.3% less water compared with FI, DI50 and PRD50 practices received 46.5% less water than FI. The soil moisture contents in the DI and PRD practices were low due to reduced irrigation quantities compared with the FI practice. However, soil moisture contents did not decrease below the wilting point in both DI and PRD practices throughout the growing seasons.

Irrigation water and soil analysis

The pH and electrical conductivity (EC) values of irrigation waters applied were measured by a pH-meter (Orion 3-Star) and EC-meter (Orion 3-Star), respectively (Tüzüner, 1990). Total suspended solids (TSS) were analyzed as described in the American Public Health Association [American Public Health Association (APHA), 1995]. Total N was determined using the Kjeldahl method [Environmental Protection Agency (EPA), 2001]. Total P was determined by measuring orthophosphate (Nollet, 2000). Calcium and Mg were analyzed by EDTA titration. The flame-photometric method was used for Na and K analysis, and B was determined by the carmine method (Richards, 1954). HCO₃ and CO₃ concentrations were measured by titration using sulfuric acid, and the Cl concentration was determined using volumetric silver nitrate (Richards, 1954). Sulfate was analyzed by a spectrophotometer (Specord 200 Plus) using barium chloride solution (Eltan, 1998). Micro and trace minerals (Fe, Zn, Cu, Mn, Cd, Ni, Pb, Co, and Cr) were quantified by an atomic absorption spectrophotometer (PerkinElmer) in water samples diluted with HNO₃ of 2.5% (Karadede and Ünlü, 2000). The membrane filtration method was used in determining fecal coliforms [Turkish Standards Institution (TSI), 2011]. Furthermore, the sodium adsorption ratio (SAR) was calculated (Kanber and Ünlü, 2010).

The pH, EC, TSS, B, Cl, Na, and HCO₃ values of the waters are given in Table 2 and were found to be suitable for irrigation, considering the standards proposed by the FAO (Ayers and Westcot, 1985). Although SO₄^2-(<4 me/L) concentration in the waters was low according to the Turkish National Water Pollution Control Regulation Standards, total N (>5 mg/L) and P (>0.65 mg/L) values were high in RW [Water Pollution Control Regulation (WPCR), 2008]. While the Fe, Mn, and Zn concentrations in RW and FW provided the best quality for irrigation, the Cu, Cd, Ni, Pb, Co, and Cr concentrations in RW indicated lower quality (WPCR, 2008). The Cu, Cd, Ni, Co, and Cr concentrations in FW also lowered irrigation water quality of this water. The SAR value was low in both water types according to the USSL classification (Kanber and Ünlü, 2010). Fecal coliform value in RW was lower than the permissible maximum value (<1000 cfu/100 mL) for raw-eaten crops (Pescod, 1992).

All the plots were sampled separately to determine the changes in physical, chemical, and biological properties of the soil during the experiment years at the end of the study (2013 and 2014). Disturbed and undisturbed soil samples were collected from the three soil layers of 0–30, 30–60, and 60–90 cm in the middle region of each plot. Soil sampling of the study area was also conducted before the initiation of the experiment to determine the initial soil properties.

The proportions of sand, silt, and clay fractions were detected by the Bouyoucos hydrometer method described by Gee and Bauder (1986). Clay contents in the clay texture soil were determined 41.2%, 44.5%, and 44.7% in 0–30, 30–60, and 60–90 cm soil layers, respectively (Table 1). The cylinder method was used to determine the soil bulk density (Blake and Hartge, 1986). The amount of water retention at field capacity (−33 kPa) and wilting point (−1.5 MPa) were determined with a pressure plate (Cassel and Nielsen, 1986). The available water contents in 0–30, 30–60, and 60–90 cm soil layers calculated from field capacity, wilting point, and bulk density values were 44.1, 47.9, and 50.6 mm, respectively (Table 1).

A saturation extract was prepared from the soils, and used to determine soil pH by a pH-meter (Mclean, 1982) and soil EC using an EC-meter (Rhoades, 1996). Soil OM was determined by using the Walkley-Black method (Nelson and Sommers, 1982). The soil CaCO₃ content was measured with a Scheibler calcimeter under dilute acid reaction (Nelson, 1982). Total soil nitrogen was determined by the Kjeldahl method considering the procedures given by Kaçar (2009). The available P₂O₅ content was measured by the Olsen method (Olsen and Sommers, 1982). Exchangeable Ca and Mg were determined by EDTA titration, a flame photometer (BWP XP) was used to analyze the exchangeable Na and K concentrations in the solution obtained through soil extraction with 1N ammonium acetate (Black, 1965). Available K₂O was calculated using K content. Cation Exchange capacity (CEC) was determined by the sodium acetate method (Rhoades, 1982). Exchangeable sodium percentage (ESP) was calculated by the ratio of exchangeable Na to CEC, and the Azomethine-H method was used to determine the boron content (Kaçar, 2009). Fe, Zn, Cu, and Mn concentrations were analyzed with DTPA extraction of soil (Lindsay and Norwell, 1978). Similarly, while Cd and Ni concentrations were analyzed with DTPA extraction of soil, Pb content with ammonium acetate extraction (Kaçar, 2009). All micromineral readings were done by an atomic absorption spectrophotometer (PerkinElmer).

Statistical analysis

The analysis of variance model in SAS software (ver. 8.1) was used for the evaluation of the effects of the irrigation water types and irrigation practices on soil chemical properties. The means were grouped using Duncan's multiple range test.

Results and Discussion

The soil chemical findings determined from wastewater irrigations under different irrigation practices were evaluated on three categories considering main chemical and biological properties of the soil, its macronutrients, and micro and trace minerals.

Main chemical and biological properties

According to Table 3, the pH and OM values in three soil layers were not statistically different under the RW and FW treatments. Similar changes in soil pH were possibly due to the similarity of the pH values of the irrigation water sources (Table 2). However, pH values showed the highest reduction in 0–30 cm soil layer under RW in comparison with the initial values (Table 1). The decreasing trend in soil pH in the RW treatments was compatible to the findings observed by Al-Omron et al. (2012) and Wang et al. (2007), who reported wastewater irrigation lowered soil pH slightly. Similarly, Kiziloglu et al. (2008) and Singh and Agrawal (2012) indicated that nitrification of ammonium in wastewater and oxidation of different organic compounds can reduce soil pH values.

Table 3.

pH, Electrical Conductivity, Organic Matter, CaCO₃, Cation Exchange Capacity, and Exchangeable Sodium Percentage Values (Mean ± Standard Error of the Mean) in Three Different Layers of the Soil Irrigated with Recycled Wastewater and Freshwater Under Different Irrigation Practices as 2-Year Averages

Soil layer	Parameter	FI		DI25		DI50		PRD25		PRD50		General mean
Soil layer	Parameter	RW	FW	RW	FW	RW	FW	RW	FW	RW	FW	RW	FW	FI	DI25	DI50	PRD25	PRD50
0–30	pH	7.85 ± 0.07	7.83 ± 0.07	7.82 ± 0.06	7.77 ± 0.07	7.85 ± 0.06	7.94 ± 0.04	7.86 ± 0.05	7.91 ± 0.03	7.87 ± 0.05	7.89 ± 0.07	7.85	7.87	7.84	7.80	7.89	7.88	7.88
	EC, dS/m	0.77 ± 0.04	0.70 ± 0.03	0.63 ± 0.04	0.64 ± 0.03	0.56 ± 0.02	0.58 ± 0.03	0.65 ± 0.04	0.64 ± 0.05	0.62 ± 0.01	0.56 ± 0.03	0.65	0.62	0.74a^**	0.63bc	0.57c	0.65b	0.59bc
	OM, %	2.41 ± 0.13	2.32 ± 0.13	2.28 ± 0.16	2.24 ± 0.14	2.20 ± 0.16	2.15 ± 0.21	2.33 ± 0.14	2.24 ± 0.16	2.19 ± 0.17	2.12 ± 0.10	2.28	2.21	2.36	2.26	2.18	2.29	2.16
	CaCO₃, %	3.73 ± 0.27	4.27 ± 0.18	3.89 ± 0.29	4.31 ± 0.20	4.19 ± 0.18	3.81 ± 0.26	3.74 ± 0.18	4.24 ± 0.20	3.54 ± 0.26	3.92 ± 0.16	3.82b^*	4.11a	4.00	4.10	4.00	3.99	3.73
	CEC, cmol/kg	32.5 ± 1.2	34.5 ± 0.9	31.4 ± 1.4	33.6 ± 0.3	31.2 ± 1.6	33.4 ± 1.0	35.5 ± 1.3	33.5 ± 1.0	31.5 ± 1.5	32.1 ± 0.4	32.4	33.4	33.5	32.5	32.3	34.5	31.8
	ESP, %	2.20 ± 0.10	1.68 ± 0.09	2.03 ± 0.15	1.60 ± 0.08	1.81 ± 0.11	1.73 ± 0.23	1.94 ± 0.17	1.86 ± 0.20	1.76 ± 0.17	1.77 ± 0.14	1.95a^*	1.73b	1.94	1.82	1.77	1.90	1.77
30–60	pH	7.85 ± 0.04	7.90 ± 0.03	7.78 ± 0.04	7.82 ± 0.04	7.81 ± 0.03	7.91 ± 0.04	7.92 ± 0.04	7.88 ± 0.03	7.90 ± 0.03	7.94 ± 0.03	7.85	7.89	7.87ab^*	7.80b	7.86ab	7.90a	7.92a
	EC, dS/m	0.77 ± 0.05	0.63 ± 0.04	0.67 ± 0.05	0.59 ± 0.05	0.70 ± 0.04	0.49 ± 0.02	0.70 ± 0.05	0.60 ± 0.02	0.64 ± 0.05	0.56 ± 0.02	0.70a^**	0.57b	0.70	0.63	0.59	0.65	0.60
	OM, %	2.38 ± 0.20	2.29 ± 0.15	2.23 ± 0.15	2.17 ± 0.14	2.12 ± 0.06	2.09 ± 0.26	2.21 ± 0.15	2.06 ± 0.12	2.16 ± 0.13	2.09 ± 0.10	2.22	2.14	2.34	2.20	2.10	2.14	2.12
	CaCO₃, %	4.11 ± 0.43	4.12 ± 0.32	3.66 ± 0.21	4.14 ± 0.17	3.99 ± 0.18	3.98 ± 0.16	3.77 ± 0.16	3.98 ± 0.21	3.68 ± 0.23	3.82 ± 0.15	3.84	4.01	4.12	3.90	3.99	3.87	3.75
	CEC, cmol/kg	35.4 ± 0.9	32.6 ± 1.0	32.2 ± 0.6	33.4 ± 0.7	33.4 ± 1.0	32.8 ± 1.0	35.5 ± 1.1	34.4 ± 1.2	34.5 ± 1.2	33.4 ± 1.0	34.2	33.3	34.0	32.8	33.1	34.9	34.0
	ESP, %	2.09 ± 0.14	2.01 ± 0.18	1.99 ± 0.13	1.77 ± 0.11	2.06 ± 0.12	1.65 ± 0.12	1.85 ± 0.09	1.67 ± 0.09	1.96 ± 0.14	1.87 ± 0.10	1.99a^*	1.79b	2.05	1.88	1.86	1.76	1.92
60–90	pH	7.87 ± 0.05	7.85 ± 0.06	7.92 ± 0.04	7.92 ± 0.03	7.89 ± 0.06	7.91 ± 0.02	7.86 ± 0.04	7.91 ± 0.03	7.91 ± 0.03	7.84 ± 0.05	7.89	7.88	7.86	7.92	7.90	7.88	7.87
	EC, dS/m	0.64 ± 0.06	0.62 ± 0.03	0.60 ± 0.02	0.58 ± 0.03	0.52 ± 0.02	0.57 ± 0.03	0.63 ± 0.03	0.58 ± 0.04	0.56 ± 0.05	0.53 ± 0.03	0.59	0.58	0.63	0.59	0.54	0.61	0.54
	OM, %	2.39 ± 0.20	2.21 ± 0.12	2.01 ± 0.14	2.0 ± 0.07	1.95 ± 0.12	1.88 ± 0.09	1.90 ± 0.17	1.87 ± 0.16	1.80 ± 0.15	1.95 ± 0.09	2.01	1.98	2.30a^**	2.00b	1.92b	1.89b	1.87b
	CaCO₃, %	3.89 ± 0.42	4.60 ± 0.22	4.30 ± 0.33	4.21 ± 0.35	3.71 ± 0.13	4.21 ± 0.23	4.45 ± 0.24	4.52 ± 0.18	3.66 ± 0.15	3.93 ± 0.22	4.00	4.30	4.24	4.26	3.96	4.48	3.80
	CEC, cmol/kg	34.5 ± 1.2	33.6 ± 1.14	35.1 ± 1.0	33.6 ± 0.9	33.5 ± 1.0	32.2 ± 0.4	35.8 ± 0.9	34.4 ± 0.5	37.3 ± 1.1	32.7 ± 0.6	35.2a^**	33.3b	34.0ab^*	34.3ab	32.9b	35.1a	35.0a
	ESP, %	2.08 ± 0.12	1.90 ± 0.05	1.92 ± 0.14	1.88 ± 0.10	1.99 ± 0.20	1.88 ± 0.14	1.70 ± 0.16	1.71 ± 0.15	1.59 ± 0.08	1.93 ± 0.07	1.85	1.86	1.99	1.90	1.93	1.70	1.76

The values marked with the same letters for different water qualities or irrigation practices in each line of the “general mean” column of each soil layer are not significantly different (^**p < 0.01 or ^*p < 0.05).

DI25, 25% deficit irrigation; DI50, 50% deficit irrigation; FI, full irrigation, FW, freshwater; OM, organic matter; PRD25, 25% deficit irrigation with partial root-zone drying irrigation technique; PRD50, 50% deficit irrigation with partial root-zone drying irrigation technique; RW, recycled wastewater.

The OM contents in the three soil layers increased under both water types compared with the preplanting values (Table 1), and higher accumulation percentages were observed in deep layers. As a supporting result, the OM accumulation in the tomato-cultivated soils can be realized due to the above- and below-ground crop residues (Loveland and Webb, 2003). Therefore, more accumulation rates for OM content in the subsoil layers indicate that these layers probably included higher crop residues.

The irrigation with the RW with higher Na contents significantly (p < 0.05) increased ESP values compared with the FW values in 0–30 and 30–60 cm soil layers (Table 3). The RW treatment also significantly (p < 0.01) increased EC values in 30–60 cm soil layer and the CEC values in 60–90 cm soil layer (Table 3). The EC values in the three soil layers compared with the initial EC values given in Table 1 increased more under the RW treatment. However, low EC values in the RW as shown in Table 2 could not increase soil EC values at pronounced levels. In parallel with our findings, many researchers also determined that soil salinity increased based on the salinity of water under wastewater irrigation (Kiziloglu et al., 2008; Alrajhi et al., 2015; Tunc and Sahin, 2016). The CEC values under the conditions that wastewater was used increased with the depth (Table 3). This increase in CEC could be resulted probably from the accumulation in below layers of the clay aggregate disturbed by the effect of high Na concentrations in wastewater used is also expressed by Oliveira et al. (2016).

The CaCO₃ content in 0–30 cm soil layer in the RW treatment was significantly (p < 0.05) lower than in the FW treatment (Table 3). Moreover, the RW treatment decreased the CaCO₃ content in surface soil layers more, considering the data before planting (Table 1). One of the benefits of reusing the wastewater in agricultural irrigation is the increase in the microbial activity in soil (Durán–Álvarez and Jiménez–Cisneros, 2014). Sahin et al. (2011) and Eroglu et al. (2012) also expressed that microbial activity increased CaCO₃ solubility. Hence, increased dissolution of the lime due to probably increased microbial activity in the wastewater irrigation conditions could decrease the CaCO₃ content under this treatment. In parallel with our findings, Kiziloglu et al. (2008) and Tunc and Sahin (2015) determined lower CaCO₃ values in the wastewater irrigation compared with FW. Lime contents in 0–30 cm soil layer in both water types were lower than the initial values, while the concentrations were higher in 30–60 and 60–90 cm soil layers (Table 1). The pH values were also lower in surface soil layer considering the initial values in Table 1. Therefore, these lower values could be resulted from an increase in CaCO₃ solubility in surface soil and an accumulation in sublayers with leaching.

The pH, EC, OM, CaCO₃, CEC, and ESP values in all soil layers were generally similar for other practices compared with FI practice (Table 3). As a similar finding to the pH in this study, Alrajhi et al. (2017) found no effect on change of soil pH under PRD and DI practices with 50% and 25% water deficits. Wetting area ratio of <1% that was applied in the drip irrigation removed the leaching in higher irrigated levels. Therefore, lesser irrigation quantities brought about lesser surface soil salinity. Soil OM content in bottom soil layer (60–90 cm) significantly (p < 0.01) decreased in the practices irrigated with lesser irrigation quantities compared with full irrigation practice (Table 3). This could be a result of root distribution and development that might be affected by the irrigation practices. To support this claim, Zotarelli et al. (2009) expressed that the crop root development may change with different irrigation quantities due to the change in wetted soil volume. Ismail et al. (2007) reported that sometimes water stress effects root system structure by promoting the lateral root growing. Alrajhi et al. (2017) determined the increased root biomass in PRD and DI practices under tomato cultivation. Therefore, it could be said that similar OM contents in 0–30 and 30–60 cm soil layers are due to the tomato roots that spread mostly into upper soil layers in PRD and DI practices probably.

The highest EC and ESP values in all soil layers were detected in full irrigation with the RW. Rising sodicity and salinity under the RW treatment according to FW may be associated with higher SAR and EC values of wastewater applied (Table 2). These results agree with the findings of Kallel et al. (2012) and Schacht and Marschner (2015), who reported that saline- and sodic-treated wastewater applications provided increases in the soil salinization and sodification. Similarly, Alrajhi et al. (2015) and Biswas et al. (2017) found higher soil EC values in wastewater irrigation compared with FW. Although, increases in EC and ESP values with the RW treatment were observed in our study, the soil EC and ESP values obtained are quite below the limit values accepted for salinity (4 dS/m) and sodicity (15%) (Kanber and Ünlü, 2010).

Soil macronutrients

Main macronutrient (total N, P₂O₅ and K₂O) concentrations were higher in the RW treatment compared with the FW treatment and the concentrations decreased with the increase of the soil depth (Table 4). Statistically similar concentrations were determined in 0–30 cm soil layer for both water types, while the concentrations were significantly (p < 0.01) higher under the RW treatment in the bottom soil layer (60–90 cm). Moreover, concentrations of macronutrients in all soil layers increased at the end of experiments according to initial values shown in Table 1. This finding that was observed in both two water types can probably be a result of the fertilizing that was done before the experiment.

Table 4.

Macronutrient, Micro- and Trace Mineral Concentrations (Mean ± Standard Error of the Mean) in Three Different Layers of the Soil Irrigated with Recycled Wastewater and Freshwater Under Different Irrigation Practices as 2-Year Averages

Soil layer	Parameter	FI		DI25		DI50		PRD25		PRD50		General mean
Soil layer	Parameter	RW	FW	RW	FW	RW	FW	RW	FW	RW	FW	RW	FW	FI	DI25	DI50	PRD25	PRD50
0–30	Total N, %	0.15 ± 0.01	0.12 ± 0.02	0.13 ± 0.02	0.13 ± 0.01	0.11 ± 0.04	0.10 ± 0.02	0.13 ± 0.02	0.11 ± 0.01	0.10 ± 0.01	0.09 ± 0.01	0.12	0.11	0.14a^*	0.13ab	0.11bc	0.12abc	0.10c
	P₂O₅, kg/da	16.3 ± 0.9	15.9 ± 1.2	13.6 ± 0.9	14.4 ± 1.0	13.5 ± 0.8	12.0 ± 0.8	14.7 ± 1.2	15.1 ± 0.8	12.4 ± 1.2	10.5 ± 0.3	14.1	13.6	16.1a^**	14.0bc	12.7cd	14.9ab	11.5d
	K₂O, kg/da	97.4 ± 8.7	82.2 ± 6.2	73.7 ± 5.5	66.5 ± 5.5	75.5 ± 4.5	68.4 ± 13.4	78.3 ± 5.8	74.0 ± 4.5	74.4 ± 7.5	73.8 ± 7.5	79.9	73.0	89.8a^*	70.1b	72.0b	76.2b	74.1b
	Ca, cmol/kg	26.1 ± 1.2	27.8 ± 1.2	24.9 ± 1.6	25.6 ± 0.8	24.9 ± 0.9	26.0 ± 0.9	26.6 ± 1.0	26.4 ± 0.9	24.7 ± 0.8	24.6 ± 0.6	25.4	26.1	27.0	25.3	25.5	26.5	24.7
	Mg, cmol/kg	4.69 ± 0.30	4.40 ± 0.34	4.89 ± 0.66	5.75 ± 0.57	5.31 ±± 0.59	4.55 ± 0.65	6.30 ± 0.73	4.63 ± 0.30	5.21 ± 0.88	5.32 ± 0.27	5.28	4.93	4.54	5.32	4.93	5.47	5.27
	Na, cmol/kg	0.71 ± 0.02	0.58 ± 0.02	0.63 ± 0.04	0.54 ± 0.03	0.56 ± 0.04	0.58 ± 0.07	0.68 ± 0.04	0.62 ± 0.06	0.56 ± 0.06	0.57 ± 0.05	0.63a^*	0.58b	0.64	0.58	0.57	0.65	0.56
	B, mg/kg	0.84 ± 0.02	0.80 ± 0.03	0.69 ± 0.06	0.70 ± 0.04	0.55 ± 0.06	0.62 ± 0.05	0.74 ± 0.06	0.76 ± 0.02	0.47 ± 0.05	0.60 ± 0.07	0.66	0.70	0.82a^*	0.69b	0.58c	0.75ab	0.54c
	Fe, mg/kg	20.9 ± 1.4	17.3 ± 1.3	19.4 ± 1.3	15.5 ± 1.1	16.0 ± 0.8	14.0 ± 1.1	19.0 ± 1.4	14.1 ± 0.7	16.7 ± 0.9	13.6 ± 1.0	18.4a^**	14.9b	19.1a^**	17.4ab	15.0c	16.5bc	15.2bc
	Zn, mg/kg	0.74 ± 0.10	0.64 ± 0.11	0.73 ± 0.08	0.49 ± 0.07	0.63 ± 0.08	0.54 ± 0.08	0.62 ± 0.13	0.65 ± 0.11	0.64 ± 0.07	0.56 ± 0.09	0.67a^*	0.58b	0.69	0.61	0.58	0.64	0.60
	Cu, mg/kg	1.17 ± 0.07	0.90 ± 0.12	0.92 ± 0.14	0.80 ± 0.09	0.89 ± 0.04	0.75 ± 0.03	0.82 ± 0.07	0.76 ± 0.02	0.85 ± 0.09	0.70 ± 0.03	0.93a^**	0.78b	1.04a^*	0.86b	0.82b	0.79b	0.77b
	Mn, mg/kg	13.3 ± 0.2	13.2 ± 0.4	12.9 ± 0.4	12.7 ± 0.4	12.9 ± 0.4	12.7 ± 0.6	12.6 ± 0.4	12.0 ± 0.6	12.8 ± 0.4	12.2 ± 0.8	12.9	12.6	13.3	12.8	12.8	12.3	12.5
	Cd, mg/kg	0.43 ± 0.003	0.39 ± 0.01	0.37 ± 0.02	0.36 ± 0.02	0.33 ± 0.02	0.32 ± 0.03	0.37 ± 0.02	0.35 ± 0.01	0.34 ± 0.02	0.30 ± 0.02	0.37a^*	0.34b	0.41a^**	0.36b	0.32bc	0.36b	0.32c
	Ni, mg/kg	2.53 ± 0.11	2.34 ± 0.14	2.21 ± 0.19	1.57 ± 0.09	2.14 ± 0.20	1.73 ± 0.17	2.14 ± 0.22	1.72 ± 0.23	2.06 ± 0.27	1.41 ± 0.21	2.22a^**	1.76b	2.43a^**	1.89b	1.94b	1.93b	1.73b
	Pb, mg/kg	0.12 ± 0.01	0.12 ± 0.01	0.11 ± 0.01	0.10 ± 0.01	0.06 ± 0.01	0.10 ± 0.01	0.11 ± 0.01	0.10 ± 0.01	0.08 ± 0.01	0.09 ± 0.01	0.10	0.10	0.12a^**	0.11b	0.08c	0.11b	0.09c
30–60	Total N, %	0.11 ± 0.01	0.11 ± 0.01	0.10 ± 0.01	0.10 ± 0.01	0.10 ± 0.01	0.09 ± 0.01	0.10 ± 0.01	0.09 ± 0.01	0.10 ± 0.01	0.08 ± 0.01	0.10	0.10	0.11a^*	0.10ab	0.10ab	0.10ab	0.09b
	P₂O₅, kg/da	12.9 ± 0.9	12.5 ± 0.8	11.8 ± 1.0	11.2 ± 0.7	10.8 ± 0.8	10.2 ± 0.5	11.6 ± 0.5	11.0 ± 0.6	10.4 ± 0.6	10.3 ± 0.4	11.5	11.0	12.7a^*	11.5ab	10.5b	11.3ab	10.2b
	K₂O, kg/da	86.9 ± 8.1	66.6 ± 7.2	72.0 ± 9.8	61.3 ± 4.8	70.3 ± 4.8	60.6 ± 7.7	69.5 ± 74.6	66.3 ± 4.8	66.7 ± 3.8	58.4 ± 4.9	73.0a^**	62.7b	76.8	66.7	65.3	67.9	62.6
	Ca, cmol/kg	26.9 ± 1.3	24.6 ± 0.7	24.5 ± 0.5	27.5 ± 1.2	26.3 ± 1.1	26.8 ± 1.0	26.9 ± 0.9	25.7 ± 0.9	25.2 ± 0.9	24.9 ± 0.7	25.9	25.9	25.8	26.0	26.5	26.3	25.0
	Mg, cmol/kg	6.39 ± 1.03	5.32 ± 0.80	5.22 ± 0.26	3.83 ± 0.50	4.63 ± 0.60	3.78 ± 0.77	5.65 ± 0.84	6.07 ± 0.57	6.31 ± 1.09	5.97 ± 0.79	5.64	4.99	5.85a^**	4.52b	4.21b	5.86a	6.14a
	Na, cmol/kg	0.74 ± 0.05	0.65 ± 0.05	0.64 ± 0.04	0.59 ± 0.03	0.69 ± 0.05	0.54 ± 0.04	0.65 ± 0.03	0.57 ± 0.02	0.67 ± 0.04	0.62 ± 0.03	0.68a^**	0.59b	0.69	0.61	0.61	0.61	0.65
	B, mg/kg	0.77 ± 0.05	0.73 ± 0.08	0.62 ± 0.03	0.66 ± 0.06	0.62 ± 0.04	0.62 ± 0.05	0.66 ± 0.06	0.62 ± 0.04	0.536 ± 0.02	0.58 ± 0.07	0.64	0.64	0.75a^**	0.64b	0.62b	0.64b	0.56c
	Fe, mg/kg	18.0 ± 1.1	16.6 ± 1.6	15.3 ± 1.5	15.0 ± 1.2	12.3 ± 0.8	14.7 ± 0.7	16.2 ± 1.2	14.9 ± 0.8	14.8 ± 1.0	14.4 ± 0.4	15.3	15.1	17.3a^*	15.2ab	13.5b	15.6ab	14.6b
	Zn, mg/kg	0.65 ± 0.10	0.48 ± 0.01	0.44 ± 0.04	0.48 ± 0.04	0.59 ± 0.11	0.44 ± 0.04	0.49 ± 0.09	0.50 ± 0.04	0.46 ± 0.03	04.9 ± 0.06	0.53	0.48	0.57	0.46	0.51	0.49	0.48
	Cu, mg/kg	1.14 ± 0.04	1.09 ± 0.04	1.06 ± 0.04	1.05 ± 0.05	0.90 ± 0.06	0.85 ± 0.10	1.08 ± 0.06	1.0 ± 0.06	0.94 ± 0.17	0.96 ± 0.06	1.02	0.99	1.12a^*	1.06a	0.87b	1.04ab	0.95ab
	Mn, mg/kg	13.4 ± 0.5	12.9 ± 0.4	12.1 ± 0.4	12.2 ± 0.3	11.9 ± 0.4	12.3 ± 0.5	11.7 ± 0.4	11.8. ± 0.5	118. ± 0.5	11.2 ± 0.3	12.2	12.1	13.1a^**	12.1b	12.1b	11.7b	11.5b
	Cd, mg/kg	0.40 ± 0.02	0.38 ± 0.01	0.36 ± 0.02	0.37 ± 0.01	0.33 ± 0.02	0.36 ± 0.01	0.37 ± 0.01	0.34 ± 0.01	0.35 ± 0.01	0.34 ± 0.01	0.36	0.36	0.39a^*	0.37ab	0.35b	0.35b	0.34b
	Ni, mg/kg	2.44 ± 0.06	2.26 ± 0.13	1.63 ± 0.30	1.68 ± 0.14	1.48 ± 0.19	1.55 ± 0.09	1.59 ± 0.22	1.86 ± 0.22	1.35 ± 0.13	1.52 ± 0.09	1.70	1.77	2.35a^**	1.65bc	1.51bc	1.72b	1.44c
	Pb, mg/kg	0.12 ± 0.004	0.11 ± 0.01	0.11 ± 0.01	0.10 ± 0.01	0.09 ± 0.01	0.08 ± 0.003	0.12 ± 0.01	0.09 ± 0.01	0.11 ± 0.01	0.08 ± 0.01	0.11a^**	0.09b	0.12a^**	0.10b	0.09c	0.10b	0.10bc
60–90	Total N, %	0.10 ± 0.01	0.09 ± 0.01	0.09 ± 0.004	0.08 ± 0.01	0.08 ± 0.01	0.07 ± 0.002	0.09 ± 0.01	0.08 ± 0.004	0.08 ± 0.01	0.07 ± 0.004	0.09a^**	0.08b	0.09a^**	0.08abc	0.07c	0.09ab	0.07bc
	P₂O₅, kg/da	12.1 ± 1.1	10.2 ± 0.5	10.6 ± 0.4	9.86 ± 0.32	9.22 ± 0.43	8.08 ± 0.69	10.2. ± 0.2	9.54 ± 0.31	9.3 ± 0.6	7.97 ± 0.79	10.3a^**	9.12b	11.1a^**	10.2a	8.65b	9.88ab	8.66b
	K₂O, kg/da	83.7 ± 9.6	60.7 ± 3.8	67.5 ± 8.3	58.0 ± 4.9	65.3 ± 5.5	58.3 ± 6.0	65.4 ± 5.3	64.4 ± 3.6	65.2 ± 3.7	55.1 ± 3.0	69.4a^**	59.3b	72.2a^**	62.8b	61.8b	64.9b	60.2b
	Ca, cmol/kg	26.2 ± 0.9	24.9 ± 0.5	26.1 ± 0.6	24.7 ± 0.7	25.3 ± 0.8	25.1 ± 0.5	27.0 ± 0.5	25.7 ± 0.4	26.5 ± 0.8	25.4 ± 0.6	26.2a^**	25.2b	25.5	25.4	25.2	26.3	25.9
	Mg, cmol/kg	4.98 ± 0.51	4.49 ± 0.86	6.35 ± 0.51	6.46 ± 0.37	5.46 ± 0.19	4.77 ± 0.64	6.34 ± 0.62	5.97 ± 0.31	7.96 ± 0.44	4.86 ± 0.57	6.22a^*	5.31b	4.74c^**	6.41a	5.11bc	6.16ab	6.41a
	Na, cmol/kg	0.71 ± 0.03	0.64 ± 0.03	0.67 ± 0.03	0.63 ± 0.03	0.66 ± 0.06	0.60 ± 0.04	0.61 ± 0.05	0.59 ± 0.06	0.59 ± 0.03	0.63 ± 0.02	0.65	0.62	0.67	0.65	0.63	0.60	0.61
	B, mg/kg	0.81 ± 0.05	0.81 ± 0.05	0.68 ± 0.06	0.72 ± 0.03	0.71 ± 0.08	0.64 ± 0.04	0.67 ± 0.05	0.76 ± 0.04	0.56 ± 0.03	0.69 ± 0.04	0.68b^*	0.72a	0.81a^**	0.70b	0.67b	0.71b	0.62c
	Fe, mg/kg	20.8 ± 2.2	18.0 ± 1.0	17.8 ± 1.6	14.2 ± 0.03	15.0 ± 0.8	15.4 ± 0.9	18.1 ± 0.8	16.1 ± 0.5	16.0 ± 1.6	17.3 ± 0.4	17.5	16.2	19.4a^*	16.0b	15.2b	17.1ab	16.6b
	Zn, mg/kg	0.55 ± 0.02	0.44 ± 0.06	0.47 ± 0.05	0.55 ± 0.02	0.48 ± 0.03	0.43 ± 0.06	0.56 ± 0.04	0.49 ± 0.03	0.51 ± 0.08	0.53 ± 0.05	0.51	0.49	0.50	0.51	0.46	0.53	0.52
	Cu, mg/kg	1.19 ± 0.13	1.05 ± 0.06	1.05 ± 0.12	0.87 ± 0.06	0.96 ± 0.10	0.93 ± 0.05	1.07 ± 0.10	0.94 ± 0.11	0.92 ± 0.14	0.94 ± 0.07	1.04	0.95	1.12	0.96	0.94	1.01	0.93
	Mn, mg/kg	14.5 ± 0.9	14.1 ± 0.2	13.6 ± 0.3	13.1 ± 0.2	13.3 ± 0.4	12.6 ± 0.4	13.3 ± 0.3	13.3 ± 0.6	12.8 ± 0.4	12.6 ± 0.2	13.5	13.2	14.3a^*	13.4ab	13.0b	13.3ab	12.7b
	Cd, mg/kg	0.44 ± 0.02	0.40 ± 0.01	0.42 ± 0.03	0.35 ± 0.01	0.36 ± 0.01	0.32 ± 0.02	0.39 ± 0.02	0.33 ± 0.02	0.37 ± 0.01	0.31 ± 0.02	0.39a^**	0.34b	0.42a^**	0.38b	0.34c	0.36bc	0.34c
	Ni, mg/kg	1.97 ± 0.18	1.97 ± 0.20	1.79 ± 0.11	1.44 ± 0.09	1.43 ± 0.08	1.38 ± 0.11	1.91 ± 0.12	1.56 ± 0.15	1.29 ± 0.09	1.42 ± 0.16	1.68	1.55	1.97a^**	1.61bc	1.40c	1.74ab	1.35c
	Pb, mg/kg	0.14 ± 0.003	0.14 ± 0.001	0.12 ± 0.01	0.13 ± 0.01	0.11 ± 0.01	0.11 ± 0.01	0.12 ± 0.01	0.13 ± 0.01	0.07 ± 0.01	0.12 ± 0.01	0.11b^**	0.13a	0.14a^**	0.13ab	0.11c	0.12b	0.10c

The concentrations of the main macronutrients decreased with the decrease of irrigation quantities (Table 4). The lowest concentrations were generally observed in the PRD50 treatment. However, PRD and DI treatments in 25% and 50% water deficit conditions provided similar concentrations. Less macronutrient concentrations under limited irrigation conditions are probably the result of lesser amount of water supplied. Alrajhi et al. (2015) expressed that soil water content effects the nutrient distribution, solubility, and leaching in PRD and DI practices irrigated with fewer irrigation quantities.

The FI practice irrigated with RW provided the highest total N, P₂O₅, and K₂O concentrations in three soil layers (Table 4). In 0–30 cm soil layer, total N, P₂O₅, and K₂O concentrations in the application mentioned above were higher by 87.5%, 96.4%, and 36.6%, respectively, than the initial concentrations given in Table 1. The PRD and DI practices in irrigation conditions with the RW increased main macro nutrient concentrations compared with the initial macro nutrient concentrations in all soil layers. These findings can be the result of the high levels of N, P, and K concentrations of the wastewater used (Table 2). Similarly, Alrajhi et al. (2017) expressed that soils irrigated with RW had higher K and P concentrations compared with tap water due to the high K and P concentrations in the wastewater. In addition, Arienzo et al. (2009) expressed that the potential for accumulation of the K in soil from wastewater applications is high since it has low leachability. However, K accumulation in the soil can be limited through uptake by plants. Arienzo et al. (2009) found that tomatoes have one of the highest K removing rates among various crops.

The results obtained showed that the macronutrient concentrations in the RW could be enough to cause considerable nutrient accumulation in soil layers in the short term in addition to fertilizing before the planting. Our findings were also parallel with the results of many researchers, who indicated increases in soil fertility in wastewater applications with the short term (Kiziloglu et al., 2008; Singh and Agrawal, 2012; Disciglio et al., 2015). Moreover, Alrajhi et al. (2015) reported that total N was the highest under RW application, while PRD practice had the lowest values compared with the DI and FI practices. They also indicated that soil N concentrations observed under the PRD and DI practices irrigated with RW were higher than those of irrigated with FW.

The concentration of exchangeable Na in the first and the second soil layers was statistically (p < 0.05 and p < 0.01, respectively) higher in the conditions irrigated with wastewater compared with FW (Table 4). The highest Na concentrations in all the soil layers were determined in the FI treatment irrigated with the RW (Table 4). Na accumulation in the soil could be explained with higher Na content of the RW compared with the FW (Table 2). Sodium accumulation in soils irrigated with wastewaters is a common result (Oliveira et al., 2016). The RW treatment also significantly (p < 0.01 and p < 0.05, respectively) increased Ca and Mg concentrations compared with the FW treatment in 60–90 cm soil layer. Higher Mg concentration could be associated with higher CEC values shown in Table 3, considering the fact that CEC is an important factor in determining the mobility of Mg in soils (Gransee and Führs, 2013). The Mg concentrations in second and third soil layers were also significantly (p < 0.01) affected by the irrigation practices (Table 4). However, the changes could not show an explainable trend. In addition, it was observed that the accumulation of Ca, Mg, and Na in our study was generally lower in the topsoil layer (Table 4). The plants take most of the required water from the upper root zone for growing (Shankar et al., 2013). Hence, lesser accumulation tendency in top soil is probably due to the intake of these minerals from the surface soil layer by plants at higher rates.

Soil micro- and trace minerals

Wastewater irrigation increased micro- and trace minerals in especially surface soil layer (Table 4). In 0–30 cm soil layer, Fe, Zn, Cu, Cd, and Ni concentrations in the RW treatment were significantly (p < 0.01 or p < 0.05) higher than the FW treatment (Table 4). The Pb concentration in 30–60 cm soil layer was significantly (p < 0.01) higher under the RW treatment compared with the FW, while Cd concentration was higher in 60–90 cm soil layer. In 60–90 cm soil layer, higher B and Pb concentrations were determined in the FW treatment. The results of many studies showed that wastewater irrigation increased the concentrations of micro- and trace minerals in soils compared with FW. Kiziloglu et al. (2007) found that Fe, Mn, Zn, B, and Cu amounts in the soils irrigated with untreated wastewater were higher than the values in FW irrigation, and the concentrations decreased with increasing the quality of wastewater. Kiziloglu et al. (2008) indicated that the concentrations of Fe, Zn, Cu, Mn, Cd, Ni, and Pb in soil were higher under wastewater irrigation conditions compared with the FW, the raw wastewater caused the highest values. Singh et al. (2012) determined that Fe, Mn, Zn, Cu, Pb, and Ni concentrations after the harvest in sewage water-irrigated soil in different crop cultivation conditions were higher than the values in the well water-irrigated soil. Tunc and Sahin (2016) also revealed that water quality had a significant effect on soil Cu, Fe, Mn, and Zn concentrations, and wastewater irrigation importantly increased heavy metal concentrations.

Higher concentrations of micro- and trace minerals were obtained in the FI practice (Table 4). Decreasing irrigation quantities resulted in lower soil mineral concentrations due to the lesser amount of minerals adding into the soil with irrigation water. Therefore, DI50 and PRD50 practices generally provided significantly (p < 0.01 or p < 0.05) lower micro- and trace mineral concentrations in all soil layers compared with the FI practice. The highest concentrations of micro- and trace minerals in all soil layers were determined in the fully irrigated RW treatment. Boron, Fe, Zn, Cu, Mn, Cd, Ni, and Pb concentrations in 0–30 cm soil layer were higher by 47.4%, 44.1%, 23.3%, 95.0%, 0.8%, 115%, 33.2%, and 33.3% than the initial soil concentrations, respectively (Table 1). The accumulation of micro- and trace minerals due to the irrigation with wastewater could be explained directly by the higher mineral concentration of the used wastewater (Table 2), or indirectly, by the increase in the solubility of the soil metals present in insoluble forms (Silva et al., 2016).

It was observed that the accumulation of B, Fe, Mn, and Cd did not show a trend throughout the soil depth in the wastewater-irrigated soil (Table 4). The Zn and Ni concentrations decreased with the soil depth, while Cu and Pb concentrations increased. There are different results about microelement accumulation with soil depth. Rusan et al. (2007) determined that the micronutrients were accumulated in the upper soil layer. Areola et al. (2011) observed similar trends for the changes in Cd, Ni, Fe, Mn, and Cu concentrations in soil layers of 0–15 and 15–30 cm in soils cultivated with different crops under the wastewater irrigation conditions. Castro et al. (2011) observed that Mn concentration was higher in 0–30 cm soil layer in the wastewater irrigation conditions, while the concentrations of Cu, Zn, and Ni increased with soil depth, and higher values were found in 60–90 cm soil. Nonetheless, Khaskhoussy et al. (2015) obtained similar values for Cd, Pb, and Ni concentrations between different soil layers under the wastewater and FW irrigation conditions. Oliveira et al. (2015) also determined that the Fe, Cu, Mn, and Zn concentrations did not show a trend according to the changing of soil depth, and mineral concentrations in each soil layers were generally similar between the full and DI practices. Tunc and Sahin (2016) found higher Cu, Fe, Mn, and Zn concentrations in the top soil layer.

The concentrations of potentially toxic elements (Zn, Cu, Cd, Ni, and Pb) in soil determined in our study were too below the maximum permissible concentrations given by Pescod (1992) and Khan et al. (2013) (300 mg/kg for Zn, 135 mg/kg for Cu, 3 mg/kg for Cd, 75 mg/kg for Ni, 300 mg/kg for Pb). Moreover, toxic element concentrations were in safe limits (150 mg/kg for Zn, 50 mg/kg for Cu, 1 mg/kg for Cd, 50 mg/kg for Ni, 70 mg/kg for Pb) considering the regulation about the use of domestic and urban sewage sludge in Turkey on soil (The Official Gazette, 2010). There was no B toxicity due to the fact that B content in the soil is below 1 mg/kg (Gupta, 2007).

Conclusions

The effects of the RW treatments in the full irrigation conditions were observed mostly on surface soil fertility from the increase in macro and micronutrients, and the fertility decreased in the deficit-irrigated DI and the PRD practices. Although the fully irrigated RW treatments increased toxic element accumulation in the soil, this accumulation was lower than the permissible limits for sustainable soil management. Soil EC and ESP values also did not rise to problematic levels in all water types and practices. Despite the higher soil fertility values appeared in the fully irrigated wastewater treatments, it could be stated that the obtained results with RW under different irrigation practices may change in long-term applications. Therefore, it could be concluded that full irrigation with wastewater could directly stimulate crop production for a few years, while more pronounced and stable nutrient accumulations in the soil could require more years under drip irrigation conditions. In addition, although it does not cause adverse effects in the short term, an increase in the heavy metal concentrations in the soil can be expected in the irrigation conditions with treated wastewaters for many years. Therefore, it can be said that for policy/decision makers or farmers, it is necessary to monitor heavy metal concentrations of the treated wastewater and the heavy metal accumulations in Vertisol soils in terms of plant and human health.

Footnotes

Author Disclosure Statement

No competing financial interests exist.

Funding Information

The authors thank Ataturk University for funding of this research project (BAP-2012/414).

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