Optimizing Oilfield-Produced Water Reuse for Sustainable Irrigation: Impacts on Soil Quality and Mineral Accumulation in Plants

Abstract

The effective management of produced water (PW), a by-product of oil extraction in Oman, is essential for sustainable water use and environmental protection. PW contains petroleum residues, heavy metals, and salts, which require treatment before safe reuse. In the Nimr oil field, PW undergoes partial treatment in constructed wetlands vegetated with buffelgrass (Cenchrus ciliaris). This study investigated the reuse potential of treated PW for irrigation through two parallel field experiments conducted at Sultan Qaboos University (SQU) and the Nimr wetlands site. At the SQU site, native halophytic plants were irrigated with three water sources: treated municipal wastewater, underground water (from an on-site well), and treated produced water. At the Nimr site, irrigation was conducted using underground water and treated PW. Two soil types were used: well-draining control soil and Nimr soil from southern Oman. The treatments included: (i) PW + control soil, (ii) PW + Nimr soil, (iii) PW + gypsum (3.5 g/kg soil), (iv) PW + biochar (10 g/kg soil), (v) underground water + control soil, and (vi) treated municipal wastewater + control soil. Biochar, produced from locally sourced buffelgrass via low-temperature pyrolysis (300 °C for 3 h), and gypsum (46.57% acid-extractable sulfate) were mixed into the soil before sowing. The impact of each treatment was assessed in terms of soil quality (salinity, boron, major cations), plant physiological responses, and mineral accumulation. PW irrigation (TDS ~ 6500–7000 mg/L) led to a sixfold increase in soil sodium and raised boron levels in plant tissues to over 200 mg/kg, exceeding livestock feed safety limits. Copper remained within acceptable thresholds (≤9.5 mg/kg). Biochar reduced boron uptake, but gypsum showed limited benefit. Neither amendment improved plant growth under PW irrigation. These findings highlight the need for regulated PW reuse, emphasizing the importance of soil management strategies and alternating water sources to mitigate salinity stress.

1. Introduction

Sustainable management of water resources is a critical concern in arid and semi-arid regions, where freshwater scarcity threatens food security and agricultural productivity. In such environments, the reuse of alternative water sources, particularly oilfield-produced water (PW)—offers a promising strategy for supplementing irrigation needs. PW, a by-product of oil and gas extraction, consists of formation water and injected fluids that return to the surface. Despite treatment, it often retains elevated concentrations of salts, hydrocarbons, and trace metals, which can present risks to both crops and soil quality [1,2,3].

To mitigate the impacts of salinity and promote sustainable land use, researchers have increasingly focused on the use of salt-tolerant crops and biomass-based soil amendments. Salt-tolerant grasses such as buffelgrass (Cenchrus ciliaris L.), Panicum maximum (Guinea grass), and alfalfa (Medicago sativa) have demonstrated varying degrees of tolerance to saline irrigation and are commonly used for forage due to their high biomass yield and resilience [4,5,6,7]. Buffelgrass, in particular, has shown strong adaptability to drought, heat, and salinity, making it a promising candidate for cultivation under water-scarce and saline conditions [8,9].

Several studies have explored the use of these grasses under saline irrigation, yet limited research has addressed their response to produced water, especially regarding mineral uptake and heavy metal accumulation in plant tissues. The integration of biochar derived from salt-tolerant grasses into saline soil management offers a potentially effective approach. Biochar not only improves soil structure and nutrient retention but may also immobilize contaminants, thus reducing their bioavailability [10].

In Oman, where oil production is high and freshwater resources are limited, the potential for reusing treated PW for irrigation is gaining attention. The Nimr Water Treatment Plant (NWTP) in southern Oman treats large volumes of PW through a series of constructed wetlands and evaporation ponds. This treated PW has been tested on various perennial species, including Acacia spp. and Casuarina, through field-scale irrigation trials using both flood-and-drain and bubbler systems [11,12,13,14,15]. Despite partial treatment, concerns remain regarding the long-term accumulation of trace elements such as cadmium, lead, zinc, boron, and copper in soils and plants—even when present at concentrations within regulatory limits [16,17,18].

In response to these concerns, this study investigates the feasibility and environmental implications of using treated PW for irrigating salt-tolerant grasses in southern Oman. Specifically, it evaluates the effects of PW on plant growth, mineral uptake, and heavy metal mobility in both soil and plant tissues under different irrigation and soil salinity conditions. The findings aim to support the development of strategies for optimizing oilfield-produced water reuse in agriculture, contributing to sustainable water management and enhanced resilience of arid farming systems.

2. Materials and Methods

This study was conducted at two locations in Oman—the Sultan Qaboos University (SQU) Agricultural Experiment Station (AES) and the Nimr oil field. A stratified random design with three water treatments was used across both sites.

All samples were analyzed at the SQU laboratory to measure boron (B), sodium (Na), potassium (K), calcium (Ca), magnesium (Mg), arsenic (As), zinc (Zn), lead (Pb), cobalt (Co), cadmium (Cd), nickel (Ni), iron (Fe), mercury (Hg), manganese (Mn), chromium (Cr), copper (Cu), and aluminum (Al) concentrations. Elemental concentrations in liquid samples, including soil extracts, roots, and plant tissues, were determined using an inductively coupled plasma optical emission spectrometer (ICP-OES) by PerkinElmer (Waltham, MA, USA) (Model: 8000) at the Central Analytical and Applied Research Unit (CAARU). For samples with particularly high element concentrations, complementary measurements were performed using a flame photometer (Model: 7890A–7697A, Make: Agilent, Santa Clara, CA, USA). Element concentration calculations were based on the amount of emitted light detected at specific mineral wavelengths.

CAARU is certified by QS Zurich AG under certificate number 11825, meeting the ISO 9001:2015 standards for management systems related to the testing of soil and water for mineral and element detection [19]. This certification ensures the reliability and accuracy of the analyses performed in this study.

2.1. Experimental Setup

2.1.1. Experiment 1: Pot Trials at Sultan Qaboos University (SQU)

Experiment 1 was conducted at Sultan Qaboos University (SQU) using a controlled pot system to evaluate plant responses to different water quality treatments. A total of 96 pots were arranged and irrigated using a drip system comprising six lines, each supporting 16 plants. Three irrigation water sources were tested: groundwater (total dissolved solids; TDS approximately 825 mg/L), produced water (PW) with a TDS range of 6500–7000 mg/L, and treated municipal wastewater (TDS approximately 895 mg/L). Water was applied uniformly across all treatments to ensure consistency in delivery and minimize variability in plant exposure.

Two types of soil were used: a well-draining control soil (EC ~ 1.96 mS/cm; TDS ~ 1.28 g/L; pH 8.1) and Nimr soil collected from southern Oman (EC ~ 12.39 mS/cm; TDS ~ 8.88 g/L; pH 7.5). To enhance drainage and reduce salt accumulation, both soils were amended with perlite and peat moss. Soil moisture content was determined gravimetrically. The physical and chemical properties of the soils are presented in

Table 1. Main characteristic of the soils used both experiments.

Soil Characteristics	Nimr Soil	Control Soil
EC (mS/cm)	12.39	1.96
* TDS (g/L)	8.88	1.28
pH	7.5	8.1
* MC %	2.91	1.145
Silt %	6.17	0.0
Clay %	0.0	0.0
Sand %	93.8	100
Texture	Sand	Sand

Note: * MC: moisture content; TDS: total dissolved solids.

, and their elemental compositions are summarized in

Table 2. Contents of elements of the soils used in Experiments 1 and 2.

Elemental Contents (mg/kg)	Nimr Soil	Control Soil
Zn	0.05	0.05
Fe	0.013	0.023
B	0.03	0.013
Al	0.087	0.043
Na	678.43	108.09
K	359.24	66.98
Ca	446.24	54.41
Mg	116.07	24.99
As-Pb-Co-Cd-Ni-Hg-Mn-Cr-Cu	* ND	ND

Note: * ND: not detected.

.

To evaluate the effect of soil amendments on plant growth under PW irrigation, different treatments were applied across specific drip irrigation lines. Agricultural gypsum containing 46.57% acid-extractable sulfate was incorporated at a rate of 3.5 g/kg of soil in the second line, while biochar was added at a rate of 10 g/kg in the third line. The biochar was locally produced at SQU-AES from buffelgrass through a low-temperature pyrolysis process (300 °C for three hours in an oxygen-free environment). Untreated Nimr soil served as the baseline for the fourth line. Both gypsum and biochar application rates were selected based on common recommendations for saline soil remediation [20,21], and were thoroughly mixed into the soil prior to planting, in line with best practices for uniform amendment incorporation [22].

Plant species tested included Panicum, Panicum maximum, buffelgrass, and alfalfa, selected for their known or potential salinity tolerance. Each of the six drip lines was subdivided into four drip position levels (DPL 1 to DPL 4), ranging from the inlet to the terminal end. All treatments were irrigated with calibrated drip systems delivering water at 50–60 mL/min for 20–30 min per session, three times daily, ensuring approximately 1–1.8 L of water per plant per day. The irrigation setup is shown in Figure 1. Figure 1 and Figure 2 illustrate the segmentation of each line into four distinct irrigation positions, starting from the water inlet (DPL 1) to the terminal point (DPL 4).

2.1.2. Experiment 2: Field Trials at Nimr Site

Experiment 2 was conducted under open field conditions at the Nimr site, using native desert soil consistent with that used in Experiment 1. The soil was similarly amended with perlite and peat moss to enhance drainage and reduce salinity stress. A total of 120 plants were irrigated using a 10-line drip system: five lines received produced water (TDS ~ 6500–7000 mg/L), and the other five lines used ground well water (TDS ~ 3000 mg/L). Each line irrigated 12 plants.

The seeds used in this study were sourced from local markets in Oman, with alfalfa of local origin, Panicum of Brazilian origin, and buffelgrass of U.S. (Laredo) origin. Seeds of Panicum and buffelgrass were germinated under two water quality conditions: reverse osmosis (RO) water (TDS ~ 0.00025 g/L) and groundwater (TDS ~ 3.0 g/L). Panicum maximum seeds were germinated in a shaded nursery using groundwater and transplanted after two weeks to assess early-stage salinity effects. The elemental compositions of soils used are presented in Table 2, while

Table 3. Element concentrations of the produced water treated wastewater, and groundwater used at SQU-AES (Experiment 1).

Element (mg/L)	Produced Water	Treated Wastewater	Groundwater
B	6.95	1.17	0.23
Zn	0.02	* ND	ND
Ni	0.03	ND	ND
Fe	0.04	0.01	0.01
Mn	0.02	ND	ND
Cu	0.01	ND	ND
Al	0.16	0.07	0.09
Mg	39.87	9.36	84.33
Ca	117.76	37.67	45.27
Na	5022.68	188.09	222.27
K	47.56	10.86	8.01

Note: * ND: not detected.

and

Table 4. Element concentrations of the produced water and groundwater used at the Nimr site (Experiment 2).

Elemental Concentrations (mg/L)	Produced Water	Groundwater
B	6.95	1.84
Zn	0.02	0.04
Ni	0.03	0.03
Fe	0.04	0.03
Mn	0.02	0.02
Cu	0.01	0.01
Al	0.16	0.28
Mg	39.87	305.44
Ca	117.76	579.3
Na	5022.6	1404.0
K	47.56	43.21

display the concentrations of key elements in irrigation water.

Mineral accumulation in plant tissues and soils was monitored throughout the growing season to assess species-specific uptake and the effects of salinity on soil quality. Plant samples were collected monthly, with three harvests at SQU and five at Nimr due to better salt leaching in sandy soils.

At the end of the experiments, root and soil samples were collected across treatments. Soil was sampled at depths of 0–15 cm and 15–30 cm to reflect the active root zone. Plant tissue samples were washed, oven-dried at 70 °C for 72 h, and ground into a fine powder. One gram of each sample was digested in nitric acid, diluted, filtered, and analyzed using inductively coupled plasma optical emission spectrometry (ICP-OES) at the Central Analytical and Applied Research Unit (CAARU), following standard procedures and calibration with certified reference materials.

Root samples followed the same preparation protocol. Soil samples were air-dried, and 300 g subsamples were saturated with distilled water for 24 h. A 50 mL extract was filtered and analyzed for salinity and elemental composition using established methods [23]. The study, conducted from July 2020 to May 2021, provided insights into seasonal variations in mineral accumulation and evaluated the effectiveness of biochar and gypsum amendments in mitigating the adverse effects of PW irrigation in arid conditions.

2.2. Statistical Analysis and Element Concentration Evaluation

Statistical analyses were performed using analysis of variance (ANOVA) with the Tukey Studentized range test at a 95% confidence level to assess significant differences (p < 0.05) across all elements in the plant shoots, roots, and soil samples. The software Minitab 19 was used for data organization and calculations. The results were used to verify the data’s normality and test the null hypothesis, either rejecting or failing to reject it.

Plant survival and overall health were monitored throughout the experiment; however, biomass quantification was not within the study’s scope. Element concentration thresholds for plants and soil were evaluated against established guidelines outlining acceptable mineral ranges, as presented in [24,25] and summarized in

Table 5. Critical limits of elements in soil and their concentrations in plants.

Elements	Critical Soil Limit (mg/kg) *	Critical Plant Concentrations (mg/kg) *
Ag	2	-
As	20–50	5–20
Cd	0.1–2.4	5–30
Cr	75–100	5–30
Cu	60–125	20–100
Hg	0.3–5	1–3
Mn	1500–3000	300–500
Mo	2–10	10–50
Ni	100	10–100
Pb	10–400	30–300
Zn	70–400	100–400
B	100	283–333 (>150 animal feeding)
Al	-	50–3410
Fe	-	>3080

Note: * Data from [24,25,26].

. In general, element levels were categorized as follows. Calcium: high at 2001–4500 ppm; very high above 4500 ppm. Sodium: high at 121–160 ppm; very high above 160 ppm. Potassium: high at 120–160 ppm; very high above 160 ppm. Magnesium: high at levels of 100 ppm or more.

3. Results

The results presented in the following sections describe the effects of water salinity accumulation levels, soil amendments, and other factors on mineral uptake in plants, as well as elemental accumulation in soil and roots. Key findings highlight variations in element concentrations under different conditions, providing insights into the influence of salinity on nutrient availability and plant health.

3.1. Plant Tissue

3.1.1. Mineral Accumulation in SQU Field Trial

Element concentrations varied significantly across harvests, reflecting changes in nutrient uptake (

Table 6. Plant tissue element (mg/kg) values and significance levels for Experiment 1.

Tissue Minerals/Factors	B		Zn	Fe	Mn	Na	Mg	K	Ca	Al
Harvest 1	60.4 B *		8.9 B	182.9 B	47.9 B	6864.4 B	525.9 B	519.1 B	516.3 B	67.6 B
Harvest 2	114.6 AB		29.7 A	402.1 A	90.9 A	NA *	NA	NA	NA	179.4 A
Harvest 3	126.1 A		24.7 A	215.3 B	98.5 A	24,634.8 A	2660.2 A	14,790.4 A	8618.1 A	110.6 B
	p-value	0.018	0.000	0.000	0.000	0.000	0.000	0.000	0.000	0.00
Wastewater	94.5 AB		19.9 A	260.0 A	69.0 A	12,894.3 A	337.2 B	7693.3 A	5024.5 A	116.7 A
Produced Water	150.8 A		24.6 A	270.9 A	78.0 A	23,726.9 A	848.8 AB	6086.1 A	3917.6 A	125.5 A
Groundwater	55.9 B		18.8 A	269.3 A	90.2 A	10,627.7 A	2015.4 A	7627.5 A	4759.5 A	115.5 A
	p-value	0.004	0.335	0.977	0.092	0.005	0.647	0.725	0.299	0.91
Panicum maximum	105.5 AB		17.6 A	264.5 A	96.4 A	21,991.5 A	1041.5 A	8167.7 AB	8596.3 B	111.1 A
Panicum	98.1 AB		20.9 A	258.5 A	106.4 A	14,209.3 A	858.5 A	6956.3 AB	2615.5 B	107.9 A
Buffelgrass	51.6 B		23.1 A	210.6 A	47.9 B	13,114.0 A	551.2 A	10,004.0 A	2396.9 B	102.5 A
Alfalfa	146.6 A		22.7 A	333.4 A	65.6 AB	13,683.7 A	1817.4 A	3414.5 B	9659.9 A	155.1 A
	p-value	0.034	0.631	0.297	0.588	0.165	0.045	0.001	0.000	0.3
Gypsum	118.0 A		18.0 A	338.2 A	93.4 A	15,469.5 A	685.9 A	5540.8 A	4602.9 A	143.1 A
Biochar	103.3 A		24.8 A	208.7 A	63.8 A	19,453.3 A	842.8 A	7974.4 A	4727.4 A	90.9 A
None	79.9 A		20.3 A	253.4 A	80.1 A	12,326.0 A	1672.6 A	7891.6 A	4371.1 A	123.4 A
	p-value	0.373	0.229	0.065	0.513	0.101	0.374	0.969	0.101	0.13
Control Soil	103.2 A		23.6 A	238.5 A	74.8 A	11,173.1 A	1530.2 A	8802.5 A	5115.5 A	117.3 A
Nimr Soil	97.6 A		18.6 A	295.1 A	83.3 A	20,326.2 A	604.0 A	5468.7 A	4018.8 A	121.0 A
	p-value	0.837	0.215	0.305	0.147	0.062	0.097	0.452	0.532	0.88
Drip position
DPL 1	82.4 A		19.7 A	279.2 A	75.8 A	7925.3 A	1225.1 A	5064.4 A	3605.8 A	132.2 A
DPL 2	107.8 A		17.1 A	275.9 A	86.2 A	13,235.4 A	846.5 A	7056.8 A	3554.0 A	112.6 A
DPL 3	104.0 A		23.3 A	235.6 A	77.9 A	18,294.2 A	1179.9 A	8946.0 A	5731.2 A	109.6 A
DPL 4	107.5 A		24.2 A	276.6 A	76.3 A	23,543.6 A	1016.8 A	7475.2 A	5377.6 A	122.1 A
	p-value	0.749	0.267	0.829	0.091	0.849	0.275	0.302	0.856	0.81
Interaction p-values
Harvest—Species	0.255		0.797	0.113	0.113	0.820	0.091	0.345	0.000	0.120
Harvest—Water	0.338		0.531	0.776	0.155	0.001	0.001	0.055	0.307	0.695
Species—Water	0.764		0.383	0.855	0.685	0.964	0.761	0.484	0.916	0.549

Note: * Different letters within each column indicate significant difference (p ≤ 0.05); * NA: not available.

). Boron levels increased steadily from 60.4 mg/kg in the first harvest to 126.1 mg/kg in the third (p = 0.018), indicating progressive accumulation. Zinc peaked in the second harvest (29.7 mg/kg) before declining (24.7 mg/kg, p = 0.000), while iron spiked in the second harvest (402.1 mg/kg) but decreased in the third (215.3 mg/kg, p = 0.000). Manganese levels increased from 47.9 mg/kg in the first harvest to 98.5 mg/kg in the third, highlighting the influence of plant growth stages on nutrient uptake.

The water source had a significant impact on boron accumulation, with the highest levels recorded in plants irrigated with PW (150.8 mg/kg) and the lowest in those receiving groundwater (55.9 mg/kg, p = 0.004). However, Zn, Fe, and Mn concentrations were largely unaffected by water type (p-values: 0.335, 0.977, and 0.299, respectively).

Plant species exhibited distinct nutrient accumulation patterns. Alfalfa had the highest accumulation of boron (146.6 mg/kg), significantly surpassing buffelgrass (51.6 mg/kg), Panicum (98.1 mg/kg), and Panicum maximum (105.5 mg/kg, p = 0.034). A similar trend was observed for manganese, with alfalfa accumulating the highest levels (106.4 mg/kg) and buffelgrass accumulating the lowest (47.9 mg/kg, p = 0.000). Zinc and iron concentrations showed no significant species-dependent variations.

Soil amendments had a moderate influence on mineral uptake. Zinc levels were slightly higher in biochar-treated soil (24.8 mg/kg) compared to gypsum-treated (18.0 mg/kg) and untreated soil (20.3 mg/kg), but the differences were not statistically significant (p = 0.229). Iron concentrations were highest in gypsum-treated soil (338.2 mg/kg) and lowest in biochar-treated soil (208.7 mg/kg, p = 0.065), suggesting a potential role of amendments in Fe availability.

No significant differences were found between the control and Nimr soil for boron, zinc, iron, or manganese (p > 0.2). However, the Fe concentrations were slightly higher in Nimr soil (295.1 mg/kg) than in control soil (238.5 mg/kg).

The macronutrient concentrations also fluctuated across harvests. The aluminum levels increased from 67.6 mg/kg in the first harvest to 179.4 mg/kg in the second and then dropped to 110.6 mg/kg in the third (p = 0.000). Sodium peaked in the second harvest (24,634.8 mg/kg), while magnesium, potassium, and calcium also showed significant variations (p = 0.000), reflecting changing nutrient availability over time.

PW irrigation resulted in a significantly higher sodium accumulation (23,726.9 mg/kg) compared with wastewater (12,894.3 mg/kg) and groundwater (10,627.7 mg/kg), with the difference being statistically significant (p = 0.005). Magnesium was highest in plants irrigated with groundwater (2015.4 mg/kg) and lowest in those receiving wastewater (337.2 mg/kg, p = 0.005). Potassium and calcium levels remained stable across water treatments.

Plant species differed in macronutrient uptake. Panicum maximum and alfalfa accumulated the most sodium (21,991.5 mg/kg and 13,683.7 mg/kg, respectively), while buffelgrass had lower levels (13,114.0 mg/kg). Potassium concentrations were highest in alfalfa (10,004.0 mg/kg) followed by Panicum maximum (8167.7 mg/kg; p = 0.045), while alfalfa also showed significantly higher calcium levels (9659.9 mg/kg; p = 0.001), indicating a greater capacity for macronutrient absorption.

Biochar-treated soil showed the highest sodium levels (19,453.3 mg/kg), whereas untreated soil had the lowest (12,326.0 mg/kg), though this was not statistically significant (p = 0.513). The magnesium was the highest in untreated soil (1672.6 mg/kg) compared to gypsum-treated soil (685.9 mg/kg; p = 0.101), while potassium and calcium remained stable across soil treatments.

Significant interactions were found between harvest time, species, and water sources. Sodium accumulation varied by water source across harvests (p = 0.001), suggesting that irrigation effects changed over time. A strong interaction between species and water source was observed for magnesium uptake (p = 0.001), indicating species-specific responses to irrigation water salinity.

3.1.2. Plant Nutrient Uptake Under Nimr Site Conditions

The element concentrations in plant leaves varied significantly across harvests, reflecting dynamic nutrient uptake. Zinc and iron peaked in the second harvest (43.7 mg/kg and 260.8 mg/kg, respectively) before declining, while boron accumulated progressively, reaching 211.0 mg/kg in the third harvest. Manganese remained stable (p = 0.137). These trends suggest that Zn, Fe, and B uptake depend on plant growth stages and soil nutrient availability, with potential depletion in later harvests (

Table 7. Plant tissue element (mg/kg) values and significance levels for Experiment 2.

Tissue Minerals/Factors	Zn		Fe	B	Mn	Mg	Ca	Na	K	Al
Harvest 1	23.6 C *		161.5 B	51.1 C	62.7 A	NA *	NA	NA	NA	75.5 A
Harvest 2	43.7 A		260.8 A	147.7 AB	70.6 A	NA	NA	NA	NA	112.9 A
Harvest 3	30.9 BC		160.4 B	211.0 A	59.8 A	4543.4 A	9097.1 A	26,780.2 A	21,328.1 AB	110.4 A
Harvest 4	33.2 ABC		117.5 B	183.5 AB	53.9 A	5657.3 A	9228.3 A	19,745.4 A	26,955.6 A	95.0 A
Harvest 5	38.2 AB		152.1 B	100.2 BC	74.7 A	4654.9 A	10,590.9 A	28,497.8 A	15,866.4 B	113.0 A
	p-value	0.000	0.000	0.000	0.175	0.587	0.667	0.005	0.137	0.104
Produced water	35.1 A		176.1 A	186.7 A	73.4 A	5360.1 A	11,037.0 A	32,412.8 A	19,083.5 A	117.1 A
Groundwater	32.7 A		164.8 A	90.7 B	55.3 B	4543.6 A	8240.5 B	17,602.8 A	23,683.2 A	85.6 B
	p-value	0.327	0.493	0.000	0.127	0.045	0.093	0.068	0.002	0.004
Panicum maximum	28.2 B		158.8 A	142.5 A	73.3 A	5313.0 AB	9038.9 A	21,558.6 A	18,568.4 AB	88.6 A
Panicum	31.6 B		193.1 A	192.8 A	78.1 A	5892.7 A	11,200.8 A	30,933.8 A	19,568.7 B	112.2 A
buffelgrass	41.9 A		159.5 A	80.8 B	41.6 B	3649.9 B	8676.6 A	22,531.0 A	26,013.0 A	103.3 A
	p-value	0.000	0.119	0.000	0.002	0.184	0.564	0.028	0.000	0.210
Germination:
Reverse osmosis	34.2 AB		179.6 A	126.4 A	55.2 A	4784.58 A	9340.7 A	19,266.6 A	24,109.9 A	106.832 A
Groundwater	39.3 A		173.0 A	147.2 A	64.5 A	4758.13 A	10,536.6 A	34,198.2 A	21,471.7 A	108.725 A
None	28.2 B		158.8 A	142.5 A	73.3 A	5313.08 A	9038.9 A	21,558.6 A	18,568.4 A	88.675 A
	p-value	0.021	0.621	0.728	0.796	0.578	0.147	0.261	0.167	0.341
Interaction p-values
Harvest—Species	0.064		0.671	0.438	0.799	0.313	0.860	0.715	0.458	0.119
Harvest—Water	0.122		0.002	0.096	0.000	0.327	0.019	0.182	0.208	0.002
Species—Water	0.462		0.650	0.302	0.487	0.892	0.415	0.827	0.622	0.346

Note: * Different letters within each column indicate significant difference (p ≤ 0.05); * NA: not available.

).

The water source significantly affected boron and manganese accumulation. Boron levels were higher in plants irrigated with PW (186.7 mg/kg) compared to ground well water (90.7 mg/kg, p = 0.000), suggesting greater bioavailability in PW-irrigated soils. Conversely, manganese levels were higher in plants receiving ground well water (55.3 mg/kg) than those irrigated with PW (73.4 mg/kg; p = 0.002), indicating species-specific uptake responses.

Plant species influenced nutrient absorption. Buffelgrass accumulated the highest zinc levels (41.9 mg/kg; p = 0.000), while Panicum had the highest boron concentrations (192.8 mg/kg; p = 0.000). Manganese uptake was lowest in buffelgrass (41.6 mg/kg), whereas Panicum maximum and Panicum had similar levels, highlighting species-dependent nutrient preferences.

Macronutrient analysis revealed dynamic shifts over time. Sodium declined across harvests, from 26,780.2 mg/kg in the first harvest to 19,745.4 mg/kg in the second (p = 0.667), while potassium peaked in the second harvest (26,955.6 mg/kg) and declined in later harvests (p = 0.005). Aluminum, magnesium, and calcium showed minor fluctuations but remained stable.

PW irrigation significantly increased aluminum and calcium accumulation. Al concentrations were higher in plants irrigated with PW (117.1 mg/kg) compared with ground well water (85.6 mg/kg, p = 0.004), while calcium was also elevated (11,037.0 mg/kg vs. 8240.5 mg/kg; p = 0.045). Magnesium and potassium were unaffected by water source.

Among plant species, Panicum had the highest magnesium accumulation (5892.7 mg/kg; p = 0.002), while buffelgrass accumulated the most sodium (30,933.8 mg/kg) and potassium (26,013.0 mg/kg; p = 0.028), suggesting its resilience to high-salinity irrigation. Germination methods had no significant impact on Al, Mg, Ca, Na, or K accumulation (p > 0.1). Significant interactions were observed between harvest time, species, and water source. Zinc and iron uptake varied across harvests depending on irrigation source (p = 0.002 and p = 0.000, respectively), while species–water interactions influenced magnesium accumulation (p = 0.019), indicating species-specific responses to irrigation sources.

3.2. Plant Roots

3.2.1. Plant Root Dynamics in SQU Field Trial

Water sources affected mineral accumulation, though not all changes were significant. Produced water irrigation led to the highest accumulation of boron (71.1 mg/kg), while wastewater increased iron (1767.9 mg/kg) and manganese (325.1 mg/kg) levels (p < 0.05). Calcium was highest in wastewater-irrigated plants (15,933.4 mg/kg; p = 0.036), suggesting different water sources influence specific nutrient uptake (

Table 8. Plant root elemental (mg/kg) values and significance levels of boron, iron, manganese, and calcium for Experiment 1.

Root Minerals/Factors	B		Fe	Mn	Ca
Wastewater	40.7 A *		1767.9 A	325.1 A	15,933.4 A
Produced water	71.1 A		633.4 B	97.6 B	14,064.8 AB
Groundwater	40.1 A		953.4 B	140.9 B	13,279.4 B
	p-value	0.081	0.007	0.024	0.036
Panicum maximum	4.1 B		714.5 A	101.7 A	3814.6 C
Panicum	24.2 B		1314.9 A	220.3 A	6983.5 C
Buffelgrass	3.4 B		1447.3 A	329.7 A	14,538.1 B
Alfalfa	171.0 A		996.3 A	99.7 A	32,367.4 A
	p-value	0.005	0.078	0.073	0.000
Gypsum	51.1 A		1235.3 A	215.3 A	16,248.0 A
Biochar	47.8 A		1058.5 A	186.8 A	12,781.0 B
None	53.1 A		1060.9 A	161.3 A	14,248.7 AB
	p-value	0.871	0.404	0.830	0.018
Control Soil	15.0 B		1091.1 A	178.4 A	8067.0 B
Nimr Soil	86.3 A		1145.4 A	197.3 A	20,784.8 A
	p-value	0.005	0.701	0.683	0.000
Drip position:
DPL 1	78.8 A		1289.8 A	199.5 A	16,324.0 A
DPL 2	50.8 A		938.7 A	221.3 A	13,739.1 AB
DPL 3	34.7 A		1038.6 A	165.7 A	12,911.1 B
DPL 4	38.3 A		1205.9 A	164.9 A	14,729.3 AB
	p-value	0.098	0.287	0.669	0.049
Interaction p-values
Soil type—Species	0.005		0.455	0.617	0.000
Species—Water	0.297		0.061	0.053	0.013

Note: * Different letters within each column indicate significant difference (p ≤ 0.05).

).

Plant species played a major role in root mineral composition. Alfalfa accumulated the most boron (171.0 mg/kg; p = 0.005) and calcium (32,367.4 mg/kg; p = 0.000), while Panicum had the highest zinc levels (0.6 mg/kg; p = 0.007). Sodium and potassium showed no significant variation across species.

Soil amendments had minimal impact, except for calcium, which was highest in gypsum-treated soil (16,248.0 mg/kg; p = 0.018). Soil type, however, significantly influenced boron and calcium accumulation, with Nimr soil supporting higher levels of both (p < 0.005).

The highest root element concentrations at final harvest were as follows: wastewater—Zn (1.43 mg/kg), Fe (2348.5 mg/kg), Mn (649 mg/kg), Al (1088.5 mg/kg); produced water—B (342.5 mg/kg), Na (58,421.5 mg/kg) (Figure 2), K (6158 mg/kg), Ca (55,647 mg/kg); and groundwater—Mg (8924.5 mg/kg).

Drip position significantly affected zinc accumulation (p = 0.035), while species–soil and species–water interactions influenced boron and zinc uptake (p = 0.005). Other interactions had no significant effects.

3.2.2. Plant Root Dynamics Under Nimr Site Conditions

The water source significantly affected zinc and copper uptake in plant roots. Plants irrigated with ground well water had higher Zn levels (51.5 mg/kg) compared with those receiving PW (17.1 mg/kg; p = 0.003). Conversely, Cu was more concentrated in plants irrigated with PW (4.5 mg/kg) than ground well water (4.0 mg/kg; p = 0.008). Other elements, including nickel, iron, and boron, showed no significant differences across water sources (p > 0.2), indicating selective mineral uptake influenced by irrigation water (

Table 9. Plant root elemental (mg/kg) values and significance levels of manganese, aluminum, magnesium, calcium, potassium, and sodium for Experiment 2.

Experiment 2 Root Factors/Elements	Mn		Al	Mg	Ca	K	Na
Produced Water	20.4 A *		145.5 A	1573.4 A	2926.5 A	1020.4 A	20,579.9 A
Groundwater	35.5 A		173.1 A	2592.1 A	3435.4 A	2210.9 A	10,313.9 A
	p-value	0.31	0.793	0.214	0.829	0.065	0.125
Panicum max	22.5 A		82.2 A	2012.0 A	1706.2 A	1370.2 A	14,606.8 A
Panicum	34.1 A		171.7 A	2441.3 A	2822.0 A	1934.5 A	21,350.1 A
Buffelgrass	27.3 A		224.0 A	1795.0 A	5014.7 A	1542.2 A	10,383.9 A
	p-value	0.79	0.585	0.696	0.510	0.627	0.260
p-values of interaction
Species—Water	0.522		0.425	0.486	0.568	0.281	0.345

Note: * Different letters within each column indicate significant difference (p ≤ 0.05).

).

The highest root element concentrations at final harvest were as follows: groundwater—Zn (82.0 mg/kg), Mn (75.0 mg/kg), Mg (3961.5 mg/kg), K (3037.5 mg/kg); produced water—Ni (7.5 mg/kg), Fe (518.0 mg/kg), B (30.5 mg/kg), Cu (5.0 mg/kg), Al (458.0 mg/kg), Ca (10,819.5 mg/kg), and Na (40,341.0 mg/kg) (Figure 3).

These findings suggest that PW leads to an increased accumulation of Cu, Al, Fe, B, Na, and Ca, while ground well water promotes a higher uptake of Zn, Mn, Mg, and K.

Plant species significantly influenced Zn and Cu levels but had little effect on Ni, Fe, and B. Panicum maximum accumulated the highest levels of Zn (51.5 mg/kg; p = 0.008), while Panicum had the highest levels of Cu (4.7 mg/kg; p = 0.006). Other elements remained stable across species, indicating that Zn and Cu uptake is species-dependent, while external conditions primarily influence other minerals.

The water source appeared to influence macronutrient uptake, with K levels being numerically higher in plants irrigated with ground well water (2210.9 mg/kg) compared to those irrigated with produced water (1020.4 mg/kg), though the difference was not statistically significant (p = 0.065). Similarly, Na concentrations were higher in plants irrigated with PW (20,579.9 mg/kg) than in those receiving ground well water (10,313.9 mg/kg), but this difference was also not statistically significant (p = 0.125). These findings suggest that while salinity may influence Na and K uptake, its effect on other macronutrients remains unclear.

Zn and Cu accumulation showed significant interaction effects between water source and plant species (p = 0.022 and p = 0.002, respectively), suggesting that species respond differently to water sources. However, for Ni, Fe, and B, no significant interactions were observed, suggesting that plant species and water source influenced these elements independently rather than in combination.

3.3. Soil Element Composition

3.3.1. Mineral Accumulation in SQU Field Trial Soil

The water source significantly influenced soil element composition. Soil irrigated with produced water contained the highest levels of Fe (0.1 mg/kg), B (5.9 mg/kg), and Al (0.24 mg/kg) (p = 0.039), while soil irrigated with groundwater and wastewater had lower concentrations. Na, K, Ca, and Mg levels also varied notably in soil. Sodium (Na) was significantly higher in soil irrigated with produced water (32,256.3 mg/kg) compared with wastewater (6611.7 mg/kg) and groundwater (6886.9 mg/kg) (p = 0.000) (

Table 10. Soil elemental (mg/kg) composition values and significance levels at the end of Experiment 1.

Soil Factors/Elements	Fe		B	Al	K	Ca	Mg	Na
Irrigated Water:
Wastewater	0.05 AB *		1.6 B	0.13 B	53.3 B	383.1 B	21.9 C	6611.7 B
Produced Water	0.1 A		5.9 A	0.24 A	265.0 A	640.7 A	131.7 B	32,256.3 A
Groundwater	0.0 B		1.3 B	0.10 B	44.4 B	380.2 B	188.5 A	6886.9 B
	p-value	0.039	0.001	0.000	0.008	0.000	0.000	0.000
Panicum max	0.07 A		3.2 A	0.17 A	90.3 A	585.2 A	152.6 A	15,264.8 A
Panicum	0.03 A		1.7 A	0.17 A	122.2 A	481.2 A	134.1 A	17,180.9 A
Buffelgrass	0.10 A		3.5 A	0.12 A	57.0 A	377.2 A	51.6 B	10,808.3 A
Alfalfa	0.08 A		3.3 A	0.15 A	214.1 A	428.4 A	117.8 AB	17,752.4 A
	p-value	0.592	0.791	0.180	0.446	0.002	0.050	0.189
Soil addition:
Gypsum	0.019 B		3.29 A	0.16 A	53.8 A	761.6 A	106.4 A	17,720.4 A
Biochar	0.12 A		3.36 A	0.16 A	164.1 A	407.3 B	124.8 A	14,532.9 A
None	0.08 AB		2.32 A	0.14 A	144.7 A	235.1 B	110.9 A	13,501.6 A
	p-value	0.017	0.626	0.060	0.000	0.441	0.441	0.138
Soil Type:
Control Soil	0.04 A		2.55 A	0.16 A	65.6 B	501.1 A	133.0 A	10,826.6 B
Nimr Soil	0.10 A		3.43 A	0.15 A	176.2 A	434.9 A	95.0 B	19,676.6 A
	p-value	0.078	0.472	0.027	0.445	0.018	0.322	0.000
Soil depth:
Top	0.07 A		3.21 A	0.14 B	121.4 A	442.6 A	122.4 A	17,087.9 A
Bottom	0.07 A		2.77 A	0.16 A	120.3 A	493.5 A	105.7 A	13,415.3 B
	p-value	0.842	0.612	0.973	0.407	0.124	0.039	0.023
Drip position:
DPL 1	0.07 A		2.7 A	0.15 A	67.3 B	477.9 A	128.1 A	16,507.9 A
DPL 2	0.08 A		3.4 A	0.15 A	62.5 B	415.4 A	103.3 A	19,339.1 A
DPL 3	0.06 A		2.3 A	0.16 A	77.9 B	454.5 A	98.4 A	13,620.4 AB
DPL 4	0.07 A		3.5 A	0.15 A	275.9 A	524.1 A	126.2 A	11,539.1 B
	p-value	0.921	0.807	0.002	0.768	0.258	0.995	0.034
Interaction p-values
Soil type—species	0.168		0.704	0.928	0.132	0.491	0.140	0.055
Species—Water	0.977		0.986	0.003	0.264	0.348	0.000	0.402
Species—Soil depth	0.386		0.920	0.575	0.949	0.938	0.202	0.256
Water—Soil depth	0.842		0.796	0.482	0.900	0.646	0.000	0.002

Note: * Different letters within each column indicate significant difference (p ≤ 0.05).

). Similarly, it contained the highest K (265.0 mg/kg), Ca (640.7 mg/kg), and Mg (131.7 mg/kg) levels (p 0.008), suggesting that produced water contributes to greater mineral accumulation in soil (Table 10).

Soil depth had a moderate effect, with Fe levels remaining unchanged, while B was slightly higher in the topsoil. Al and Na concentrations were significantly higher in the bottom and top layers, respectively. Plant species had a minimal impact on Fe, B, and Al but influenced Na, K, Ca, and Mg levels. Panicum maximum had the highest Mg levels (152.6 mg/kg), while buffelgrass had the lowest (51.6 mg/kg; p = 0.002).

The factorial plot indicates that sodium concentration is significantly influenced by interactions between soil type, grass species, water treatments, and soil depth, with Nimr soil generally showing higher Na levels than control soil (Figure 4). Water treatments (fm, pw, and ww) exhibit distinct effects, particularly in interaction with grass species, where certain species respond differently to water availability. Additionally, soil depth influences Na accumulation, with variations observed between the top and bottom layers, highlighting the complexity of Na distribution in the experimental setup.

Biochar-treated soil had higher Fe levels (0.12 mg/kg) than gypsum-treated (0.019 mg/kg) and untreated soil (0.08 mg/kg) (p = 0.017). Gypsum increased Ca levels (761.6 mg/kg) compared with untreated soil (235.1 mg/kg) (p = 0.000). Nimr soil had significantly more Na (19,676.6 mg/kg) and K (176.2 mg/kg) than the control soil (p = 0.027), while Mg levels were slightly higher in the control soil (p = 0.018). Na was influenced by water source and soil depth (p = 0.002), and K was influenced by plant species and water source (p = 0.000). Most interactions were not significant, indicating independent effects. The top-soil layer, across all plant types and soil amendments, had the highest sodium level (47,250.0 mg/kg) when irrigated with PW (Figure 5).

3.3.2. Mineral Accumulation in Nimr Site Trial Soil

Water sources played a major role in soil element concentrations. Soil irrigated with produced water had significantly higher boron (5.0 mg/kg vs. 1.9 mg/kg; p = 0.001), magnesium (1155.2 mg/kg vs. 552.4 mg/kg; p = 0.015), and sodium (13,112.5 mg/kg vs. 5548.7 mg/kg; p = 0.064) levels (

Table 11. Soil element composition values (mg/kg) and significance levels at the end of Experiment 2.

Soil Factors/Elements	Zn		Ni	Fe	B	Cu	Al	Mn
Produced Water	0.46 A *		0.04 A	0.14 A	5.0 A	0.07 A	0.31 A	0.70 A
Groundwater	0.53 A		0.045 A	0.10 A	1.9 B	0.05 A	0.30 A	0.62 A
	p-value	0.697	0.561	0.163	0.222	0.155	0.001	0.500
Top	0.53 A		0.04 A	0.17 A	4.2 A	0.07 A	0.32 A	0.91 A
Bottom	0.46 A		0.04 A	0.07 B	2.7 A	0.05 A	0.28 B	0.42 B
	p-value	0.697	1.000	0.002	0.094	0.002	0.060	0.002
Panicum max	0.37 A		0.042 A	0.10 A	2.94 A	0.05 A	0.30 A	0.78 A
Panicum	0.64 A		0.045 A	0.14 A	3.66 A	0.06 A	0.31 A	0.68 A
Buffelgrass	0.48 A		0.045 A	0.13 A	3.91 A	0.06 A	0.30 A	0.53 A
	p-value	0.463	0.829	0.624	0.543	0.834	0.758	0.309
Germination:
Reverse Osmosis	0.50 A		0.043 A	0.10 B	2.83 A	0.05 A	0.29 A	0.49 A
Groundwater	0.62 A		0.04 A	0.17 A	4.74 A	0.08 A	0.32 A	0.72 A
None	0.37 A		0.04 A	0.10 AB	2.94 A	0.05 A	0.30 A	0.78 A
	p-value	0.595	0.542	0.037	0.104	0.126	0.060	0.139
Interaction p-values
Germination—Water	0.702		0.068	0.159	0.044	0.36	0.323	0.735
Germination—Depth	0.717		0.542	0.729	0.429	0.951	0.241	0.140
Water—Depth	0.692		0.711	0.934	0.967	0.363	0.381	0.986
Species—Water	0.902		0.829	0.832	0.792	0.642	0.547	0.894
Species—Depth	0.344		0.532	0.972	0.628	0.364	0.375	0.201

Note: * Different letters within each column indicate significant difference (p ≤ 0.05).

) compared with soil irrigated with ground well water. However, Zn, Ni, Fe, Mn, Cu, and Al concentrations in soil showed no significant differences between the two irrigation sources.

Soil depth significantly affected certain elements. While Zn, Ni, and Fe levels remained stable, boron (0.17 mg/kg vs. 0.07 mg/kg; p = 0.002) and manganese (0.91 mg/kg vs. 0.42 mg/kg; p = 0.002) levels were higher in the topsoil. Major nutrients such as Mg, Ca, K, and Na were also more concentrated in the top layer (p = 0.044), suggesting greater accumulation at shallower depths.

Plant species had no significant influence on Zn, Ni, Fe, B, Mn, Cu, Al, Mg, Ca, K, or Na concentrations (p = 0.355), indicating that element levels remained stable regardless of vegetation type. The top–bottom soil layer, across most plant types and germination methods, had higher sodium levels under produced water irrigation compared with groundwater (Figure 6).

3.4. Plant Tissues and Root Mineral Uptake

In Experiment 1, differences in element concentrations were observed between the roots and shoots of the tested plants. Zinc, manganese, sodium, calcium, and magnesium had similar concentrations in both plant shoots and roots. Alfalfa showed higher calcium uptake in shoots across all irrigation levels compared with other plant species. In contrast, aluminum concentrations were consistently higher in roots than in shoots, regardless of irrigation level. Moreover, the results showed that aluminum and iron were higher in plant roots than shoots.

Experiment 2 revealed variations in element uptake between plant roots and shoots. Zinc and copper had similar concentrations in both shoots and roots. However, boron, manganese, magnesium, sodium, potassium, and calcium had higher concentrations in plant shoots than in roots due to nutrient uptake dynamics. This accumulation leads to a reduction in plant development. Moreover, the element content in all the plant shoots increased with irrigation using produced water, except for potassium, where the increase was higher with ground well water than with produced water. Aluminum and iron exhibited similar trends in the balance of concentrations between the plant shoots and leaves, as observed in Experiment 1.

The addition of gypsum to the soil increased the calcium concentration in the topsoil layer compared to the other soil samples, which is due to the content of calcium in the gypsum in the form of calcium sulfate dihydrate (CaSO₄ 2H₂O). Nevertheless, all elements accumulated in higher amounts than the original sample concentration when produced water was used for irrigation. However, soils irrigated with groundwater showed an increase in magnesium compared to soils irrigated with other water sources and the initial soil concentration.

In both experiments, plants absorbed minerals differently in roots and shoots. Zinc, manganese, sodium, calcium, and magnesium showed similar levels, while aluminum accumulated in roots. Both experiments demonstrated higher sodium (Figure 7 and Figure 8) and boron (Figure 9 and Figure 10) levels in the roots and shoots when produced water was used.

Soil salinity, as indicated by electrical conductivity (EC) levels exceeding 15.5 dS/m, can significantly reduce biomass yield and affect soil structure stability (

Table 12. Experiment of one soil extract salinity and pH measurement. Abbreviations: PW = Produced Water; Pmax = Panicum maximum; P = Panicum; Buffel = Buffelgrass; Alfa = Alfalfa; Top = top-soil sample; Bottom = Bottom-soil sample; TDS = Total dissolved solids; EC = Electrical Conductivity; pH = potential of hydrogen.

Sample	TDS (g/L)	EC (mS/cm)	pH
PW-Pmax-gypsum-Top	18.27	21.8	7.4
PW-Pmax-gypsum-Bottom	12.67	15.95	7.4
PW-P-gypsum-Top	22.2	24.1	6.9
PW-P-gypsum-Bottom	14.7	15.42	7.2
PW-Buffel-gypsum-Top	20.0	20.8	6.9
PW-Buffel-gypsum-Bottom	13.63	14.0	7.0
PW-Alfa-gypsum-Top	19.78	22.9	7.4
PW-Alfa-gypsum-Bottom	13.75	16.77	7.2

). Several samples in the dataset exhibit high EC values, with the highest recorded at 27.38 dS/m in PW-Alfa-Biochar-Top, followed closely by PW-Pmax-Biochar-Top (26.6 dS/m) and PW-P-gypsum-Top (24.1 dS/m). These elevated salinity levels can lead to osmotic stress, making it difficult for plants to absorb water, ultimately reducing growth and productivity. Additionally, excessive salt accumulation can cause soil dispersion, negatively impacting water infiltration and aeration, largely due to excessive sodium. The pH in most samples ranges from 7.0 to 7.6, indicating a neutral to slightly alkaline nature; however, PW-Buffel-Biochar-Top shows a higher pH of 8.1, which may further affect nutrient availability.

4. Discussion

This study provides a comprehensive assessment of how various irrigation water sources and soil amendments affect mineral dynamics and plant responses in arid environments, with an emphasis on sustainability under saline conditions. PW, known for its high TDS, notably influenced both soil and plant chemistry—particularly by elevating concentrations of sodium (Na), boron (B), magnesium (Mg), and potassium (K). These findings are consistent with earlier studies [23,27], which reported similar adverse impacts of saline water irrigation on soil health and nutrient balance. Soils irrigated with groundwater and wastewater exhibited overall lower potassium concentrations compared to those irrigated with PW, likely due to the combined effects of soil degradation, salinity, and sodicity, as also noted in [27]. Additionally, higher concentrations of several elements—including iron, boron, manganese, magnesium, sodium, and potassium—were observed in the topsoil layer, suggesting accumulation due to limited leaching under arid conditions.

A particularly concerning outcome was the sixfold increase in TDS in Nimr soil, suggesting intense salinization. This is consistent with Rengasamy [28], who reported that sodicity, driven by excess Na, leads to soil dispersion and reduced permeability, impeding water infiltration and nutrient uptake. Persistent accumulation of Na and manganese (Mn), especially in Nimr soil, indicates a risk of long-term soil degradation under continuous PW application, corroborated by [29], which emphasized the irreversible structural changes caused by saline-sodic water.

The study also found elevated boron concentrations in plant tissues—exceeding 200 mg/kg in some treatments—raising concerns about fodder safety for livestock. While B is an essential micronutrient, excessive uptake can be phytotoxic and harmful to animals. These findings are supported by [25,26,30], which noted that high B levels impair enzymatic function and cellular integrity in plants and pose health risks to herbivores.

Gypsum, a common soil amendment, showed limited effectiveness in mitigating salinity-induced stress, slightly improving calcium (Ca) levels but not sufficiently reducing Na accumulation. This aligns with [22], which reported that the efficacy of gypsum diminishes under highly saline conditions. In contrast, biochar application appeared to reduce B accumulation in plant tissues, likely due to its high surface area and cation exchange capacity, which enhance ion retention. These findings are consistent with [20,21] and recent studies in [31], emphasizing biochar’s role in modulating soil–plant ion interactions.

Despite these interventions, EC levels in several PW-amended treatments exceeded 15.5 dS/m—well above the FAO recommended threshold of 4 dS/m for most crops—indicating persistent osmotic stress. Such stress is known to impair photosynthesis, enzyme activity, and overall plant metabolism, ultimately reducing biomass production, as reported in [32,33].

Elemental distribution patterns further revealed that Na and B accumulated in both roots and shoots under PW irrigation, while aluminum (Al) and iron (Fe) were more prevalent in roots. This may reflect limited translocation or adsorption at the root surface, as described in a recent study [25]. The lack of detection of toxic heavy metals such as lead (Pb), cadmium (Cd), and mercury (Hg) is reassuring; however, trace levels of copper (Cu) (~9.5 mg/kg) suggest the need for monitoring potential long-term buildup and bioaccumulation risks.

Site-specific conditions also played a significant role in outcomes. At SQU, plant performance declined after three harvests due to increased salinity, whereas the sandy soils at Nimr supported five harvests—indicating better salt leaching. These findings reinforce the hypothesis that soil texture and drainage capacity are critical factors influencing plant resilience under saline conditions, as highlighted in [34,35].

Overall, the study affirms prior research on the potential and limitations of PW reuse in agriculture. While biochar offers promise for mitigating B toxicity, neither amendment fully resolved the salinity challenges posed by PW. Notably, the lack of a combined biochar–gypsum treatment leaves room for future experimentation on synergistic effects. Future work should focus on integrated management strategies—combining multiple amendments, salt-tolerant genotypes, precision irrigation, and seasonal planning—to enhance resilience and sustainability in saline-irrigated systems, especially in the context of water scarcity and climate variability in the Gulf region.

5. Conclusions

This study demonstrates that treated produced water (PW) can be reused for irrigating salt-tolerant forage species in arid environments, but only under controlled conditions. PW irrigation significantly increased soil salinity and mineral accumulation, especially sodium and boron, leading to reduced plant performance. Soil texture influenced outcomes, with sandy soils allowing better salt leaching than finer soils. Biochar reduced boron uptake, while gypsum contributed calcium; however, neither amendment fully mitigated salinity stress. These findings underscore the need for site-specific management strategies that combine amendments, regulated irrigation scheduling, and alternating water sources. Future work should focus on long-term impacts, amendment combinations, and food chain safety assessments to support sustainable water reuse in agriculture.