Efficacy of different amendments and residual effects on nickel toxicity and nutritional quality in wheat/rice continuous system and health risk assessment in Ultisol

ABSTRACT Rice straw (RS), biochar (BC) and lime (LM) have proved to be effective immobilizing agents in acidic contaminated soil. Up-to-date scientific data is lacking regarding residual effects of these amendments in acidic soils and wheat/rice cropping system. The objective was to analyze the efficacy of amendments to improve grain yields, nutritional quality, and reduce Ni bioavailability of wheat/rice rotation system. A pot experiment was conducted to examine the efficacy of RS, BC 10 and 20 g kg−1 and LM 10 g kg−1 application rates on plant growth, nutritional quality and Ni contents in wheat and rice tissues and grain. Amendments enhanced Ni immobilization, declined their uptake by plants and accumulation in the grains of both crops. Rice straw biochar reduced several factors like health risk assessment, Ni uptake and bioconcentration factor. BC 20 g kg−1 increased shoot, root and grain yields, while enhanced essential nutrients in the wheat/rice cropping system compared to untreated soil. Soil amendments significantly decreased toxicity characteristics leaching procedure (TCLP) extractable Ni by 42.25% and 42.62% and simple bioaccessibility extraction test (SBET) by 42.1% and 45.25% in post- wheat/rice crops. This process enhanced the safety of wheat/rice rotation cropping system to be consumed and mitigated security risks.


Introduction
Toxic elements (TEs) that contaminate agricultural soil constitute one of the globally major hot concerns regarding food security and human health (Cui et al. 2016;Seleiman and Kheir 2018;Shoial et al. 2020;Seleiman et al. 2020a). Specifically, soil contamination poses an increasing threat to health and agricultural sustainability in China (Zhao et al. 2015;Rasool et al. 2022). According to Ministry of Environment Protection (MEP) and Ministry of Land Resources (MLR) of the People's Republic of China surveys, 19.4% of agricultural soil sites were exceeding the Chinese Soil Environmental Quality Standards including, Cd, Ni, Pb, Cu and Zn (MEEPRC 2018). Of all the TEs, nickel (Ni) has been recognized as 3 rd ranked with a percentage of 4.5% of the soils tested samples, making it one of the most critical pollutants in the agroecosystems of China Ali et al. 2019a;Tauqeer et al. 2022a).
The contamination of arable lands and cereal-based foods with excess Ni may result predominately from the use of cadmium batteries electroplating (Shahbaz et al. 2018b), phosphate fertilizers (Sohail et al. 2020), smelting, nickel steel, contaminated manures i.e. biosolids and raw sewage water for irrigation (Kamran et al. 2019). Consequently, low levels Ni plays a significant role in seedling growth and development (Mosa et al. 2016;Turan et al. 2018aTuran et al. , 2018b. However, elevated Ni concentrations in soil constitute a major challenge because Ni typically has high availability, mobility, and toxicity (Shahbaz et al. 2018b) and is easily accumulated by cereal-based crops. Compared to fruits, vegetables, and rooted crops, cereal crops such as wheat and rice crops exhibit a great potential to accumulate Ni from the soil, resulting in compromised growth, smaller yield, and poorer quality (Shahbaz et al. 2018b). Wheat and rice are mainly used as staple foods around the globe, and more than half of the world's population depends on rice and wheat to contribute to meeting their daily caloric intake Alkharabsheh et al. 2021a;Tauqeer et al. 2022b). During the last decade, wheat/rice rotation in Ni contaminated soil is a hot issue for several human diseases such as allergic dermatitis, kidney disease, lung fibrosis, food safety and sustainable crops yield (Rosskopfova et al. 2013;Khan et al. 2017).
Therefore, amelioration of Ni acidic contaminated soil is a devasting task to minimize Ni toxicity from the soil while at the same time ensuring reduced Ni uptake by wheat-rice rotation are important processes for retaining a healthy environment and healthy people. Ensuring the nutritional quality of wheat-rice requires the incorporation of various amendments in soil, and this can be achieved by minimizing Ni uptake in wheat-rice and immobilizing/stabilizing Ni in contaminated soil.
Use of traditional techniques such as soil excavation, landfilling, and soil washing for soil remediation have been developed but they are time-consuming, expensive and disturbing the environment (Hamid et al. 2019). For these reasons, it is important to develop techniques that can immobilize/stabilize Ni in situ, and the phytoremediation remediation technique is cost-effective and eco-friendly (Shaaban et al. 2018;Azhar et al. 2019). Alternatively, using organic and inorganic amendments could be considered for the immobilization of Ni soils by curtailing Ni availability in polluted soils (Ahmad et al. 2015;El-Naggar et al. 2018). China is one of the leading rice-producing countries . In China, the annual cultivated area of rice has reached 30 million hectares, and which is one-third of the total area of grain crops. In 2018/2019 rice consumption was nearly 143 million metric tons, respectively. So, efficient and rational utilization of the abundant rice residues is also a problem that China is facing. An alternative environmentally friendly management can be turning crop straw into biochar and returning the product to soils to improve soil fertility and avoid environmental pollution (Shu et al. 2016;Zhang et al. 2017). During the last decade, biochar (BC), the pyrolyzed form of rice straw/waste biomass recommended as a potential organic conditioner, showed that to dilute the bioavailabilities of TEs including Ni (Nawaz et al. 2019;Wang et al. 2021;Zubair et al. 2021) and improve the soil nutrient supply, thereby increasing crop yields and quality (Shahbaz et al. 2018b). Addition of BC into soils and waste water has been considered a key environmental management option for mitigating sustaining agricultural productivity by improving soil properties and functions for hundreds of years (Shaaban et al. 2018;Shen et al. 2018;Ali et al. 2019b;Seleiman et al. 2020b;Alkharabsheh et al. 2021b).
Studies by Shahbaz et al. (2019) and Bashir et al. (2018a) showed that rice straw biochar incorporated into the natural nickel and cadmium contaminated soil could significantly decrease bioavailability and bioaccessibility in maize and Chinese cabbage. Similarly, Ni uptake and translocation in maize (Lahori et al. 2020), rice (Ramzani et al. 2016) and wheat (Abbas et al. 2018) have been indicated to decrease by the addition of BC. These mechanisms are held responsible due to BC physiochemical attributes, such as alkaline in nature, high cation exchange capacity, water holding capacity, porosity, and surface area (Paz-Ferreiro et al. 2014;Salam et al. 2019). Similarly, biosorption techniques are widely accepted due to their cost-effective, environment-friendly and economic feasibility which can be exploited for several TEs irrespective of their toxicity and mobility due to high capacity and less time to remove TEs from aqueous without producing any toxic metabolites. Shen et al. (2017) used BC to sorb Ni from an aqueous solution. Nowadays, scientists' findings showed that biochar is a promising solution owing to its high removal efficiency and low-cost material to bind Ni in soil and aqueous (Shen et al. 2017;Wang et al. 2021).
Lime stone (LM) is a calcium-rich and oldest inorganic substance that could enhance HMs restriction contaminated soils (Cui et al. 2016;Rehman et al. 2017). The addition of LM in contaminated soil showed a consistent modification on soil pH, which plays a dynamic role in decreasing TEs mobility (Woldetsadik et al. 2016;Hamid et al. 2019;Ali et al. 2019a). Mahar et al. (2017) concluded that application of lime in co-contaminated soil significantly raises soil pH, decline availability and leachability of TEs due to the formation of hydroxyl groups and different surface negative charge on lime.
Numerous studies have successfully done restriction/immobilization of Ni contaminated soils using different types of immobilizer substances. However, information regarding the role of biochar and lime used to immobilize Ni contaminated Ultisol and enhance wheat-rice rotation grain yields by mitigating Ni toxicity are gap. Based on the above discussion, the present wheat/rice rotation pot study was conducted with the hypothesis that sustainable agriculture production from contaminated soils can be made possible by using friendly low-cost amendments. We hypothesized that RS, BC and LM could effectively alleviate Ni toxicity in both crops by reducing Ni content, promoting plant growth and nutritional quality and thus decreasing human health risks. Therefore, the objectives of the study were to 1) analyze the efficacy of all amendments to improve grain yields, nutritional quality, 2) study Ni uptake and translocation and health risk assessment, 3) supply efficacy of amendments regulating Ni bioavailability in Ultisol, and 4) biochar restrict or sorb Ni from aqueous solution.

Characterization of study soil, and amendments
Current pot study used soil is an Ultisol. Bulk soil was obtained from Jiangxia district, Wuhan city, Hubei Province, China (30° 17.804′ N, 114° 19.246' E). Obtained soil was transferred to the laboratory, air-dried at room temperature, ground and passed through a 2 mm sieve. Pre-sowing, soil samples physicochemical properties were analyzed i.e. soil pH, electrical conductivity (EC) , soil organic matter (SOM), cation exchange capacity (CEC) and soil particle distributions (Houba et al. 2000), available phosphorus and potassium (Ramzani et al. 2016). Soil's pH and EC were determined in 1:2.5 and 1:5 soil:water (m/V) ratio by using a Mettler-Toledo FE20 pH meter and DDS-370A EC meter, respectively (Ali et al. 2019a). The soil texture was classified as silty clay loam with an acidic nature (pH 5.3) and a small amount of organic matter (14.79 g kg −1 ) (Table 1). Moreover, all the amendments were alkaline in nature. Biochar had the highest carbon contents (470.7 g kg −1 ) followed by RS (364 g kg −1 ) and LM (121.2 g kg −1 ) ( Table S1). The total concentration of Ni in soil and amendments was analyzed by Agilent AA-240FS atomic absorption spectrophotometry (AAS) after using the standard Aqua Regia (HCl-HNO 3 -HClO 4 ) digestion method (Houba et al. 2000). These soil characteristics are presented in Table 1.
Straw from rice (Oryza sativa L.) and its derived biochar (BC) were chosen in this study due to their easily available and highly residues production in China, especially in the current area study conducted (Shaaban et al. 2018b;Wang et al. 2021). Rice straw (RS) was collected from the experimental station of Huazhong Agriculture University, Wuhan, Hubei Province. Firstly, RS was washed with tap water, rinsed in distilled water to remove impurities, then air-dried and chopped into pieces <1.5 mm and stored in a polythene bag prior to the pot study. RS biochar was produced at 500°C pyrolysis at a rate of 20°C min −1 and adjusted for 2 h by using a TDW-2001 1300 high temperature muffle furnace. Finally, obtained RS derived biochar was ground to pass through a sieve of 0.15 mm, and stored in a polythene bag for further physiochemical characteristics. Total C, H, O and N contents were analyzed by using Vario PYRO cube and Isoprime100 (Germany) elemental analyzer.
Meanwhile, lime (LM) was purchased from Zhongxiang Phosphorus Co. Ltd., Hubei. Physicochemical characteristics of used amendments are summarized in Table S1. The pH and EC of the amendments were determined in 1:10 solid to water (m/V) ratio suspension by using a Mettler-Toledo FE20 pH meter and DDS-370A EC meter, respectively.

Experiment set up
The pot study was conducted in the open experimental area to evaluate the effect of rice straw, biochar and lime over wheat/rice growth, Ni toxicity and health risk assessment. The soil was synthetically contaminated with nickel (Ni) 135 mg kg −1 of soil (three times over permissible limits by NiSO 4 ·6H 2 O addition) and then incubated for two months in a dark room at 25°C. The polluted soil was amended with rice straw (RS), its derived biochar (BC) with 2 levels and lime (LM) with 1 level, and homogeneously mixed except control (CK) (only Ni polluted soil), and further incubated for 30 days at 65% moisture. Each plastic pot was filled with 5 kg of amended soil. The treatments were (i) control (CK), without amendments, (ii) RS 10 g kg −1 , (iii) RS 20 g kg −1 , (iv) BC 10 g kg −1 , (v) BC 20 g kg −1 , and LM 10 g kg −1 . The statistical experiment was arranged by following a completely randomized design (CRD) and each treatment was replicated. There were three pots in each replication. In each pot, eight wheat seeds (variety Zhen mai 9023) were sown, and after germination three plants were maintained in each pot. The application rates of nitrogen (N), phosphorus (P), and potassium (K) fertilizers were 0.3 g N kg −1 soil by CO(NH 2 ) 2 , 0.15 g P 2 O 5 kg −1 soil by Ca(H 2 PO 4 ) 2 , and 0.2 g K 2 O kg −1 soil by K 2 SO 4 , respectively. During the whole growth period, pots were regularly irrigated with DI water to avoid drought stress. The wheat plants from each pot were harvested at the maturity stage. Prior to harvesting various agronomic parameters like plant height, spike length and yield were recorded, and soil and plants samples were prepared for analysis.
After 1 month of the harvest of wheat, rice was successively grown again in the same pots without further incorporation of the amendments to evaluate the residual effects of amendments on the bioavailability of Ni for wheat/rice cropping system. Seeds of rice cultivar (Huang Zhong) were sterilized in H 2 O 2 solution were grown in the field area of the University, and 25 days old three rice seedlings were transplanted in each pot. Prior to rice seedling transplanting, fertilizers were added into soils evenly P and K all at once, while urea was applied in two splits, and then the soils in pots were submerged. Recommended N:P: K fertilizers (0.3: 0.15: 0.2 g kg −1 soil) were given as CO(NH 2 ) 2 , Ca(H 2 PO 4 ) 2 , and K 2 SO 4 , respectively. Each pot was kept flooded with DI during the entire growth period and harvested at the maturity stage.

Post-harvest soil and plants analysis
After the completion of wheat and rice experiment, the bioaccessible Ni was assessed in the rhizosphere soil that existed wheat/rice crop in each treatment through a simplified bioaccessibility extraction test (SBET). This test has been recently described in Liu et al. (2019). Briefly, 0.5 g sieved soil from each sample was added in a 50 ml centrifuge tube, and 25 mL of 0.4 M glycine solution (pH 2.5) was added. The suspension was kept on agitation on an end-over-end and shaken at 30 rpm for 1 h at 37°C ± 1. Likewise, bioavailable Ni concentration was examined by extracting with 0.01 M CaCl 2 (2:20 m/V) mixtures as followed by Houben et al. (2013). The mixtures were shaken in an orbital shaker for 2 h at 180 rpm at 25 ± 2°C. Once the suspensions were separated by centrifuged at 1871 × g for 20 min and each solution was filtered, stored at 4°C and analyzed for Ni concentration (mg kg −1 ) using AAS. Meanwhile, the soil pH was determined using the method above. Fully matured wheat and rice plants were harvested using a sharp cutter and divided into spikes, shoot, roots and grain. The grain yields of each treatment were recorded with a digital weighing balance. While, roots and shoots were washed carefully with distilled and deionized water, after which they were oven-dried to at 70°C to achieve a constant weight. Next, the oven-dried root, shoot and grain samples were ground separately using a stainless-steel grinder. These ground samples were then subjected to Ni concentrations. The Ni content of both plants roots, shoots and grain was determined by the wet digestion method which has been described by Jones et al. (2016). Briefly, 0.25 g dried ground plant sample (root, shoot and grain) were digested with a mixture of HNO 3 -HClO 4 acid (3:1 V/V) in a 100 mL conical flask and retained overnight. The digestion was done using a hot plate at 150-180°C until 2-3 mL suspension remained, then the suspension was diluted to 25 mL with DI. Diluted supernatants were filtered and stored at 4°C and their concentrations of Ni were measured using AAS. The total NPK contents in plant digest samples were measured on inductively coupled plasma mass spectrometer (ICP-MS-7890A), while the total N concentrations were determined using a flow injection system (Auto-analyzer, Seal, Germany).

Element's uptake and biochar Ni removal efficiency
For wheat/rice cropping system, tissues NPK and Ni concentrations (g kg −1 ) were multiplied with the plant tissues dry biomass (g) as described by Zia ul Hassan et al. (2016) in the following formula: Wheat and rice obtained grain, shoot and concentration, uptake was measured as described above and used for quantification of Ni translocation and harvest index were calculated according to equations given below as recommended by references (Sohail et al. 2020;Azhar et al. 2019), as follows: Ni translocation indexðTIÞ% ¼ wheat or rice grain Ni contents wheat or rice grain þ shoot þ root Ni contents � 100 (2) Ni harvest index ¼ wheat or rice grain Ni contents þ wheat or rice shoot Ni contents wheat or rice grain þ shoot þ root Ni contents � 100 (3) While Ni bioconcentration factor (BCF) was measured by equation proposed by Zhuang et al. (2007), for which post-harvest soil and plant total Ni concentration (mg kg −1 ) were calculated.
Bio À concentration factor BCF ð Þ ¼ Ni conconten harvested plant tissue mg kg À 1 ð Þ Ni content post harvested soil mg kg À 1 ð Þ The average daily intake of toxic metal (DITM) was determined by the following formula by Nawab et al. (2019) as described.
Daily intake of toxic element DITE ð Þ ¼ C TE � C factor � D daily food intake = B average weight (5) Where C TE is a wheat/ rice grain concentration (mg kg −1 ), C factor is the conversion factor (0.085), D daily food intake is the daily use of grains taken as 400 g per person per day and B average weight is the average weight of the body which was as taken as 70 kg per person. The health risk assessment (HRA) for each selected toxic element was determined by estimating the daily intake of toxic elements including Ni (DITM) and with oral reference dose of Ni (RF D ). The RF D value for Ni was taken as 0.91 mg kg −1 body weight day −1 as described previously (MEEPRC 2018;Nawab et al. 2019).
Ni immobilization index was measured as proposed by Shaheen et al. (2019) given below equations: Ni stabilization indexð%Þ TCLP Ni Control À TCLP Ni in amended soil TCLP Ni Control (7) Currently study also shows that biochar has the ability to bind Ni from the soil solution and can decrease bioavailable Ni to wheat/rice cropping system. For this purpose, the removal efficiency of Ni by biochar was examined. The 0.4 g biochar was mixed with 25 mL of 0.01 M NaNO 3 solution with various Ni concentrations, i.e. 20-200 mg L −1 in a 50 mL centrifuge tube. The solution initial pH was adjusted at 5.0 and the centrifuge tubes were shaken for 24 h at 220 rpm at 25°C. After the reaction, all mixtures were centrifuged at 1871 × g, filtered and their concentrations of Ni were determined using AAS. The Ni removal efficiency in aqueous solution was calculated as described by Ahmad et al. (2015), as follows: Ni removal efficiency in aqueous % ð Þ ¼ Initial Ni concentration À Final Ni concentration Initial Ni concentration � 100 (8)

Statistical analysis
The obtained data were statistically analyzed by using a statistical program, Statistic version 8.1. A one-way analysis of variance (ANOVA) using Tukey's test was performed to detect significant differences between each treatment at P ≤ 0.05

Soil pH post-wheat and rice harvest
The pot study confirmed that the incorporation of RS, BC and LM significantly (P < 0.05) improved the chemical properties of both wheat and rice post-harvestcontaminated soil. Changes in post-harvest soil pH significantly (P < 0.05) differed among used amendment types and their application rates (Figure 1). Soil pH values were differently affected in amendments and their application rates at the harvesting stage of both crops as compared to unamended soil (CK) (Figure 1). The lowest soil pH was noted in RS 10 and 20 g kg −1 soil application rates, while LM amended pot examined an increase in soil pH in post-harvest wheat 6.86 and rice crop 7.31, respectively, as compared to CK (5.28).

Efficacy of amendments on Ni bioavailability, bioaccessibility and Ni immobilization index
The phytoavailability of soil Ni was determined after the harvest of wheat and rice crops at the maturity stage. The TCLP extractable soil Ni significantly decreased in the amended soil ( Figure 2). After wheat harvest, the lowest TCLP extractable Ni was noted with BC (18.04 mg kg −1 ) compared to CK (31.24 mg kg −1 ). However, after rice harvest, relative to the CK, the TCLP extractable soil Ni decreased by 26.65%, 42.62% and 38.62% when RS, BC and LM were applied at 20 g kg −1 and 10 g kg −1 soil application rates, respectively.
An in-vitro technique was used to examine the Ni bioaccessibility after the harvest of wheat and rice. The SBET-extractable soil Ni was significantly reduced in the presence of organic and inorganic amendments (Figure 2). The greater reduction of 30%, 42.1% and 34.9% in wheat harvest soil, and 30.2%, 45.25% and 39.33% in rice harvest soil were recorded at the 20 g kg −1 soil application rate of RS, BC and 10 g kg −1 rate of LM, respectively compared to CK.
Efficacy of soil-applied organic and inorganic amendments to immobilize Ni index was calculated at the harvesting stage of both crops (Figure 3). Especially, post wheat-rice biochar enhanced the immobilization of soil Ni relative to other treatments and untreated soil. The enhancing order of Ni immobilization index was 34.7%, 42.1% and 40.2% at wheat, and 27.9%, 42.9% and 37.7% at rice harvest at maturity stage, when RS, BC and LM were applied at 20 g kg −1 and 10 g kg −1 soil application rates prior to 1 st crop, relative to control.

Plant growth and essential nutrients accumulation of wheat and rice
Plant growth data presented in Figure 4 showed that applied all amendments significantly improved the dry biomass and grain yields of wheat and rice. Plant growth parameters i.e. plant height and spike length, root and shoot biomass and grain yield markedly improve due to Ni uptake by changing phytoavailable Ni, while indirect influences were on root and shoot growth by providing macro-nutrients. These parameters were observed maximum in BC and RS amended plants. An increase in wheat spike length, 39.10%,47.82%, and 7.6%, plant height by 29.52%, 40.33% and 12.64%, root dry biomass by 51.85%, 92.5% and 50%, shoot dry biomass by 60.31%, 101% and 51.32% and grain yield by 1.48-fold, 2.41-fold and 1.92-fold higher in presence of RS, BC 20 g kg −1 and LM 10 g kg −1 soil application rates, respectively, while, these attributes enhanced in rice spike length by 57.6%, 64.39% and 34.8%, plant height by 33.51%, 37.3 and 19.6%, root dry biomass by 31.28%, 125.77% and 53%, shoot dry biomass 73.63%,140% and 28.39% and grain yield by 1.37-fold, 2.34-fold and 1.45-fold higher for the above corresponding treatments at the rice maturity stage.
The grain N, P and K concentration and accumulation of both crops were examined lowest in unamended pots (Table 2). Wheat N, P and K contents were increased by 49.98%, 31.58% and 33.16% in the presence of BC treatment compared to crops grown in the unamended soil. Similarly, rice essential nutrient contents were increased as a result of amendments in the following increasing order BC>RS> LM >unamended.

Nickel contents in plants tissues
Nickel concentrations in the roots, shoots and grain of wheat and rice had the lowest values in amended soil when compared to CK ( Figure S1). The maximum Ni concentration in both crops was found in plants growing in untreated soil. Wheat root Ni reduction by 83.86%, 21.91% and 76.44% were noted in the BC and RS at 20 g kg −1 and LM at 10 g kg −1 application rates, respectively relative to CK. Similarly, incorporation of the amendments in the contaminated soil decreased the Ni concentration in the shoot by 39.46%, 62.31% and 48.45% with RS, BC 20 g kg −1 and LM at 10 g kg −1 application rates, respectively, over the untreated soil. Likewise, BC 20 g kg −1 treatment showed the lowest Ni content in wheat grain by 74.01% relative to control. Wheat grain Ni content was decreased in the following order: BC > LM > RS > CK. Similarly, the incorporation of all amendments was significantly (P < 0.05) decreased Ni grain uptake compared to control (Table. S1). Meanwhile, BC 20 kg −1 application rate illustrated its highest residual potential and significantly lowest root, shoot and grain Ni content reduction by rice crop. The maximum decreases in rice root by 66.47%, shoot by 57.74% and grain 71.61% in Ni concentrations were found in BC 20 g kg −1 application rate, when compared to control. The potential efficiency of residual amendments to decrease rice grain Ni content was noted in the following order: BC > LM > RS > CK.

Bioconcentration factor and health risk assessment
The addition of amendments has shown a different effect on BCF (Figure 2), translocation index, and health risk assessment in both crops (Table S2). All amendments significantly reduced BCF for shoot and grain in both crops as compared to unamended (Figure 2). The lowest Ni grain translocation index of 60.68% was calculated in BC 20 g kg −1 application rate when compared to the unamended (Table. S2). Similarly, harvest index, daily intake metal and health risk assessment of Ni were maximum for crops grown in Ni contaminated soil, and lowest values were reported in BC 20 g kg −1 application rate (Table. S2).

Discussion
Research has demonstrated that organic and inorganic amendments to soil can efficiently immobilize metals in soil to reduce their toxic effects on plants (Turan 2020(Turan , 2021(Turan , 2022. Application of rice straw, biochar, and lime to Ni contaminated Ultisol revealed a significant reduction in Ni phytoavailability, bioaccessibility and Ni contents in wheat and rice plants. The present study displayed that application of RS, BC and LM altered soil chemical properties and enhanced growth and grain yields of wheat and rice in Ni contaminated Ultisol. Post-harvest of wheat and rice, the soil pH changed due to LM decomposed into two forms and released OH ions upon the reaction of alkaline material with irrigated pots and resultantly enhanced soil pH (Houben et al. 2013;Woldetsadik et al. 2016). This modification of acidic soil pH was due to increased negative charges and enhanced precipitation of Ni cation. Biochar application also exhibited a significant increase in soil pH (Figure 1) at both crop harvesting. The increased soil pH be in response to the hydrolysis of CaCO 3 into hydroxyl ions (Hamid et al. 2019).
Our results were well substantiated with the findings Ali et al. (2019a), who indicated modification of soil pH with the addition of liming agents i.e. LM due to the sufficient amount of Ca 2+ which could ameliorate the acidity of contaminated soil. Likewise, Shen et al. (2017) concluded that during biochar production through pyrolysis, the transformation of base cations into carbonates, oxides and hydroxides may contribute to elevated soil pH. The reason accountable for enhancement in soil pH could be ash content, more functional group and release of base saturating cation Ca 2+ , Mg 2+ and K + present on BC and LM (Shen et al. 2017;Zhang et al. 2017;Shaaban et al. 2018). These results are in accordance with previous acidic contaminated soil reported by  and ) they suggested that elevated soil pH may be due to the carbon mineralization, hydroxyl group production and release of basic cations. Post-wheat and rice, TCLP-extractable Ni significantly changed in applied amendments and also influenced the soil Ni immobilization (Figure 2). The TCLP extractable-Ni fell with the application of RS, BC and LM amended soil. These variations in Ni stabilization probably depended on their difference in physiochemical properties of applied amendments large surface area of BC and LM through different mechanisms such as precipitation with mineral, exchange with cations and complexation with functional groups (Rehman et al. 2017;Zhang et al. 2017;El-Naggar et al. 2018). These decreases can be explained by a modification of soil pH due to alkaline amendments (Table S1) being applied in acidic contaminated soil have a good contribution leading to a lower TCLP-Ni extractability. These findings are in accordance with previous studies where the applied BC and LM especially in acidic contaminated soil (Mahar et al. 2017;El-Naggar et al. 2018;Bashir et al. 2018a).
Indeed, it is important to note here that the addition of BC and LM might decline the available/ soluble proportion of TEs in contaminated soil through precipitation because of CO 3 and complex formation with stable OC of BC having a strong affinity for Ni (Mahar et al. 2017;Shen et al. 2018). Overall, the results of the present study indicated the possible mechanism of Ni immobilization could be followed: BC and LM are carbon sequestration materials and alkaline in nature could raise soil pH; micro or meso-porous structures, plenty of surface functional groups; and sorption of Ni on the surface area which contributed decline Ni availability in contaminated soil. Bashir et al. (2018b) noted a 38% reduction in TCLP extractable soil concentration, respectively, when adding rice straw biochar in contaminated soil and comparing this to the untreated soil. Previous studies stated that the application of different organic and inorganic amendments (agriculture wastes, biochar and lime materials) has the potential to decrease TEs leachability in polluted soil, due to their physicochemical characteristics (Ramzani et al. 2016;Mahar et al. 2017;Rehman et al. 2017). Recently, Nawab et al. (2019) and Shah Sohail et al. (2020) found that the incorporation of organic amendments, such as rice straw biochar in naturally contaminated soil improved cereal crops growth, yields as well as significantly decreased extractable Ni and Cd, daily intake metal and health risk assessment and our findings are in accordance with them after applied biochar.
Similarly, SBET was assessed by measuring Ni bioaccessibility in a gastric/intestinal pH kept at 2.5, and significant decline bioaccessible Ni (Figure 2). The lower SBET-Ni is explained by the precipitation of Ni as insoluble compounds, processes of complexation, adsorption of toxic elements on soil components due to the intestinal environment, characterized by a modification of pH and the excess amount of carbonates, resultant the metal being less available Liu et al. 2019). Similarly, trends were investigated in a recent study described by (Bashir et al. 2018a) and (Janus et al. 2019), they revealed remarkable decreases of bioaccessible metals concentration by 30.5% and 140%, using the same technique, when being added biochar in contaminated soil, as compared to CK. Current study findings showed the concentrations of SBET-Ni were significantly declined as the concentrations of Ni reduced in SBET-Ni, after the addition of BC and LM at various rates after harvesting both crops. Consequently, these findings suggested that bioaccessibility (real health risks) of Ni ingested soil was mainly attributable to the decrease of soluble friction of Ni in contaminated soil. Numerous, physicochemical characteristics of soil, like carbon contents, cation exchange capacity, and texture can manipulate toxic elements bioaccessible because of its interaction with other soil mineralogy (Khan et al. 2017;Mahar et al. 2017).
Excessive concentration of Ni in soils influences plant growth and hence results in a reduction of plant growth and yields (Mosa et al. 2016;Shahbaz et al. 2019). The lowest plant growth was observed in the unamended due to deficiency of essential nutrients to plants under stress, and these were in accordance with Mosa et al. (2016). In the present study, applied amendments in contaminated soils, directly and indirectly, influence plant growth parameters; firstly, these amendments bind bioavailable fraction of Ni on their binding sites and reduce Ni availability to plants. Secondly, applied amendments decompose resulting in more adsorption sites and releasing a sufficient nutrient (Rehman et al. 2016;Shah et al. 2018;Wang et al. 2021), which may promote the plant growth and grain yield, sufficient nutrients in polluted soil. Similarly, LM application to contaminated soil quickly ameliorates the acidity of the soil, improves soil fertility status (Woldetsadik et al. 2016;Hamid et al. 2019). The much-enhanced in both crop growth and grain yields are evidence that is consistent with other researchers' findings for the dry biomass and grain of plant species (Jones et al. 2016;Torres et al. 2016). This specifically refers to the incorporation of BC in Ni contaminated soil. Similarly, an increment in the growth, biomass and grain yields of wheat and rice are grown on contaminated soil with the incorporation of BC and LM (Ramzani et al. 2016;Shahbaz et al. 2019;Mujtaba Munir et al. 2020). However, LM and RS treatment promoted statistically lower yield and NPK concentrations response of both crops compared to the BC treatment (Figure 4).
The results depicted that applied amendments significantly reduced the Ni concentrations in tissues and grains of both crops. These findings suggested that the incorporation of BC and LM has the potential to enhanced Ni sorption on amended soil, which could be responsible for reducing Ni availability/solubility to wheat and rice plants relative to CK. The lowest Ni absorption by plant tissues and grain was caused by the minimum concentration of extractable Ni in the amended soil, which directly influence soil health, i.e. modification of soil pH and Ni induced into the amended soil. In the current study, BC and LM application significantly reduced the Ni contents in both crops plant tissues and grain, and these findings are consistent with what earlier studies have done, where BC and LM shrunk the contents of toxic metals including Ni in shoots, roots and grains of sunflower, maize and wheat (Shahbaz et al. 2018b(Shahbaz et al. , 2019, rice (Ramzani et al. 2016;Hamid et. al 2019) rapeseed (Houben et al. 2013). On the other hand, Shah et al. (2018) concluded that BC application in polluted soil including Ni contaminated soil is a better and low cost option for declining Ni contents in plant tissues and grain. Shahbaz et al. (2019) revealed that the addition of biochar was able to decrease the Ni extractability up to 37% and Ni grain uptake up to 34%, respectively, relative to unamended. Similarly, Ni uptake was decreased in treated plants relative to untreated plants. After absorption by the root, Ni is mainly retained in the roots and a small quantity is translocated to shoot and then to wheat and rice grains (Ramzani et al. 2016). This reduction in Ni uptake in both crops with incorporated amendments may be associated with an enhanced Ni Immobilization and decreased Ni bioavailability (Ahmad et al. 2015). The possible mechanism associated with the reduction of Ni contents uptake in both crops' tissues might be associated to the neutralization of hydrogen ion (H + ), which untimely decreases Ni bioavailability, bioaccessibility, increase in soil (-) charge, presence of Ca 2+ in BC and LM and through different chemical processes such as precipitation, and adsorption with amendments (Ramzani et al. 2016;Shen et al. 2017;Hamid et al. 2019).
Furthermore, amendments also declined Ni bioconcentration factor ( Figure S2), Ni harvest index, daily intake of Ni and health risk assessment of both crops. Similar findings were noted in the cultivation of wheat and rice rotation system (Azhar et al. 2019). According to our results, the BCF values were calculated less than one in amended soil as compared to CK. These findings are almost similar to Rehman et al. (2016), who revealed the lowest BCF of plants grown on the amended soil as compared to CK. This finding showed that amendments could immobilize Ni for wheat-rice rotation system which justified the biochar application in the Ni contaminated soils to ensure food safety.
The effectiveness of biochar to removal efficiency of Ni was affected by various concentrations of Ni at initial pH. BC indicated promising results in terms of their removal efficiency in aqueous solution ( Figure S3). BC showed 99.69-95.85% Ni removal efficiency under 20-200 mg L −1 concentration. According to the obtained result, it is inferred that the BC has the potential to remediate Ni in polluted soil as well as in aqueous solution. The acidity and alkalinity of the solution have great influences on the sorption of Ni. In acidic conditions, mostly TEs are present and their mobility is higher relative to alkaline. Our results inferred that initial pH was ideal for Ni removal from wastewater. The result of the present study suggested that Ni could bind to BC that helped in 95.85% removal efficiency after 24 h in 200 mg L −1 concentration was noted in aqueous solution. Interestingly, we cannot neglect the concentration of Ni, because increased concentrations have enhanced Ni removal efficiency depending on the pH and actual TEs concentration in the solution. The results of the present study are similar to the findings from (Ahmad et al. 2015) and (Shen et al. 2017).