The contamination of heavy metals (HMs) in soil is a serious threat to sustainable crop production and soil quality because these metals are non-degradable and persist in soil for longer durations (Kamran et al., 2019). HMs accumulation deteriorates the physicochemical and biological properties of soil, which results in poor nutrient availability and ultimately decreased crop yield (Mehmood et al., 2018b). Naturally, HMs are the constituent of sediments, adsorbed on soil organic matter, and their toxicity increased as free ions in soil solution (Abbas et al., 2020). Modern agricultural practices, especially excessive use of inorganic fertilizers and agrochemicals, have polluted soil and ultimately resulted in enhanced environmental degradation and threatened sustainability (Ditta et al., 2015; Murtaza et al., 2021b). Moreover, the application of organic waste manure, sewage sludge, and irrigation with industrial effluents contributes significant amounts of HMs into agricultural systems (Kamran et al., 2019; Bashir et al., 2020a,b).

Among HMs, chromium (Cr) is considered a major environmental pollutant due to its severe toxicity and recalcitrant nature (Ali et al., 2018). Industrial effluents from electroplating, catalytic manufacturing and wood preservation, leather tanning, and alloy preparation contain large amounts of Cr, which are the major sources of Cr contamination in the soil (Govil and Krishna, 2018). In the environment, Cr exits in various oxidation states, but Cr(III) and Cr(VI) are dominant in soil (Junaid et al., 2016; Bashir et al., 2020a). Cr(III) is an essential micronutrient for animals and involved in cell metabolism (de Oliveira et al., 2014), relatively stable, and 10–100 times less toxic as compared to Cr(VI) (Tepanosyan et al., 2020). Cr(VI) is a highly soluble, potentially mobile across membranes with strong oxidation potential, toxic, allergenic, carcinogenic, and irritant that damages the liver, kidney, and lungs (Bashir et al., 2020a,b; Tepanosyan et al., 2020).

The application of contaminated water with a higher concentration of Cr to the agricultural land not only results in detrimental effects on plant growth and loss of production, food security, and animal health but also affects the overall ecosystem (Mehmood et al., 2021). Cr phytotoxicity severely affects seed germination, decreases root growth and shoot growth, and ultimately reduces biomass production (Bashir et al., 2020a,b). Cr disrupts photosynthesis, nutrient utilization, water relation, and enzymatic activity which produces reactive oxygen species (ROS) that oxidize lipids, protein, and nucleic acid and ultimately result in plant death (de Oliveira et al., 2014; Bashir et al., 2020b; Murtaza et al., 2021b).

Various physical (e.g., soil incineration, landfill, excavation, and soil flushing), chemical (e.g., oxidation–reduction), and biological techniques (e.g., phytoremediation and biodegradation) have been used for remediation of metal-contaminated soils or conversion into less toxic form and reduced bioavailability to plants (Mustafa et al., 2020; Sabir et al., 2020; Irshad et al., 2021). Physical and chemical remediation techniques are laborious, time-consuming, and energy-consuming, and considerable toxic waste products are the main cause of limited application despite high efficiency and utility (Mustafa et al., 2020; Sabir et al., 2020). Among organic amendments, farmyard manure, animal waste, and compost are used to reduce HMs uptake for a shorter duration because these amendments improve soil physicochemical properties, nutrient uptake, soil aggregation, and maintain soil quality (Mehmood et al., 2018a; Chukwuka et al., 2020; Hu et al., 2020; Murtaza et al., 2021a). It is, therefore, a need of the hour to develop efficient in situ immobilization techniques that should be cost-effective, less destructive, and environment friendly for soil restoration.

In recent years, biochar (BC) is gaining popularity for the restoration of metal-contaminated sites and carbon sequestration in the soil for longer periods due to its recalcitrant nature, cost-effective, and ecofriendly approach (Mehmood et al., 2018a; Kamran et al., 2019). BC is a fine-grained black color porous material with carbonaceous nature produced from organic wastes (e.g., crop residues, sewage slug, animal wastes, and farmyard manure) at high temperatures under limited or no oxygen supply (Abbas et al., 2020; Hu et al., 2020). BC has unique physicochemical properties such as porous structure, high surface area, variety of oxygen-containing functional groups that make it an excellent material for soil fertility improvement, carbon sequestration, wastewater treatment, metal stabilization, and organic pollutant degradation in soil (Mehmood et al., 2018a,b; Chukwuka et al., 2020; Hu et al., 2020; Murtaza et al., 2021a). BC has a stable carbon pool that influences soil physicochemical and biological properties and directly increases soil organic carbon contents (Sarfraz et al., 2019; Ijaz et al., 2020; Ullah et al., 2020). The application of BC in agricultural soil improves aeration, increases nutrient availability, and increases soil organic matter, a microbial activity that ultimately increases crop production, decreases fertilizer requirement and nutrient leaching, and reduces soil erosion by controlling soil pollution (Doumer et al., 2016; Rizwan et al., 2021). BC reduces metal mobility through physical adsorption, ion exchange, surface precipitation, and metal complexation (Sarfraz et al., 2019; Irshad et al., 2021); cation exchange; and electrostatic interaction in metal-contaminated soil (Brassard et al., 2016). The adsorption property and high carbon contents of BC make it an excellent material to minimize Cr toxicity in the terrestrial ecosystem (Jia et al., 2017).

Various studies reported that combined application of compost and BC could be a better approach as BC increases compost stability and improves the productiveness of the soil, HM adsorption, and production of crop and carbon sequestration (Agegnehu et al., 2015; Coelho et al., 2018). The mixing of BC and compost adds value in increasing soil fertility by different aspects such as reducing the volatility of ammonia from manure or minimizing methane emission (Coelho et al., 2018). Compost provides high nutrients to the soil compared to BC, whereas BC could be reducing the decomposition rate of compost (Ditta et al., 2015). Many studies showed the combined application of BC and compost had a synergistic impact in improving soil structure, water-holding capacity, and nutrient contents under field conditions with reduced fertilizer use and nutrient loss from soil (Agegnehu et al., 2015). BC co-composting can reduce compost decomposition, decrease nutrient loss in the environment, improve soil structure by direct organic matter addition, and increase microbial biodiversity and activity in the soil. We hypothesized that co-composted BC could act as a better ameliorant for improving growth, physiology, and antioxidant machinery by retrieving the harmful effects of Cr in Cr-contaminated soil. The objectives of this study were to investigate the impact of compost, BC, and co-composted BC on growth, physiological, antioxidant, and biochemical attributes of Brassica grown under Cr-contaminated soil and the phytostabilization efficiency of compost, BC, and co-composted BC in addition to the Cr-contaminated soil and associated health risk assessment of Brassica grown under Cr stress.

Results showed that the application of compost, BC, and co-composted BC significantly (p < 0.05) improved the growth attributes of Brassica in uncontaminated and contaminated soil (Cr-25). The soil under Cr stress (Cr-25) showed reduced shoot and root length, shoot fresh and dry weight, and root fresh and dry weight of Brassica (Figure 1). Many researchers have reported reduced morphological parameters under Cr stress (Adrees et al., 2015; Maqbool et al., 2018). Cr toxicity inhibits cell division and elongation in plant root cells (Adrees et al., 2015), thus reducing root surface area for enhanced water and nutrient uptake from the soil (Medda and Mondal, 2017; Ahmad et al., 2020) and ultimately growth and biomass production. Our findings are in line with the study by Jun et al. (2009) that Cr-contaminated soil reduced shoot length, root growth, and biomass production in cereals, vegetables, and forages.

Our results showed that plant growth significantly improved with the application of compost, BC, and co-composted BC. Novak et al. (2019) observed growth improvement when different combinations of BC and compost were used under Cd-contaminated soil. Abbas et al. (2020) observed that a combination of BC (3%) and AMS (5%) increased shoot length (93%), root length (222%), shoot dry weight (60%), and root dry weight (164%) of maize grown in Cr-contaminated soil. Arshad et al. (2017) reported that the pine wood chip BC produced at 450°C significantly improved maize root and shoot fresh weight, root length, and root surface area. Similarly, various studies have reported that BC application improved plant growth under a Cr stress environment (Agegnehu et al., 2017; Arshad et al., 2017). BC addition improves nutrient retention and water uptake and increases organic carbon (Rizwan et al., 2016; Cheng et al., 2020), thus enhancing soil fertility and boosting vegetative growth (Ali et al., 2018). In addition, the combined compost and BC influenced soil properties, such as pH, cation exchange capacity (CEC), soil organic matter, and nutrients availability, and ultimately increased soil fertility and plant growth (Agegnehu et al., 2017). In this study, an increase in plant growth parameters might be due to decreased Cr bioavailability with enhanced nutrient uptake under BC-amended Cr-contaminated soil. It may also be the BC -mediated conversion of Cr(VI) to Cr(III) in soil, which is relatively less mobile and less toxic (Rizwan et al., 2016).

Cr toxicity severely affects plant physiology and our results showed that the application of compost, BC, and co-composted BC significantly (p < 0.05) improved physiological attributes of Brassica in soil under Cr stress (Cr-25). Cr toxicity severely affects chlorophyll contents by disrupting chloroplast and discontinuing electron transport chain reaction, relative water contents, and electrolyte leakage in leaves. Many studies have shown that Cr toxicity damages photosynthetic machinery and produces a large amount of ROS in plants, which results in reduced growth (Ali et al., 2018; Bashir et al., 2020a).

Results of this study demonstrated that the application of compost, BC, and co-composted BC significantly reduced Cr uptake from soil under Cr contamination (Cr-25). Mustafa et al. (2020) observed that BC applied at the rate of 1 and 2% (w/w) reduced Cr uptake by 21 and 41%, respectively, under 500 μM of Cr stress. Similarly, Ali et al. (2018) observed that the application of BC efficiently removed Cr(VI) from wastewater. Cheng et al. (2020) noticed bamboo BC reduced Cu, Ni, Hg, and Cr uptake from water and soils. The reduction in Cr uptake might be due to the high surface area of BC that facilitates sorption, ion exchange, electrostatic interaction, complexation, precipitation, and co-precipitation (Rizwan et al., 2016). BC surface contains phenolic, hydroxyl, and carboxylic functional groups that facilitate metal binding (Mehmood et al., 2021; Murtaza et al., 2021a).

Under normal conditions, plants produce ROS in a controlled amount but under stress conditions; their production increases many times, which leads to oxidative stress, membrane permeability, DNA damage, and even cell death (Ahmad et al., 2010, 2019; Kohli et al., 2019). Plants have developed certain antioxidant enzymes to scavenge ROS and produce resistance against stress (Ditta and Khalid, 2016; Niamat et al., 2019). It was observed in many studies that the application of organic amendments enhanced resistance of plants against abiotic stresses (Seneviratne et al., 2017; Ali et al., 2018; Niamat et al., 2019). Our results showed that the application of co-composted BC indirectly influenced the antioxidant activity by reducing Cr bioavailability. In our findings, CAT activity was increased under Cr stress (Cr-25) in the control treatment. BC addition increased CAT activity (38.79%) in Zea maize and Brassica rapa (Ali et al., 2018; Bashir et al., 2020b). Similarly, Arshad et al. (2017) observed significant reductions in antioxidant enzyme activities under increasing levels of Cr stress. BC addition markedly improved antioxidant enzyme activities under Cr stress, which might be due to reduced Cr uptake (Seneviratne et al., 2017). Our results showed CAT, GSH, GR, GP, GPX, and GST activities in Brassica leaves increased under Cr stress (Cr-25), and the application of compost, BC, and co-composted BC reduced antioxidant activities. As discussed earlier, it was observed increased antioxidants activity in B. napus under Cd-spiked soil (Sabir et al., 2020).

This study results showed the co-composted BC significantly reduced Cr uptake in the Brassica plants. Novak et al. (2019) observed that the combined BC and compost reduced Cd uptake in plants. Seneviratne et al. (2017) also noted a significant reduction in cadmium (19.4%) and lead (22.0%) uptake when BC at the rate of 40 t ha^–1 was applied in Vigna radiata. It was observed by many researchers that BC reduced HMs bioavailability by manipulating soil pH, precipitation, sorption, and changing HMs redox state (Ditta and Khalid, 2016; Rizwan et al., 2016; Cheng et al., 2020). BC contains a variety of oxygen-containing functional groups (such as carbonyl, carboxylic, hydroxyl, and phenol) that might be involved in complexation with HMs and cation exchange process with metals cations present on BC surface (Seneviratne et al., 2017; Niamat et al., 2019). With time, BC undergoes an oxidation process and new reactive sites are formed on BC surface that facilitates HMs immobilization (Bian et al., 2014) and ultimately results in reduced HMs uptake in plants. Such mechanisms of applied amendments might have resulted in reduced values of metal uptake in Brassica plants grown in this study and hence improved phytostabilization efficiency (Figure 7) and reduced health-associated risks (Figure 8). This was further evidenced by the negative correlations observed among Cr concentration in roots and shoots and growth and physiological parameters recorded for Brassica (Figure 6). The BCF and BAC values have further shown that the Brassica has the potential to extract Cr at higher concentrations and hence can be grown in metal-contaminated soils. Moreover, the values for DIMs and health risk index (HRI) were <1, which indicated that the consumption of Brassica was safe with no associated health risks.

The results showed that Cr stress (Cr-25) significantly reduced growth, physiological and biochemical parameters of Brassica under the control treatment without any amendment. The application of compost, BC, and co-composted BC significantly alleviated Cr stress and enhanced various growth, physiological, and biochemical parameters. With the application of co-composted BC, the maximum increase in growth, physiological, and biochemical parameters of Brassica was recorded in comparison to the control treatment without any amendment under Cr stress (Cr-25). Similarly, the application of co-composted BC maximized the phytostabilization efficiency as revealed through the minimum values of BCF, BAF, and BAC of Brassica as compared to the control treatment under Cr stress (Cr-25). Moreover, the same treatment significantly reduced the health risks associated with the consumption of Brassica under Cr stress (Cr-25) as clear from the lowest values of DIM, HI, CR, and THQ. In conclusion, the application of co-composted BC could serve as an effective amendment in the alleviation of Cr stress in Brassica via phytostabilization and reduction in associated health risks.

The original contributions presented in the study are included in the article/supplementary material, further inquiries can be directed to the corresponding authors.

AM, WX, and MN: conceptualization. BT: methodology. BT, AD, and MB: software. AM, MN, and MZN: validation. MB and JK: formal analysis. ZS and QS: resources. BT and QS: data curation. MN: writing—original draft preparation. AM, JK, and MR: writing—review and editing. AM and WX: supervision. MN and AD: project administration. WX and MB: funding acquisition. All authors have read and agreed to the published version of the manuscript.

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.