The current study showed that combined application of microbes (T. harzianum L. and B. subtilis L.), inoculation, and biochar significantly influenced the physico-chemical characteristics of soil, improves maize performance, and reduces Cd bioavailability under Cd contaminated soil. Higher toxicity of Cd leads to significantly minimized root growth, shoot growth, and overall biomass of maize (Table 4S, 5S, Fig. 1, 2). These findings are in line with the previously published literature (Abbas et al. 2018; Zhang et al. 2019; Farooq et al. 2020), reporting that photosynthetic activity, chlorophyll content, essential nutrients uptake, root morphology, and plant biomass were reduced under Cd stress. Reduction in the uptake of the essential nutrients, physiological and morphological traits of the plant is regulated by Cd which can be transferred to the aboveground part of the plant by replacing essential ions i.e., Ca due to having the same chemical behavior, ionic radius, and charges (Haider et al. 2021a,b), which may have direct effect on plant metabolic processes, resulting in growth inhibition, and contributing to yield reduction (Abbas et al. 2017; El-Naggar et al. 2020). The biochar having high adsorption potential helped to ameliorate the toxicity of Cd (Younis et al. 2016), contributing to improve the maize root morphological characteristics in soil, which results to enhance the shoot biomass of maize (Table 4S, 5S). The soil used in current study are calcareous (Table S1), therefore, induces soil alkalinity because of the deposition of biochar’s mineral elements (Zulfiqar et al. 2019). The soil of current maize study was a complete mixture of loam and sand having alkaline pH of 8.64. Incorporation of biochar in soil as a soil improver positively influenced the soil pH (Table S6). Numerous studies have shown that the soil pH is greatly impaired by the higher rate biochar application in soil (Bashir et al. 2018; Haider et al. 2020). The present study showed that soil nutrients were increased by biochar incorporation in Cd-contaminated soil (Table 6S, Fig. 1S). This may be due to appropriate application of synthetic fertilizers and biochar, which helps in improving the soil organic matter and nutrients availability in the Cd contaminated rhizosphere soil. Furthermore, soil physical properties i.e., porosity, aggregation capacity, and water storage capacity were greatly enhanced by addition of biochar in Cd-contaminated soil (Yuan et al. 2019). Improvement of soil properties may improve soil fertility by increasing the availability of nutrients, and reducing the nutrients leaching (Abbas et al. 2018; Haider et al. 2020). Similarly, addition of biochar in Cd-polluted agricultural soils, results to improve the concentrations of essential nutrients P, C, K, N, and Ca in soil which further improved the plant biomass (Zhang et al. 2018; Zhang et al. 2019). Biochar is a renewable porous carbonaceous resource loaded with phosphate, nitrate, and ammonium that enhances the fertility status of agricultural soil (Ding et al. 2016). Biochar can also be utilized as a source of nutrients to improve the fertility status of soil in various agronomic crops i.e., rice (Oryza sativa L.), wheat (Triticum aestivum L.), maize (Zea mays L.), alfalfa (Medicago sativa L.), and soybean (Glycine max L.), due to the inclusion of soluble nutrients and the labile fraction of biochar comprising organically bound nutrients for mineralization (Rehman et al. 2016; Abbasi et al. 2017; Zhang et al. 2019; Haider et al. 2021b).
Photosynthesis activity is regarded as a critical signal for detecting Cd toxicity in plants (Sabae et al. 2006; Gallego et al. 2012). Current study showed that contamination of Cd results into decrease in maize growth, photosynthesis pigments, and gas exchange observations and the reduction of these observations were escalated with a higher Cd-concentration in contaminated soil (Fig. 1, 2). The reduction in photosynthesis activity, pigments, and gas exchange parameters in many horticultural and agronomics crops (Younis et al. 2016; Rizwan et al. 2018), may be due to ultra-structural alternations in Cd-induced plants and a decrease in plant nutrients uptake (Zhang et al. 2019). It was reported that oxidative stress seriously affects various plant metabolic processes and cellular functions and causes nucleic acid disruption (Haider et al. 2021a,b). In current study, toxic Cd concentration enhanced the activities of EL, H2O2, POD, and MDA while decreasing the activities of SOD and CAT in maize leaves as compared plants treated with no Cd-contamination (Fig. 3, 4). The increase in oxidative stress and the decrease in the SOD and CAT synthesis under higher toxic Cd concentration in leaves may be attributed to the reduced anti-oxidative capacity of maize due to Cd toxicity. Furthermore, results indicate that application of biochar as an amendment in Cd-contaminated soil significantly decreased the activities of EL, MDA, H2O2, and POD, while improving the SOD and CAT synthesis activities of maize leaves as compared with control (Fig. 3, 4). The current study concur with the findings of Farooq et al. (2020) reporting that higher contamination of Cd in agricultural soils, significantly regulates the antioxidant and oxidant activities of various crops i.e., castor (R. communis L.), mustard (B. juncea L.), wheat, maize, and rice (Farhangi-Abriz and Torabian, 2017; Haider et al. 2021a,b). Application of biochar in Cd-contaminated soil minimizes the bioavailability of Cd to plants that results to minimize the synthesis of H2O2 and EL as compared to soil contaminated with Cd and having no biochar (Abbas et al. 2018). Similarly in another study, incorporation of biochar significantly minimized the activity of MDA in spinach (S. oleracea L.) leaves cultivated under Cd-contaminated soil (Younis et al. 2016).
Biochar can absorb anthropogenic toxics i.e., trace metals, petroleum hydrocarbons, steroid hormones, and other organic contaminate, from water or soil (Haider et al. 2020). Biochar has a high aromatic structure, and pH, active surface functional groups and porous structure (El-Naggar et al. 2020). These features of biochar play a crucial role in the remediation mechanism of organic and inorganic pollutants by precipitation, complexation, electrostatic interaction, ion exchange, and physical adsorption (Shabaan et al. 2018). Furthermore, the adsorption of biochar is largely dependent on its large specific surface area, microporous structure, active functional group, and pH which can further be modified by variation in pyrolysis temperature, retention time, and feedstock used for biochar preparation (Haider et al. 2020). The SEM graphic of biochar pyrolyzed from feedstock i.e., maize straw showed obviously the observable plant structure residues, showing the derogation of cellulose and lignin results place due to pyrolysis temperature such as 550°C. The SEM graphic of the biochar materials clearly shows in the current research showed that the composition of the biochar is complicated. The numerous pyrolysis processes of the parent feedstock are produced by the variation and heterogeneity in the composition of biochars. The cellulose structure of maize straw biochar can be classified as prismatic, fibrous, or spherical, because there are visible and possibly associated signs of porous and fibrous longitudinal structures of plant cell walls. In addition, the pyrolysis of biochar by cow and poultry manure observes the coarseness in composition showing small pores and sand micro-particles having uneven porosity. The deviation and difference in the feedstock’s absorbent composition may have a substantial effect on the adsorption potential of trace metals remediation in metal contaminated agricultural soils by biochar (Bashir et al. 2018; Yuan et al. 2019).
Remediation of Cd contaminated soils can be accomplished using, biological, physical, and chemical approaches. Physical and chemical remediation methods give a quick remediation time, but are costly and cause secondary pollutants (Zhang et al. 2019). Microbial-remediation depends on rhizosphere competent microbial flora and root exudates secreted by plants. Plant roots secrete exudates including organic acids, alcohols, and sugars that serve as a source of energy in the form of carbohydrates for the microflora of soil and improve microbial activity and growth (Yaghoubian et al. 2019). Certain root exudates may also serve as chemo-tactic signals for microbial flora. Additionally, plant roots help to loosen the soil structure and improve water transport in the rhizosphere, thereby further boosting microbial activities (Chellaiah, 2018). Microorganism survival in the presence of toxic metals in soil depends on structural and biochemical properties, genetic and/or physiological adaptation, modification of trace metals in the environment, and its specification, toxicity, and availability (Sabae et al. 2006; Ahmad et al. 2014). Microorganisms’ inoculation in trace metal contaminated agricultural soil greatly remediates the toxicity of Cd and enhanced the plants growth and photosynthesis activity (Lata et al. 2019).
Inoculation of T. harzianum L. and B. subtilis L. in Cd-contaminated maize soil, has been observed to significantly minimize the toxicity of Cd in the root zone of maize and enhance the root growth of crops like mustard, soybean, alfalfa, wheat, and rice (Hammer et al. 2015). Previous research have shown that inoculation of Bacillus sp., can significantly remediate the contamination of trace metals from urban waste and industrial effluents by accompanying mechanisms i.e., biosorption, biotransformation, biomineralization, and bioaccumulation (Sakthipriya et al. 2015; Ejaz et al. 2021). Similarly, inoculation of Pseudomonas sp., was also effective for appropriate bioremediation of Cu, Cd, Cr, Hg, Ni, Pb, U, and Zn (Hammer et al. 2015). In order to mitigate the toxic effect of Cd in the soil, microbes have adapted a series of pathways such as(1) by metal ions pumping exterior the cell membrane (Ahmad et al. 2014), (2) inside accumulation and cell sequestration of trace metal ions (Lata et al. 2019), (3) conversion/transformation of toxic trace metals into a less toxic form (Yaghoubian et al. 2019), and (4) adsorption/desorption of toxic metals (Zhang et al. 2019). Current experiment observations revealed that integration of microorganism’s inoculation and biochar was considered as more valuable in improving the maize growth under Cd contaminated soil as contrasted to alone application of biochar or microbes. Biochar is porous in nature which serves as a habitat for the optimum survival of microorganisms in the rhizosphere (Zhang et al. 2018). The coupling of both microorganisms and biochar in contaminated soil may provide habitat for microorganisms in soil that significantly affects the metabolic process in plant directly or indirectly (Han et al. 2016), and further leads to enhance the microbe population with respect to microbial density and abundance (Haider et al. 2021b). In addition, biochar can increase the soil’s cation exchange ability and conserve nutrients for microbial growth by the absorption of nutrient cations from biochar functional groups (Hussain et al. 2021). Moreover, the porous structure of biochar help in the adsorption of trace metal ions in it those are further degraded into non-toxic form by the activity of metal remediating-microorganisms in soil (Rizwan et al. 2016). Integrated addition of biochar with microorganisms i.e., T. harzianum L. and B. subtilis L. enhanced the remediation potential in soil against contaminants like Cd because of positive synergistic response of biochar on microorganisms growth (Zhou et al. 2018; Zhang et al. 2019). In the present study, the improvement with the intermixed treatment of biochar and microorganisms might be due to the same mechanisms. An adequate application of biochar can improve the soil properties i.e., water retention, cation exchange capacity, aeration conditions, and pH which enhance the growth of microorganisms in the rhizosphere contaminated with trace metals, which is a major component of bioremediation in trace metal contaminated agricultural soils (Yuan et al. 2019).