Several studies described that biochar addition improves the physical attributes of soil via reducing bulk density, increasing porosity, and enhancing water retention and aggregation (Baiamonte et al., 2015; Ding et al., 2016b; Carvalho et al., 2020; Seitz et al., 2020). Different kinds of biochars, when added in a sufficient amount into different soils, substantially amend the various soil physical attributes (Table 1). Soil bulk density (BD) is an important indicator of soil physical condition because it indicates the arrangement and packing of soil particles (Singh et al., 2019; Bhat et al., 2022). Low soil BD enhances soil composition, decreases soil compaction, and improves WHC and nutrient release (Tang et al., 2022). Biochar application decreases both the bulk and particle densities of soils (Munoz et al., 2016). This decrease could be attributed to the lower BD (.6 g cm⁻³) and particle density (1.5–2.0 g cm⁻³) of biochars as compared to the 1.25 g cm⁻³ BD and 2.4–2.8 g cm⁻³ particle density of soils (Yu et al., 2019). Biochar BD varies depending upon the type of feedstock and preparation conditions. For instance, BD of wood biochar is 1.30 g cm⁻³, woody forest residue biochar is .09 g cm⁻³, straw biochar is 1.30 g cm⁻³, maize cob biochar is .29–.36 g cm⁻³ and rice husk biochar is .37 g cm⁻³ (Sun and Lu, 2014; Obia et al., 2016; Zhang et al., 2021).

The reduction in soil BD after biochar addition results in an overall increase in soil porosity (Qin et al., 2016). The increase in soil porosity is attributed to the biochar porosity (70%–90%) and further contributed by increased soil aggregation, reduced bulk density, reduction of soil packing and interaction with mineral soil particles (Blanco-Canqui, 2017). Biochar application elevates the total porosity especially the micropores because they help to form porous material (Yang C. et al., 2021). The micropores enhance the water retention, whereas macropores improve the drainage (Baiamonte et al., 2019). The soil porosity was observed to increase in the range of 5–25 µm after biochar amendment (Rasa et al., 2018). Increase in soil porosity can enhance the movement of gases, water, and heat in the soils (Yu et al., 2019). The type of feedstock used for biochar production significantly influences the soil porosity pattern (Yang Q. et al., 2021). Woody biochars contain higher porosity compared to the biochars prepared from crop residues, which causes difference in the biomass cell structure, composition, size, and shape (Edeh et al., 2020). Soil WHC is also affected by the porous structure of the biochar (Bhat et al., 2022). The high porosity and specific surface area (SSA) increase the WHC, reduce the soil water permeability resistance, and change the water flow direction and residence time in soil (Abrol et al., 2016). The improvement in soil WHC after the addition of biochar is attributed to the rise in pore space of soil and biochar mixture. This mixing resulted in increased number of water holding sites due to large number of pores which increased the WHC of soil amended with biochar (Yang C. et al., 2021).

Biochar application may change the soil’s chemical attributes, such as increasing soil organic carbon, CEC, and pH (Abbasi and Anwar, 2015). However, alteration in soil chemical attributes by biochar application is mainly dependent upon biomass, biochar production temperature, types of soils, and biochar application rates (Table 2). It is considered that alkaline biochars (having pH > 7), when added to soil, elevates the pH of soil (Roberts et al., 2015). The biochar pH is largely dependent upon the type of feedstock used, and it may vary from acidic to alkaline. The biochar produced from different agricultural residues, raised the pH of soil from 4.59 to 4.86, 4.8 to 6.3, and 4.3 to 4.6 (Abhishek et al., 2022). Levesque et al. (2021) observed enhanced soil pH by 1 unit after application of Acer saccharum-biochar into clay soil. It was attributed due to the biochar’s high ash content and pH, great liming factors contributing to enhance pH of soil. He et al. (2021) applied rice straw biochar to notice the change in the attributes of acidic paddy soil. They observed that pH buffering capacity and resistance to paddy soil acidification were efficiently improved with biochar application. Biochar application after the wet-dry cycle increased the pH of acidic paddy soil (Hafeez et al., 2021). They proposed that biochar is a dominant solution to remediate acidic soil. The basic mechanism is that the weak acid functional groups on the surface of biochar mainly appear in the form of organic anions under neutral and alkaline soils. However, under soil acidification, these organic anions protonated with H⁺ and converted into neutral molecules, thus inhibiting the soil acidification and decreasing pH of soil (Wu et al., 2020).

Mostly studies focus on the effect of biochar application in acidic soils due to its potential of increasing pH (Novak et al., 2014; Edenborn et al., 2015), however discussion regarding the effect of biochar on calcareous soils is very limited (Zhang D. et al., 2015). This could be attributed to the buffering capacity of the calcareous soils resisting the alkaline effects of biochar (Usman et al., 2016; Al-Wabel et al., 2017).

CEC is an indirect measure of soil capacity to retain nutrients and water (Alkharabsheh et al., 2021). The biochar CEC is determined as the number of cations adsorbed on the surface of biochar (Zhang et al., 2021). Feedstock material, pyrolysis temperature, and functional groups are the key factors that govern the biochar CEC (Murtaza et al., 2021b). The CEC of biochars reduces with the elevation of pyrolysis temperature owing to the loss of functional negatively charged groups and low pyrolysis temperature (300°C–450°C) (Palansooriya et al., 2019). Sufficient availability of O₂-containing functional groups on the surface of biochar leads to anionic surface charge, which increases the CEC of soil after biochar application (Lu et al., 2022). Biochar application significantly increased the CEC by 3%–40% compared to control soil (Laird et al., 2010). Similarly, the CEC of extensively weathered soil was enhanced from 7 to 11 c mol kg⁻¹ after applying biochar from the tamarind plant (Jien and Wang, 2013; Wang et al., 2022). Fang et al. (2016) reported that the CEC of corn straw biochar produced at 450°C was 26.36 c mol kg⁻¹ and decreased to 10.28 c mol kg⁻¹ at 700°C. Munera-Echeverri et al. (2018) revealed that the CEC was low until the temperature surpassed 420°C due to the nutrient content in the feedstock changed with temperature. Abhishek et al. (2022) presented that the high CEC facilitated the heavy metals removal from the polluted soil.

Soil organic matter (SOM) is a crucial factor affecting the soil health (Battaglia et al., 2021). The biochar amount and stability used as a soil improvement play a key role in enhancing the SOM (Alkharabsheh et al., 2021). Adding biochars derived from silver grass, umbrella tree, rice straw, and crop residues caused an increase of SOM content by 42%–72% in sandy soils and 32%–48% in loam soils (El-Naggar et al., 2018). In another investigation (2 years) Adekiya et al. (2020) found that application of hardwood biochar at the rate of 30 Mg t ha⁻¹ increased the SOM content by 77, 18, and 9% compared to control (un-amended soil), 10 and 20 Mg t ha⁻¹ biochar addition. Application of straw biochar and rapeseed stalk biochar also improved the SOM content in a red soil (Yang and Lu, 2020).

Amending soils with biochar also improves the soil organic carbon (SOC) content (Alkharabsheh et al., 2021). However, such increases in SOC are largely dependent on type of feedstock, pyrolysis temperature and soil types (Al-Wabel et al., 2017). Generally, biochars prepared at low pyrolysis temperatures contain a higher labile C compared to biochars fabricated at high pyrolysis temperature (Lévesque et al., 2018). Yin et al. (2014) in an incubation study observed a higher accumulation of SOC with rice-straw biochar prepared at 250°C than biochar generated at 350°C. This could happen due to the contribution of partially pyrolyzed portion of the biomass in biochar (produced at low temperature) to SOC. Furthermore, high temperature treated biochars carry large content of fixed carbon (aromatic C-C bonds) making biochar more stable, whereas low temperature biochars contain more labile substrates (C-H bonds) (Cardinael et al., 2017). Biochar enhances the SOC either by decomposition via soil microbes or through preservation of existing natural SOC (Al-webal et al., 2017). Overall, the effect of biochar addition on soil chemical properties is solely depend upon the type of feedstock, pyrolysis temperature, soil types and biochar application rates. Therefore, to get maximum benefit from biochar amendment, special attention should be paid to the biochar production conditions.

Biochar applications not only amend the soil physiochemical attributes but also alter soil biological attributes (Table 3) (Manirakiza et al., 2021). Biochar application affects the activity and community structure of soil microbes due to its large SSA, pore space, functional groups, surface volatile organic compounds, minerals and porosity (Gao and Deluca, 2016; Zhu et al., 2017). These alterations could modify the soil structure, reduce nutrient leaching, increase the nutrient cycles, form labile carbon compounds for microbial growth, increase aggregation, enhance nutrient immobilization and retention, and accelerate the plant growth mechanism (Murtaza et al., 2021a; Xu et al., 2021). Biochar particles and pores provide a habitat for the intrusion of filamentous microbes and fungi (Yin D. et al., 2021). Biochars, rich in sugars and yeasts, promote the growth of bacteria (Gram-negative) and fungi (Kocsis et al., 2022). Biochar alkalinity may stimulate the growth of Gram-negative and Gram-positive bacteria (Zimmermann et al., 2012; Osman et al., 2022). Sun et al. (2012) observed the bacterial populations were more dynamic and active in biochar-treated soil than fungal populations under field conditions; this may be attributed to the biochar’s higher surface area and biochar carbon (Liu et al., 2022). Moreover, pyrolysis temperature, biomass type, and carbon content strongly influence dehydrogenase activity, microbial habitats, N immobilization and mineralization (Ameloot et al., 2015).

The high SSA and porous nature of biochar serve as favourable habitats for diverse soil microbes (Ye et al., 2017). The porosity and SSA of biochar largely depend on feedstock type and pyrolysis temperature, high temperatures result in higher porosity and SSA (Palansooriya et al., 2019). The microbes become attached to the biochar micropores by electrostatic forces, hydrophobic attraction, or the precipitate formation, thus resist leaching, and increase their abundance (Abhishek et al., 2022) Moreover, soluble substances such as water, alcohol, sugars, acids and ketone molecules present in micropores and mesopores of biochar alter the microbial composition and abundance in soils (Adnan et al., 2020). An increment in the microbial activity and population of filamentous fungi, Bacillus species and Pseudomonas species have been observed with biochar addition in pepper cultivated soils. An application of maize stalk biochar at the rate of 50, 100, and 200 Mg ha⁻¹ resulted in a 6.6%–31.2% higher fungal abundance compared to un-amended soil (Yao et al., 2017). In another study, Karimi et al. (2020) reported 20%–124% increase in soil microbial biomass in a calcareous soil supplemented with corn residue biochar at 1% and 2% (w/w) compared to control. Domene et al. (2014) found that the microbial abundance could enhance from 366 µgCg⁻¹ (control soil) to 730 µgCg⁻¹ (biochar treated soil). Also, microbial abundance was increased by (5%–50%) with the increase in biochar application rate (from 0%–14%) for various incubation times. Jin et al. (2016) found that litter-derived biochar enhanced the activity of phosphomonoesterase and decreased the activity level of acid phosphomonoesterase in silt loam and clay loam soils, respectively. An elevation in N and P retention in soil was noted due to change in microbial community structure and increased microbial activity in response of biochar application (Palansooriya et al., 2020). Biochar also found to play a crucial role in biological N fixation in legume crops by regulating different mechanisms including, increasing nodule formation, immobilizing N, enhancing P supply and altering the soil pH (Mia et al., 2014; Semida et al., 2015; Partey et al., 2016; Semida et al., 2019). Most probably, changes in resources (C and nutrients), physico-chemical factors, water availability or access to habitat may accelerate the competition among soil microbial communities which causes an alteration in community structure and composition (Semida et al., 2019).

Despite having the positive role of soil microbes in the soil, various soil pathogens can negatively impact the crop growth in the form of diseases (Bass et al., 2019). Biochar showed substantial potential to rectify the problems created by the soil pathogens. Jaiswal et al. (2018) reported that biochar can deactivate and immobilize the enzymes involved in cell wall deterioration and detoxified the metabolites produced by Fusarium oxysporum f. sp. radicis lycopersici and, protected the crop plants against soil pathogens. In another investigation, Gao et al. (2019) reported that the soil of tomato plants infected with Ralstonia solanacearum bacteria was improved with wheat straw biochar and severity of bacterial wilt was reduced with increase in total C, N, C:N ratio, K, P, pH and electrical conductivity. Biochars can potentially inhibit pathogenicity in plants by improving resistance, enhancing nutrient content, and detoxifying and adsorbing harmful chemicals in the polluted soils (Schmidt et al., 2021; Tan et al., 2022).

Biochar’s effects on enzymatic activity in soils depend on the nature of the substrate-enzyme interfaces in the presence of biochar, which are linked with biochar surface area and porosity (Bailey et al., 2011). Biochar with higher porosity and surface area would most likely decrease the extracellular enzymatic activity, given that the functional groups on the biochar would tend to bind the enzymes and substrates and therefore interfere with substrate diffusion on the active sites of the enzyme (Osman et al., 2022). Biochar treatment has both positive and negative effects on soil enzymatic activity. These impacts depend on biochar application rate and soil type (Table 3). Soil enzymes indicate the soil quality because they are directly related to soil microbial activity and biogeochemical cycling of nutrients (Palansooriya et al., 2019). For instance, increase in dehydrogenase activity was observed in different soils amended with different types of biochars (Bhaduri et al., 2016; Irfan et al., 2019). Such increase in dehydrogenase activity could be attributed to the labile organic matter and a high content of volatile matter of biochars (Gasco et al., 2016b). An increase in the activity of extracellular enzymes (β-glucosidase, a-glucosidase, β-xylosidase, and β-d-cellobiosidase) involved in soil sulfur (S) and C cycling was noted in a fluvo-aquic soil amended with biochar (Wang et al., 2015). In another study, elevation in dehydrogenase enzyme activity was observed in a red soil treated with .5% (w/w) bamboo and oak-wood biochar (Demisie et al., 2014). In the same study, β-glucosidase activity was increased in soil supplemented with only bamboo biochar at .5% and 1%. However, increase in urease activity was noted with oak wood biochar (.5% and 2%) and bamboo biochar (.5%) (Demisie et al., 2014; Palansooriya et al., 2020). Several studies (Ameloot et al., 2014; Paz-Ferreiro and Fu, 2016; Singh, 2016) also revealed that biochar manifest differential effects on enzyme activities across different types of soils. A manure-derived biochar decreased the activity of acid-phosphomonoesterase in clay-loam soil whereas activity level of alkaline-phosphomonoesterase was increased in silt-loam soils (Jin et al., 2016). Recently, a global meta-analysis (Pokharel et al., 2020) on biochar application and soil enzyme activities demonstrated an increase in the activities of extracellular enzymes including, phosphatase (25%), urease (23%) and dehydrogenase (20%). On the other hand a decrease of −13%, −7%, and −6% in phenol oxidase, β-glucosidase, and acid phosphatase, respectively was also observed. It is obvious that soils blended with biochar showed augmented proliferation of soil microbes benefitting the soil in several ways. However, more studies are needed to explore the long term effects of biochars on soil microbes and unveil the ecological roles of biochars.

The effects of biochar application on seed germination (Table 5) consisted of inhibition to activation. A high application rate of biochar can have destructive impacts, while a low application rate of biochar can be stimulatory. This section will explain the mechanisms affecting the germination and seedling growth described in the literature.

Seed germination starts with water absorption and ends when the radicle appears from the seed coat. The four main aspects regulating the effect of biochar treatment on seed germination are 1) release of phytotoxins, 2) salts released from biochar in soil solution, 3) alteration in porosity and WHC of soil, and 4) release of karrikins (germination-regulator hormone) (Joseph et al., 2021). Biochar type, pyrolysis condition, and dose have diverse effects on germination rate and speed. The specific sensitivity of different plant seeds to toxins, water availability, hormones, and salinity may cause variable results (Hasannuzzaman and Fujita, 2022). For instance, in a pot experiment, wood-derived biochar at 80 Mg t ha⁻¹ suppressed the tomatoes’ germination. At the same time, sewage sludge, wheat husk, and paper residue-derived biochar added with the same dose had no impact on lettuce, cucumber, cress, tomatoes, and lentil seed germination (Gasco et al., 2016a). Other investigations that used a range of manure and woody biochars at a rate of 10 Mg t ha⁻¹–40 Mg t ha⁻¹ observed nil or positive impacts on germination (Van Zwieten et al., 2010; Khan et al., 2014; Mete et al., 2015; Gasco et al., 2016b; Das et al., 2020).

Uslu et al. (2020) reported the negative effects (inhibition of seed germination) in different fodder crops at high biochar application rate (120 Mg t ha⁻¹) in a laboratory experiment. Aqueous extracts of various biochars have accelerated seed germination and seedling growth (Zheng et al., 2017). Seed growth and development of early seedlings can be affected due to biochar impacts on soil physical attributes. For example, by enhancing soil aeration and decreasing soil bulk density, biochar can provide oxygen for germination and to improve seedling development through seeding emergence (Obia et al., 2018). Biochar’s chemical impacts on water and soil solution can affect the seed and early seedling growth. For instance, by increasing pH, the alkaline biochars enhance the heavy metals and Al toxicity that can decrease root development in acidic-soils (Shetty et al., 2020). A high dose of biochar with high concentration of soluble salts could suppress seed growth and development by osmotic stress (Sun et al., 2017; Shetty et al., 2020). Kochanek et al. (2016) reported that the biochars comprising organic molecule karrikins, can promote seed growth and seedling development.

French and Iyer-Pascuzzi, (2018) demonstrated that the gibberellin pathway accelerates germination and seedling development through wood-derived biochar in tomato genotypes. Polyphenols and phenols released from biochar can efficiently break the seed dormancy, improve germination, and accelerate the seedling development mechanisms (Reynolds et al., 2018). Biochar comprises organic contaminants, PAHs, and heavy metals generated during the partial combustion process that can suppress germination and seedling development due to a high application rate (Gasco et al., 2016a; Gasco et al., 2016b; Das et al., 2020). Nevertheless, lower application rates, and low concentration of free radicals could be favourable as reactive oxygen can interact with various hormones of plants that stimulate the germination process (Gomes and Garacia, 2013).

It can be summarized from above discussion that mostly biochars and biochar based formulations are helpful in promoting seed germination and growth of seedlings when applied at moderate rates. However, application of biochars at relatively higher rates e.g., more than 40–50 Mg ha⁻¹ could restrict seed germination and early growth due to the release of phytotoxic organic compounds and soluble salts. Furthermore, the mechanisms responsible for positive effects of biochar include water-soluble organic compounds that accelerate the seed germination and growth, chemical reactions that dismiss the inhibitory effects of phytotoxic compounds and heavy metals. The aforementioned effects largely depend upon the temperature, biochars prepared at low temperatures contain higher amounts of organic molecules and promote seed germination and seedling growth at low application rates. Therefore, the abovementioned factors should be considered prior to the application of biochar in the crop fields to get optimum benefits.

Biochar application either decreases or increases plant growth (Deenik et al., 2010). According to previous literature, the changes in plant growth (increase or decrease) depend on soil, biochar type, and biochar preparation temperatures (Figure 2). For example, Kwapinski et al. (2010) reported inhibition of corn growth through soil treatment with biochar derived from silver grass prepared at 400 °C. However, growth was promoted by biochar obtained at 600°C. Moreover, soil treatment with biochar at an application rate of 68 t ha⁻¹ significantly enhanced the growth of cowpea, wheat, and rice (Vaccari et al., 2011; Zheng et al., 2017). In comparison, applying 10% animal manure-derived biochar reduced the sunflower plant height, number of leaves, achenes, and stem diameter (Furtado et al., 2016). Corn-derived biochar pyrolyzed at 400°C added at 20 t ha⁻¹ did not considerably increase the growth of Glycine maxon loam soil (Hafeez et al., 2022). Changes in soil properties have often resulted with biochar addition, leading to increased plant growth (Solaiman et al., 2010). Solaiman et al. (2012) reported that the application rates and biochar type considerably impacted the growth of clover, mug beans, and wheat in the laboratory experiment. In a glasshouse experiment, the growth of radish, soybean, and wheat was enhanced with papermill derived biochar at the application rate of 10 t ha⁻¹ (Van Zwieten et al., 2010). Furthermore, biochar addition to hostile sandy-soil increased the growth of corn by increasing photosynthesis rate, plant-soil water relations, decreased bulk density, and enhanced moisture retention (Haider et al., 2015). Thus, biochar addition may control the poor germination and plant growth induced by poor soil attributes (Furtado et al., 2016).

Artiola et al. (2012) reported that pine-derived biochar applied at 2% in a pot study with a sandy-loam and alkaline soil had poorly affected lettuce growth in 2% biochar-treated soil. Moreover, rice plant growth, dry mass weight, and tiller number significantly increased in various varieties grown in treated soil with rice straw biochar at the rate of 15 kg ha⁻¹. While the wheat growth was positively affected by 5% biochar application (Akhtar S. S. et al., 2015). Plant dry and fresh weights of pepper and tomato except romaine were increased by application of poultry manure-based biochar-to loam soil at the rate of 400 kg ha⁻¹ (Vaccari et al., 2015). Burke et al. (2012) reported that the growth of cotton was promoted by hardwood-derived biochar when applied at the rate of 5–10 t ha⁻¹. The growth and yield of Chenopodium quinoa and lettuce were enhanced by 300% in a hostile loam-sandy soil treated with 2% biochar (Trupiano et al., 2017).

Luigi et al. (2022) showed that biochar seems to promote the development of the tomato seedlings, especially at concentrations ranging from 1% to 20% (w/w with peat) without showing any antimicrobial effects on the beneficial soil bacteria at the tomato rhizosphere level and even improving their growth, because the application of biochar enhanced the soil pH as well as the retention of both the soil water and nutrients (Yang et al., 2022). Biochar ameliorated substrate characteristics (available N increase of 17% and total C increase 13%), resulting in a promotion effect on plant root, shoot, and leaf morphology, the biochar-treated plants had a greater number of leaves (38 and 68 at the vegetative and fruit stages, respectively) than the untreated plants (32 and 49, respectively). The biochar also increased leaf area with a rise of 26% and 36% compared with the values measured in the untreated plants. Moreover, the amendment increased twofold root length, root surface area, and root, stem, and leaf biomasses in comparison with untreated plants (Simiele et al., 2022), could have a promoting effect on plant growth as an indirect consequence of its positive effect on growth medium parameters such as water holding capacity and pH enhancement, increased nutrient availability (Malik et al., 2022). Xi et al. (2020) reported that the 2% (w/w) rice biochar application increased soil available N and K, resulting in taller lettuce plants, with longer roots, stronger leaves and stems, as well as greater leaf area. The growth improvement may be related to the impact of biochar on the physicochemical characteristics of the soils. Similarly, Huang et al. (2019) proposed that rice straw biochar contributed to the increase of total soil N content, making it more available to Phragmites communis and promoting its growth. But rice biochar can also stimulate C and N cycling by changing the microbial community. For example, increased rape shoot biomass (from 2.31 to 4.23 g) after rice biochar application was the result of an improvement of soil chemical conditions (soil pH and cation exchange capacity) and nutrient availability (total C and N), together with changes in the associated microbiota (Gomez et al., 2022).

Biochar-extracted liquor [1%–5% (w/w) in water] also promoted plant height and root growth in rice seedlings. The mechanism of action proposed was based on the overexpression of the ABP1 gene and the accumulation of its protein product. Accordingly, molecular modeling showed a molecule on the biochar surface that was able to interact with the ABP1 protein (Gelova et al., 2021). Liu Z. et al. (2021) found that application of 1% rice straw biochar, enhanced the N use efficiency of rice plants and resulted in increased shoot and root biomass (26%–29%), which were attributed to the enhancement of soil microbial biomass after biochar treatment. The application of wheat straw biochar (5–40 t ha⁻¹) promoted the growth of Lens culinaris L. by increasing the organic C content and improving other physicochemical properties of the soil (Khorram et al., 2018). Taken together, biochar application positively augmented the growth and development of plants by increasing the availability of nutrients, water, SOC and improving the soil biological activities.

Various physiological aspects do or do not respond to biochar addition (Table 6) due to factors such as biochar and soil type (Asai et al., 2009; Singh et al., 2020). For example, soil treatment with biochar decreased the content of leaf chlorophyll in rice plants grown in poor-quality soil (Asai et al., 2009). Younis et al. (2015) conducted a pot experiment. They observed that transpiration (42%) and photosynthetic rates (45%), protein content (20%), anthocyanin (60%), lycopene (30%), carotenoids (29%), chlorophyll (40%), and stomatal CO₂ level (22%) were improved. The concentration of amino acids and sugars were decreased with the increasing application rate of cotton biochar from 3%–5%. Increased P uptake, availability, and corn growth after biochar application were also observed (Mau and Utami, 2014). Compared with the control, an increase in chlorophyll contents, rate of photosynthesis, and stomatal conductance of the jute plant were found after biochar was mixed into the soil at the rate of 3 kg m⁻² (Seehausen et al., 2017).

Haider et al. (2015) described that adding biochar in hostile sandy soils increased plant growth via increasing photosynthesis rate and plant-soil water relation under drought and well-watered conditions. In corn and wheat grown on loamy soil, biochar positively influenced physiological parameters. Where biochar application at the rate of 5% positively impacted xylem Na⁺ and K⁺, stomatal conductance, and photosynthesis rate more than the control. However, biochar application did not affect the photochemical ability of the photosystem-II (Akhtar S. S. et al., 2015; Seehausen et al., 2017). Various physiological parameters such as leaf nutrient level, leaf gas exchange, water status, and nutrient recovery of apple plants were positively affected via biochar addition (Eyles et al., 2015). Stomatal conductance, photosynthetic ability, vapor pressure and transpiration rate, and N and P leaf concentration significantly increased compared to control under biochar treatment (Eyles et al., 2015; Hafeez et al., 2017).

A significant rise in P, Mg, K, and N contents in tomato plants was found after biochar addition at the rate of 14 t ha⁻¹ (Vaccari et al., 2015). Moreover, water use efficacy, transpiration and assimilation rate, and leaf water potential were positively influenced in lettuce grown in biochar treated soil (Trupiano et al., 2017). Akhtar S. S. et al. (2015) observed that biochar increased soil sorbing Na⁺ content and enhanced xylem K⁺ content, and decreasing N uptake, thereby elevating the potato yield (Akhtar S. S. et al., 2015). The biochar application significantly enhanced biomass and photosynthetic pigments development in plants. The treatments also increased membrane stability index by 45.12% and enhanced water using efficiency by 218.22%, respectively. The increase in antioxidant activities was 76.03%, 29.02%, and 123.27% in superoxide dismutase, peroxidase, and catalase, respectively (Tanveer et al., 2022). It was due to the application of biochar decreases the Pb and As toxicity and enhanced the production of the photosynthetic pigment in Salix viminalis L. They enhanced the production of chlorophyll, biomass and, gas exchange attributes in plants (Visconti et al., 2020) and antioxidants was increased by biochar because of the biochar reduces oxidative stress by the synthesis of ascorbate peroxidase, glutathione reductase, superoxide dismutase, and catalase (Kaya et al., 2020). EL Naggar et al. (2021) used of rice straw biochar (15 t ha⁻¹) and found the enhancement in photosynthetic pigments.

These results were associated with the maintenance of the integrity of cell membranes and the reduction of the oxidative damage of leaf tissues by enhancing catalase (CAT), peroxidase (POX), superoxide dismutase (SOD), and glutathione reductase (GR) activities (Gomez et al., 2022). Biochar significantly increased net photosynthetic rate, transpiration rate, stomatal conductance, and water use efficiency during the plant growth period, relative to control and shown that biochar has great potential in improving chlorophyll fluorescence (Wang S. J. et al., 2021). That’s probably because biochar has the effect of increasing the chlorophyll content of leaves (Feng et al., 2021), which can ensure the synthesis of various enzymes and electron transporters in the process of carbon assimilation, thereby improving the function of leaf photosynthesis (Hou et al., 2021).

Biochar application effects on crop productivity are variable due to feedstock composition, pyrolysis conditions, soil properties, and crop and experimental conditions. Various studies show that biochar addition has beneficial impacts on the productivity of different crops (Table 7). For instance, the yield of corn was enhanced by 40% after the addition of salwood-derived biochar (Yamato et al., 2006), 114% by corncob and wood biochar (Cornelissen et al., 2013), and 98% by the treatment of biochar-derived from manure (Uzoma et al., 2011). Compared to controls, the lantana and pine needles-derived biochars enhanced the grain yield of Triticum aestivum by 6%–24%. This is attributed to more efficient enzymatic activities and P and N uptake via grains (Bhattacharjya et al., 2015). Biochar addition at the rate of 40 t ha⁻¹ in sandy loam soil increased rapeseed yield by 36% and potato yield by 53% (Liu S. et al., 2020).

Biochar application also enhances the crop productivity grown in alkaline soil, depending on the pH of applied biochar (Zhang H. et al., 2015). For instance, biochar application to alkaline soil (8.38 pH) at the rate of 20 t ha⁻¹ and 40 t ha⁻¹ enhanced the yield of maize by 18% (Vaccari et al., 2015). Major et al. (2010) presented that the maize yield was significantly increased after wood-derived biochar application (at the rate of 20 t ha⁻¹) to alkaline soil (9.2 pH). The positive impacts of biochar application on alkaline soil depend on the type of biochar. Biochars produced at slow pyrolysis have low pH due to a large amount of aliphatic and volatile compounds (Spokas and Reicosky, 2009). Also, biochar addition has a negative or no impact on crop productivity in alkaline soils. For example, Hansen et al. (2016) conducted a pot study for biochar application at the rate of 1% in sandy loam soil (pH 9.8). Biochar addition exhibited no impact on the growth and yield of the barley crop. In a field experiment, adding wood biochar prepared at fast pyrolysis showed no effects on the corn yield in alkaline nature soil under water stress conditions (Foster et al., 2016). Marks et al. (2014) described that the poplar and pine wood-derived biochar by fast pyrolysis and gasification significantly inhibited the yield of ryegrass and lettuce at the 19 t ha⁻¹ addition rate in calcareous soil (pH 8.9). Generally, biochar effects on crop yield are more prominent in acidic soils, well-weathered and low fertile soils dominated by sesquioxides and kaolinite (Sohi et al., 2010; Xu et al., 2013).

In acidic soils, the positive effects of biochar addition on crop productivity are attributed to increasing soil CEC because of biochar’s large surface area and porous structure. It amends the physical attributes of soil by increasing the soil WHC and decreasing the soil bulk density. Additionally, it enhances nutrient use efficiency and supply of essential nutrients, controls nutrient loss, accelerates microbial activity and their functions, stabilizes phototoxic elements in soil, and reduces the impact of biochar, such as increasing pH (Singh H. et al., 2022). Biochar addition increased the nodulation, biological nitrogen fixation, and yield of various legume species, including soybean, alfalfa, and red clover (Lai et al., 2022). Mete et al. (2015) reported an increase in biological N fixation of bean with the biochar application at the rate of 78 and 100 t ha⁻¹ Mia et al. (2014) observed that biochar addition increased the total biomass, nodule number, and biological N fixation. Possible processes driving the impacts of biochar addition on biological N fixation in legumes such as 1) higher pH of soil improve the Mo availability, an essential nutrient needed in biological N fixation mechanism, 2) Higher concentration of available N immobilization which is associated to biological N fixation enhancement, 3) Strong impact of biochar on nodule production via efficient use of Nod and flavonoids feature, 4) Biochar addition accelerates the concentration of available P to phosphorus-deficient soils (Chen K. et al., 2022; Chen X. et al., 2022).

Various studies have reported that biochar addition had negative or no effects on the productivity of crops. For example, biochar application at the rate of 15 g kg⁻¹ to silty and sandy soils did not enhance the corn yield (Borchard et al., 2014). Nelissen et al. (2015) found no effect of biochar application on the barley crop yield in sandy soil. Lai et al. (2013) observed that the Walnut hull-derived biochar produced at fast pyrolysis did not affect the yield of lettuce, Swiss chard rice, and bell pepper, despite the available K and pH of the soil being considerably higher. Kloss et al. (2014) conducted a greenhouse experiment and observed a decrease in mustard and barely yield after adding the vineyard pruning, wheat straw, and woodchips-derived biochars in chernozem, cambisol, and planosol soils but the yield of red clover remained unaffected. Furthermore, maize-derived biochar application at various rates to fertile soils did not impact the corn yield under field and pot experiments (Guerena et al., 2013).

Simiele et al. (2022) reported that the biochar-treated plants showed a higher number of flowers and fruits, although the mean fruit biomass and morphology remained unchanged. Additionally, higher values of Trans-and cis-lycopene, total soluble solids, and titratable acidity were found in the biochar-treated plants when compared with the untreated ones (Simiele et al., 2022). The promotion of fruit quantity and quality could be attributable to the high total P content in the biochar-treated substrate and the high total N concentration in roots of the biochar-treated plants observed at high level. Indeed, according to other reports, there might be a relationship between the phosphorous and nitrogen contents in both growing substrates and plant tissues and the promotion of fruit production by improving the vegetative and reproductive properties of tomato plants (Hameeda et al., 2019), also attributed to the increased values of lycopene, titratable acidity, and total soluble solids when biochar was used as a soil amendment (Guo et al., 2021). The continuous application of 20 t ha⁻¹ of rice biochar to a rice field resulted in plant growth promotion and an increase of 14%–26% in soil N uptake, 7%–11% in internal N use efficiency, and a 6% in grain yield (Gomez et al., 2022). Bai et al. (2019) showed an enhanced yield (up to 35%) in different rice-wheat rotated soils, probably due to the release of plant macronutrients and micronutrients contained in the rice biochar. Nan et al. (2020) reported a clear improvement in soil bacterial cooperative relationships after treatment with rice biochar in a 4-year field trial. The complexity of the rhizosphere bacterial community was enhanced, most probably due to an increase in total soil C content, alongside with an increased total N content and soil available K and magnesium (Mg), which increased rice yield up to 14.5%. Yin D. et al. (2021) observed an increased rice yield (38%–41%) after the application of N-enriched rice straw and waste wood biochar, due to increased levels of soil C and N contents, as well as iron (Fe) availability. In addition, the application of biochar from wheat straw (20 t ha⁻¹) in rice fields increased yield by 17%, as a consequence of a higher N and P supply, together with an improvement of more than 10% in the N use efficiency (Liu et al., 2021). Overall, improvement in soil physical, chemical and biological properties due to the biochar application caused an increase in crop yield or productivity. However, further studies should focus on optimization of biochar preparation conditions based on soil type, crop species and experimental settings. This could facilitate accuracy of biochar in terms of biochar type, preparation conditions and methods, application time, application rate, and recovery processes and it may help in promoting the application of biochar across diverse environmental conditions at large scale.

In plants, absorption of metals occur through the root cortical cells via competitive absorption of essential elements and adopted by symplastic and apoplastic pathways (Tangahu et al., 2011; Haider et al., 2021). Generally, mass flow is responsible for the transport of contaminants to the surface of the roots (Tran and Popova, 2013). These metals may be absorbed via apical portion of the root or the entire surface depending on the nature of the metal. Furthermore, metal uptake also relies on root development and capacity (Begum et al., 2019; Haider et al., 2021). Many investigations have revealed that biochar application is greatly helpful in reducing the absorption of soil contaminants (trace metals) in plants (Palansooriya et al., 2020; Joseph et al., 2021; Murtaza et al., 2022b; Haider et al., 2022). Chen et al. (2018) observed the incorporation of biochar into soils, resulting in an average reduction in plant tissue concentrations of Zn, Cu, Pb, and Cd by 17%, 25%, 39%, and 38%, respectively. Various studies indicated a substantial reduction in heavy metals bioavailability after using biochar at higher rates, such as 10 Mg ha⁻¹ (Wang L. et al., 2020). Biochar surface with oxygenated-functional groups can stimulate the immobilization of heavy metals through different mechanisms (physisorption, electron shuttling, reduction, anion attraction, cation attraction, precipitation, and ion exchange) (Xu et al., 2019). Liming effects of biochar increase the pH of acidic soil, enhancing negatively charged exchange sites on the clay particles and raising cationic metals (Joseph et al., 2021).

Lei et al. (2019) presented that the biochars derived from manure contain higher Ca content than plant-based biochars and, therefore, can immobilize the Cu²⁺ and Cd²⁺ via ion exchange. Stable residues generated in the biochars with higher P can immobilize the lead (Pb) by the β-Pb₉(PO₄)₆ formation. In contrast, higher calcite and alkalinity in biochar promote the insoluble Pb₃(CO₃)₂(OH) ₂ formation (Li et al., 2016). Biochar surface particles containing C-coated minerals are mainly efficient in reducing heavy metals’ bioavailability (Kumar et al., 2020). Willow-derived biochar at high temperatures facilitate adsorption of heavy metals from sewage sludge via physisorption and chemisorption mechanisms (Bogusz et al., 2019). Khan et al. (2013) reported that various feedstocks that carry high heavy metal contents could decrease the bioavailability of the heavy metals in some soils. For instance, biochar derived from sewage sludge reduced the bioaccumulation of Pb, Ni, Cu, Co, Cr, and As but enhanced Zn and Cd in acidic soil. Biochar can improve the anionic metalloid mobility by reducing the positively charged sites, which reduces the arsenic binding sites with increased soil pH (Vithanage et al., 2017).

Nkoh et al. (2022) reported that the effect of biochar application to polluted soils is the reduction of pollutant uptake by plants, with some exceptions for Fe and Mn. The reductions were estimated at 22.8% (Mn), 33.0% (Ni), 18.3% (Zn), 3.03% (Pb), 41.5% (As), 56.0% (Cr), 25.8% (Cu), and 26.2% (Cd). The underlying mechanisms for this reduction in the bioavailability of heavy metals in soils are diverse with charged metals being fixed via ion exchange, physical entrapment on biochar’s surfaces and changes in soil chemistry (Li et al., 2022). Amending heavy metal polluted soils with biochar reduced the overall daily intake of heavy metals (12.5%), hazard quotient (30.0%), and cancer risk (30.6%). However, these effects can be quite diverse depending on biochar properties, soil properties and the chemistry of concerned heavy metals (Nkoh et al., 2022). For maize plants grown on Pb-polluted soil, biochar treatment reduced the bioavailability of Pb (II) by 71% and the exchangeable Pb(II) by 99%. Compared to the un-amended soil, biochar treatment significantly decreased the associated Pb (II) toxicity to the maize plant (Zhu et al., 2020). Lebrun et al. (2020) examined the growth of Salix viminalis in arsenic and Pb (II)-contaminated soils amended with biochar, iron grit, and compost. They found that biochar-treated soil provided a favourable growing environment for the plant by considerably reducing the toxicity and bioavailability of the contaminants. Also, the phytotoxicity of Cd (II) to rice plants was significantly decreased when biochar was added to polluted soil (Yue et al., 2019). Natasha et al. (2022) observed that biochar application to soils has the potential to decrease the uptake of Zn, Pb, Cu, Cd, Ni and As, by 22%, 28%,38%, 40%, 44% and 48%, respectively in plants. In this study, with more data points, they estimated a 26.2% (Cd), 25.8% (Cu), 56% (Cr), 41.5% (As), 3.03% (Pb), 18.3% (Zn), 33.0% (Ni), and 22.8% (Mn) reduction rate of heavy metals uptake by plants when grown on biochar amended soils.

Biochars containing large surface area, sufficient pore volume and abundant functional groups play crucial role in heavy metal sorption (Ahmad et al., 2018). However, electrostatic interaction and sorption precipitation between heavy metals and biochars are the main mechanisms of heavy metal sorption governed by biochar (Lian and Xing, 2017). Additionally, surface coprecipitation, metal ligand complexation and ion exchange also contribute to the metal sorption on biochars (Ding et al., 2016a). Furthermore, the sorption affinity and capacity of heavy metals largely depend upon surface functional groups rather than pore volume and surface area (Yu et al., 2019). Oxidation of bichar induces carboxlic functional groups on biochar surface which elevated the adsorption capacity of Al³⁺, as oxygen enriched functional groups acted as coordinated sites for the Al³⁺. Cd adsorption on biochar was mainly regulated by the ion exchange (Lian and Xing, 2017; Haider et al., 2022). The sorption of Pb³⁺ on biochar surface was attributed to: 1) interaction of heavy metal with surface functional groups; 2) exchanges of heavy metal with cations (Ca²⁺, Mg²⁺) of biochar (Yu et al., 2019). Generally, biochars prepared at middle and low temperatures exhibit the highest adsorption capacity for metal cations (Xiao et al., 2018).

Biochar application appeared as a promising approach for mitigation of heavy metal contamination in plants, which may lead to a higher agricultural productivity and protecting plant community. However, biochar remediation efficiency is largely dependent upon the biochar type, plant species, biogeochemical properties of soil, and specific trace metal. Therefore, future strategies need a comprehensive analysis on determining the optimal methods of biochar production, type of biochar, plant species, popularization, and improving emphasis on suitability, adsorption potential, and sustainability of biochar as an optimum remediation tool against heavy metals while safeguarding the food quality.