Production of BC reduces CO₂ in the atmosphere because the decay or decomposition of organic materials naturally omits CO₂ into the atmosphere, while converting those materials into carbon-neutral products eliminates the decomposition process. Figure 3 illustrates how BC is made from different agricultural materials. Woolf et al. (2010) showed that sustainable implementation of BC could offset up to 12% of global anthropogenic CO₂. Composting manure is a major GHG source, but incorporating 10% (w/w) BC during composting reduced GHG emissions substantially (Yin et al., 2021). Application of BC to soil facilities for carbon capture and storage (CCS) is associated with carbon sequestration, soil, and water remediation, decreased GHG emissions, improved soil fertility, crop production, and waste management and recycling (Haider et al., 2022). Compared to other organic materials, the internal structure of BC has a remarkably high degree of stability, providing long-term soil fertility (Agegnehu et al., 2017). Mixing BC in soil at 13.5 t/ha (3% of the upper 30 cm layer) can provide carbon storage space for two centuries (Matovic, 2011). Biochar application to horticultural fields will also contribute to CCS. However, one study showed that BC application increased soil CO₂ fluxes and reduced N₂O fluxes with no effect on CH₄ fluxes (He et al., 2017). The variable results could be attributed to multiple factors, including the differences in physical and chemical properties of BC and soil, variations in crop species in the tested field, climate differences, and the duration of the reported studies. The incorporation of BC into horticultural substrates mitigates CO₂, CH₄, and N₂O emissions. In a microcosm incubation study, total CO₂, CH₄, and N₂O emissions decreased by 50%, 21%, and 66%, respectively, in a substrate amended with biochar compared to one without BC (Lévesque et al., 2018).

Biochar application can improve soil physical, chemical, and biological properties (Yu et al., 2019) ( Figure 4 ). A meta-analysis indicated that BC amendment decreased soil bulk density by 3%–31% (mean 7.6%) in 19 of 22 soils (Omondi et al., 2016). Fruit production usually occurs in hilly and low mountain areas where soil compaction is a problem. Peake et al. (2014) reported that BC addition reduced soil compaction by more than 10%. Soil bulk density is negatively associated with soil porosity, so a decreased bulk density increases soil porosity. Biochar application can increase soil porosity by 8.4% (Omondi et al., 2016) and improve soil structural quality and aggregation (Blanco-Canqui, 2017). In general, soil water retention improves with reduced bulk density, increased porosity, and improved structural quality.

Biochar application can increase pH in acidic soils, improve soil organic carbon, increase nutrient holding capacity due to the role of functional groups, reduce the adverse effects of heavy metal adsorption, and increase the nutrient elements supplied by the BC (Yu et al., 2019; Chang et al., 2021). A meta-analysis of 59 reports published from 2012 to 2021 showed that BC application increased soil pH, cation exchange capacity, and organic carbon by 46%, 20%, and 27%, respectively (Singh et al., 2022). Biochar application to soils also increased the relative abundance of bacteria involved in soil carbon and nitrogen cycles, soil organic carbon decomposition, and plant disease suppression and significantly increased invertase and catalase activities (Li et al., 2021). Biochar application resulted in favorable conditions for soil microorganisms, improving microbial biomass carbon (C_mic) (Luo et al., 2014). Biochar application has been reported to relate to unique characteristics of BC, including altered root growth strategy, rhizosphere soil nutrient availability (Liu et al., 2022), and improved soil enzyme activities (Li et al., 2022a).

Peat is commonly used for horticultural purposes. However, the resultant depletion of peatlands has led to an increasing environmental concern about non-renewable resource peat extraction (Zulfiqar et al., 2019a; Zulfiqar et al., 2019b). Hence, there is a continuous search for sustainable growing media substitutes, such as biochar ( Figure 5 ). Ferlito et al. (2020) evaluated conifer wood BC as a growing medium component for citrus nurseries. The authors found that BC (25%) could be used in growing media comprising 50% sandy volcanic soil and 25% black peat or combined with compost (1:1) for healthy citrus production. In Lavandula angustifolia potted plants, Fascella et al. (2020) incorporated different BC concentrations (0, 25, 50, 75, and 100%, by volume) into peat mixtures. The treatments with 25, 50, and 75% BC improved agronomic traits and flower production in L. angustifolia. In lettuce (Lactuca sativa), Mendez et al. (2017) replaced 10% (by vol.) of peat with sewage sludge BC, improving biomass production by 184%–270% compared with 100% peat-based substrate due to enhanced microbial activities and NPK concentrations. Choi et al. (2018) reported that growing medium mixes comprising 20% pine bark and 80% BC (by vol.) boosted fresh and dry weights in chrysanthemum (Chrysanthemum nankingense) compared to the control with no BC. In the potted houseplant Syngonium podophyllum, Zulfiqar et al. (2019b) reported that BC incorporation (up to 20%) improved plant growth by improving leaf gas exchange, NPK concentrations, total soluble proteins, and soluble phenolics in leaves. Lévesque et al. (2020) reported improved growth of tomato and sweet pepper in peat-based growing media incorporating 10% or 15% (v/v) BC, with improved plant water use efficiencies, N and P uptake efficiencies, and increased fruit dry weights. In potted Alpinia zerumbet, Zulfiqar et al. (2021b) evaluated the impact of BC incorporation in a peat–perlite-based growing medium; growth improved with 30% BC (v/v) alone or in combination with 5% compost by regulating NPK uptake and leaf gas exchange traits. In Rhododendron delavayi Franch., Bu et al. (2022) evaluated the impact of two BC (v/v) applications in peat-based growing media, reporting improved plant and root growth, photosynthesis, and contents of carotenoids, polyphenols, and anthocyanins. The authors suggested using peat-based substrates containing 20% wood chip BC or 30%–40% rice husk BC for good growth.

Leaching nutrients, particularly N and P, from container substrates is a concern in ornamental plant production (Chen and Wei, 2018), as continuous leaching year-round can cause surface and/or groundwater contamination. Engineered Mg-enriched biochar can adsorb >100 mg P/g substrate (Yao et al., 2013), and Mg- and Fe-enriched LDH (layered double hydroxide) biochar can adsorb nitrate close to 25 mg/g (Xue et al., 2016). Biochar application to soil can reduce nitrate and ammonium leaching (Karhu et al., 2021). Thus, selected biochars could be used to sustain nutrients in substrates and soils and reduce leaching.

Applied BC can interact with plant growth regulators, enhancing plant growth. In a study conducted with Chinese cherry “Manaohong” (P. pseudocerasus), BC application increased indoleacetic acid (IAA), gibberellin A3 (GA), and zeatin levels and decreased abscisic acid (ABA) levels compared to non-treated plants. Moreover, high-throughput sequencing analysis showed the activation of IAA and inactivation of ABA signal transduction at the gene level, revealing that BC improves plant growth by regulating phytohormone signaling (Yang and Zhang, 2022). Langeroodi et al. (2019) reported that BC application decreased ABA in pumpkin seedlings under deficit irrigation relative to control plants. Akhtar et al. (2014) reported that a cotton seed shell and Oryza sativa husk BC mixture decreased ABA in potted tomato plants subjected to deficit and partial root-zone drying irrigation. While more research is needed, the positive interactions of BC with IAA, GA, and zeatin may indicate that phytohormone availability could increase plant growth.

Photosynthetic pigments affect plant photosynthetic rates and are vital determinants of healthy growth and productivity. Application of BC to horticultural crops enhances photosynthetic pigments and rates under normal and stressed conditions. For instance, Yang and Zhang (2022) reported increased chlorophyll content of the ornamental plant Centaurea cyanus L. in response to BC supplementation relative to control plants. Likewise, BC and humic acid treatments improved chlorophyll content in calendula (Calendula officinalis L.) compared to controls (Karimi et al., 2020). In fenugreek (Trigonella foenum-graecum), Jabborova et al. (2021a) reported that BC increased chlorophyll pigments by 30% and carotenoids by 60%. In Berberis integerrima, BC applications recovered the chlorophyll contents in Cd-stressed plants compared to non-treated plants. Jabborova et al. (2021c) also reported that BC increased chlorophyll pigments by 35% in ginger (Zingiber officinale) compared to the control. In salt-stressed cowpea (V. unguiculata (L.) Walp.), BC improved chlorophyll content by 6% and by 76% when combined with osmopriming (Farooq et al., 2020). In drought-stressed perennial ryegrass (Lolium perenne L.), rice husk BC improved chlorophyll content and drought stress tolerance. S. podophyllum plants grown in soilless growing media amended with BC had increased chlorophyll content compared to the non-amended control (Zulfiqar et al., 2019b). In tomatoes, Massa et al. (2019) reported decreased chlorophyll SPAD values under normal and nutritional stress conditions using BC. Similarly, Zulfiqar et al. (2021b) reported decreased chlorophyll content in A. zerumbet in response to BC supplementation in a soilless growing medium. SPAD values were also higher in tea plants in response to BC application than in untreated plants (Yan et al., 2021). In sweet basil, BC applications, especially at higher concentrations (2% and 3%), improved chlorophyll contents compared to low concentrations (1%) and control plants (Jabborova et al., 2021b). In sum, BC applications can improve photosynthetic pigments in different plants. However, further studies are needed to elucidate the specific mechanism involved.

As mentioned above, BC application generally increases photosynthetic pigment contents and thus enhances leaf gas exchange characteristics. For example, Guo et al. (2021b) showed that BC application to tomato plants under deficit irrigation improved leaf gas exchange, as exemplified by the higher P_n , T_r , and g_s than control plants. Wang et al. (2021) reported that BC significantly improved Pn, Tr, gs, and water use efficiency in field-grown peanut. In cabbage seedlings under water deficit, a low BC concentration (5%) enhanced g _s, P_n, C_i , and T_r more than a high BC concentration (10%). A wheat straw-based BC application upregulated leaf gas exchange traits (g_s , P_n , C_i , and Tr) and carboxylation use efficiency in A. zerumbet grown in a sandy loam soil (Zulfiqar et al., 2021a). In tea, BC application improved the photosynthetic rate by 17% compared to untreated control plants (Yan et al., 2021). Langeroodi et al. (2019) reported that BC improved leaf gas exchange traits in pumpkin (Cucurbita pepo) relative to control plants. Likewise, cotton seed shell and rice husk BC increased the leaf photosynthetic rate in potted tomato plants subjected to deficit and partial root-zone drying irrigation (Akhtar et al., 2014).

Incorporating BC into growing media or soil can improve crop tolerance to drought stress. Wood pellets BC added to a growing medium (30% v/v) improved tomato seedling tolerance to drought (Mulcahy et al., 2013). Agbna et al. (2017) reported that water-stressed tomato plots amended with 25 t/ha of wheat straw BC had similar fruit yields as unstressed control plots, accompanied by increased plant height, leaf numbers, and fresh and dry plant weights. Egamberdieva et al. (2017) evaluated BC-based Bradyrhizobium inoculum on the growth of drought-stressed lupin (Lupinus angustifolius L.), noting improved drought tolerance associated with revived growth, increased nutrient uptake, and improved symbiotic performance. Li et al. (2018) reported that 30 t/ha BC and deficit irrigation saved water without affecting total tomato productivity in a field experiment. In the same experiment, increasing the maize straw BC application rate from 10 to 40 t/ha increased plant dry biomass and fruit yield. However, higher BC application rates had no positive effect or decreased plant growth and yield. Moreover, a cost–benefit analysis showed that BC applications of 10, 20, and 40 t/ha positively affected net profit. In contrast, 60 t/ha negatively affected net profit compared with the unamended control. A quadratic function fitting net profit and BC application rate showed an optimal application rate of 27 t/ha (Li et al., 2018).

In a two-year study on pumpkin (C. pepo) under reduced irrigation, Langeroodi et al. (2019) reported that BC-amended plants had decreased ROS, an elevated antioxidant defense system, and improved photosynthesis and overall growth relative to unamended plants. Egamberdieva et al. (2020b) evaluated the impact of maize BC on drought-stressed broad beans (V. faba L.), reporting improved growth, yield, and nutrient uptake relative to unamended plants. Abd El-Mageed et al. (2021) studied the impact of acidified BC on drought-stressed V. faba under field conditions in a two-year experiment, reporting that BC addition improved plant growth relative to unamended soil. Biochar application to drought-stressed cabbage plants ameliorated the adverse effects of drought stress by increasing leaf water relative content and photosynthetic activities, with a 5% BC application improving shoot and root fresh and dry weights compared to unamended control plants (Yildirim et al., 2021). Abd El-Mageed et al. (2021) reported a 39% increase in seed yield of drought-stressed V. faba in BC-amended soil under field conditions compared with unamended soil. In a greenhouse experiment, Akhtar et al. (2014) reported that 5% (w/w) cotton seed shell and O. sativa husk BC increased the fruit yield of potted tomato plants subjected to 30% deficit and partial root-zone drying irrigation by 6% and 13%, respectively, compared to fully irrigated control plants. Improved drought tolerance with BC application could be attributed to increased porosity and pore volume in BC-amended soil, increasing the capacity to hold available water under drought stress. Further research is needed to address the effect of different soil porosities and pore volumes with BC application on crop growth under drought stress.

An estimated 831 million ha of arable land globally is affected by salinity and is expected to increase due to ongoing irrigated farmland growth and climate change (Wu et al., 2022). Growing crops on saline soils in locations where water supply is frequently limited is necessary to fulfill the food demands of an expanding population (Zulfiqar, 2021). Biochar incorporation into soil can help ameliorate the adverse impacts of salt stress on plants (Egamberdieva et al., 2022). For example, Mehdizadeh et al. (2020) reported that BC application improved growth and leaf mineral nutrients and decreased leaf Na⁺ and Cl^– contents in salt-stressed summer savory. In cowpea (V. unguiculata (L.) Walp.), BC application improved salt stress tolerance by elevating chlorophyll content, amylase activity, and total soluble sugars and decreasing Na⁺ and MDA contents and total antioxidant activity (Farooq et al., 2020). In salt-stressed spinach, combined Trichoderma and BC ameliorated the adverse effect of salt stress by decreasing ROS, altering phytohormones, and improving the physiological condition of plants (Sofy et al., 2021). Ghassemi-Golezani and Farhangi-Abriz (2021) reported enhanced salt tolerance in safflower (Carthamus tinctorius L.) in response to BC-based metal oxide nanocomposites of manganese (Mn) and magnesium (Mg), with improved root growth, root density, plant height, plant biomass, leaf area, chlorophyll content, branch and flower numbers, potassium, Mg, and Mn contents, duration of the reproductive stage, and seed and oil yields and decreased Na⁺ content. BC can also improve plant’s ability to cope with salt-induced oxidative stress. For example, Song et al. (2022) recently reported that organic amendment incorporation, including BC, to salt-affected soil reduced oxidative stress and improved morpho-physiological conditions by promoting Na⁺ exclusion and K⁺ accumulation in hybrid Pennisetum, relieving stomatal restriction, increasing leaf pigment levels, electron transport efficiency, net photosynthesis, and root activity, and reducing oxidative damage.

In another study, Ghassemi-Golezani and Abdoli (2022) evaluated the impact of BC-based rhizobacteria for mitigating salt stress in rapeseed (Brassica napus L.), reporting improved nutrient contents, non-enzymatic antioxidants, main and lateral root lengths and weights, main/lateral root length ratio, specific root length, root diameter, shoot length and weight, leaf area, chlorophyll content, and seed and oil yields, and reduced sodium contents, ROS generation, lipid peroxidation, and enzymatic antioxidant activities in plant tissues. Ghassemi-Golezani and Rahimzadeh (2022) tested BC-based nutritional nanocomposites for improving salt stress tolerance in dill (Anethum graveolens), reporting decreased sodium contents that reduced the levels of osmolytes, antioxidant enzyme activities, ROS, lipid peroxidation, NADP reduction, ABA, jasmonic acid, and salicylic acid in leaves. Moreover, BC addition, particularly BC-based nanocomposites, to salt-affected soil enhanced plant organ biomass, photosynthetic pigments, leaf K, Fe, and Zn contents, leaf water content, hill reaction, ATPase activities, oxygen evolution rate, endogenous indole-3-acetic acid, and essential oil yield (Ghassemi-Golezani and Rahimzadeh, 2022). Ekinci et al. (2022) also observed BC-induced salt stress tolerance in cabbage seedlings (B. oleracea var. capitata), decreasing malondialdehyde (MDA), hydrogen peroxide (H₂O₂), proline, and sucrose contents while increasing leaf and root nutrient contents of cabbage seedlings (except Na and Cl) and regulating ABA contents. BC incorporation also causes osmotic adjustments in plants under salt stress. For instance, BC application ameliorated salt stress in borage (Borago officinalis L), a moderately salt-tolerant medicinal plant, by improving plant water status and osmotic adjustment capacity linked to increased photosynthetic pigments, antioxidant activation, osmolyte accumulation, and increased K⁺ content and K⁺/Na⁺ ratio (Farouk and AL-Huqail, 2022). Biochar-ameliorated salt stress tolerance could be due to: (1) BC increasing the available water holding capacity of soil and improving soil chemical properties by limiting Na availability to plant roots; (2) BC generally contains high K; and (3) both factors leading to increased K uptake and decreased Na uptake, ameliorating the adverse effects of Na. However, molecular research on BC-mediated salt tolerance is needed to provide more detailed information on salt stress tolerance driven by BC amendment.

Heavy metal pollution in soil has become more challenging with increasing urbanization and rapid improvements in agricultural and industrial technologies, posing major threats to human health and the environment. Biochar has gained popularity as a potential substance for successfully immobilizing heavy metals in polluted soil and, thus, reducing their uptake by plants. Controlling metal absorption in horticultural crops is critical for human consumption and maintaining the productivity and quality of horticultural produce. In this context, many studies have found that BC helps reduce heavy metal stress in horticulture crops. Quartacci et al. (2017) reported that BC addition to contaminated soil restored flavonoids and antioxidant activity and increased phenolic acids, total phenols, and anthocyanins compared to the control. Almaroai and Eissa (2020) reported that BC decreased the transfer of heavy metals (Pb and Cd) from non-edible parts to edible parts of tomato (Solanum lycopersicum) grown in metal-polluted soil, increasing tomato yield by 20%–30%. Ibrahim et al. (2022) tested three BC types (Casuarina, mango, and Salix as feedstocks) for their impact on summer squash (C. pepo L.) grown in contaminated soil. Casuarina-based BC applied at low concentration (2%) had the highest growth rate, while the high concentration (4%) decreased root and shoot uptake of Cd, Co, Cr, Cu, Ni, Pb, and Zn by regulating soil pH, organic matter, and electrical conductivity. Moreover, the 4% Casuarina-based BC had the highest reduction in the bioconcentration and translocation factors of these heavy metals. Thus, BC amendment in contaminated soils can improve yield and the quality of produce by reducing the uptake of heavy metals such as Cd, Cu, and Zn (Tahervand et al., 2022), presenting a novel phytoremediation strategy for contaminated soils by phytostabilizing toxic metals in situ while reducing their bioavailability to crops and leaching into groundwater.

Acidic soils are known for their lower crop productivity than neutral or alkaline soils, which is attributed to poor fertility, mineral toxicity (e.g., Al, Mn, and Fe), and other nutrient deficiencies. Some horticultural fruit crops and landscape plants often grow in acidic soils, significantly affecting their growth. Biochar can elevate soil pH due to its alkalinity. A BC amendment can also increase plant tolerance to soil acidity. For example, Yan et al. (2021) reported that BC application improved the growth of tea plants by enhancing leaf area, photosynthesis, and mineral nutrients in acidic soil. Yin et al. (2022) compared the effects of BC and hydrochar made from cow manure and reed straw on the growth of lettuce under acid soil conditions, reporting that cow manure-based BC amendments improved lettuce growth by increasing soil pH, P and available K contents, soil bacterial activity, and the abundance of beneficial bacteria compared to reed straw and hydrochar.