Perspective Chapter: The Role of Biochar in Soil Amelioration

Abstract

Many techniques have been employed in restoring the health of physically, chemically and biologically degraded soils. Some of these techniques are expensive, time consuming and may involve soil excavation or chemical treatments with numerous washes in some cases. There is a novel technique that is cheap, can restore the properties of a degraded soil, mitigate climate change and sequestrate carbon in the soil. That technique is the biochar technology. In this review, we’ll look at biochar technology as an ameliorant in improving impoverished soils. Biochar is a carbon-rich substance that is produced when biomass (feedstock) is subjected to a thermal decomposition process under limited oxygen called pyrolysis. Biochar can be used to ameliorate soil acidity and alkalinity depending on the feedstock. It has advantages such as increasing cation exchange capacity, soil carbon and nutrient in the soil. Biochar can be inoculated with specific organisms for pollutant breakdown and acts as a habitat for naturally occurring microbes; by binding pollutants in the soil through the process of bioaccumulation, sorption, electrostatic attraction and precipitation, it acts as a remediation agent. However, the feedstock, pyrolysis temperature, and heating period can all affect the properties of biochar and its biological processes.

1. Introduction

With the increasing loss of soil particles through erosion, nutrient loss through leaching and organic matter, maintaining agricultural output is a major concern on a global scale. The modern agroecosystem has a huge challenge to continuous production of food while conserving the soil fertility with the recent rise in population, which is approaching the historic number of 8 billion [1]. According to Kopittke et al. [2], about 99% of food consumed by man comes from the soil, which also serves a number of ecosystem services [3]. As a result, degradation of soil nutrients and productivity poses a danger to global food security [1]. To reclaim degraded soil, amendments are applied to the soil. One of the novel amendments that are recently used is biochar.

Biochar is a pyrolyzed material that is mass-produced by pyrolyzing organic material (waste from plants and animals) at elevated temperatures in an oxygen-free atmosphere [4, 5]. The most prevalent approach to producing biochar for crop growth and environment is through the slow pyrolysis (350–700°C) of organic materials in the presence of limited oxygen. Alterations in temperature, raw material, moisture and pyrolysis time are major factors to be considered in the production of biochar. A wide range of organic raw materials such as agroforestry sources (wood, tree bark, wood shavings, roots and shells), livestock-related sources (beddings and animal waste products), agricultural waste-products (corn stalks, rice husk, root residues and forage crop), waste products from industries (carbon black and bagasse), agro-urban biosolids and materials that contains carbon can be used for the production of biochar [6]. The main components of biochar are cellulose and lignin, which are derived from plant biomass and have a high carbon concentration but low hydrogen and oxygen content. In recent years the usage of diverse sources of biochar has received numerous attentions, due to its function as soil amendment to minimize soil compaction and nutrient/water holding capacity and increase soil porosity due to attached functional groups, pore properties, high adsorption capacity and surface activity [7]. In general, biochar produced at pyrolysis of reduced temperature range of 100–300°C results to increase in the amount of acidic functional groups, adsorption capacity, yield and porosity tend to increase when applied in the soil, according to Sun et al. [8]; while at elevated temperatures range of 600–700°C, there is an increase in pH above 7, amount of major functional groups present and fixed carbon content, following applied biochar produced at that temperature range. Due to biochar’s increased surface area, stable carbon content and high porosity, its application can provide numerous benefits in soil amelioration [6, 9]. The considerable porous carbon rich material with an increased specific surface area could be applied as soil amendments to absorb organic/inorganic contaminants to include pesticide residues, herbicides and heavy metals, thereby boosting the quality of the soil for environmental and agricultural purposes [10, 11].

2. Impact of biochar on soil physical properties

The effectiveness of applying biochar can be well illustrated on a coarse textured sandy soil, as they reduce bulk density, increase water retention, and improve the soil physical properties [6, 12]. In addition, the feedstock, the soil type used to make biochar determines it’s efficient as a soil amendment. Improvement of soil aggregate stability after the usage of biochar to the soil as amendment is of a known significant effect [13, 14]. Improvement in aggregate stability and soil structure of soil in a one year (mustard red clover) and two years fallow period amended with wheat straw, vineyard pruning and wood chip biochar, (application rate 3% by soil weight and pH 8.3–9.7) was recorded. However, the result was more effective on soils with coarse textured compared to fine textured soil as stated by Burrell et al. [15]. In an upland red soil cultivation system for rapeseed and potatoes, wheat straw biochar (pH 10.35, application rate 2–40 t ha⁻¹) boosted soil aggregate stability and microaggregate content while also enhanced the yields of rapeseed and sweet potatoes [16]. Although the impact seems to be short-lived (2 years) and at higher rates of biochar application (>10 t/ha) and more effective in sandy soils with small size biochar [17], biochar amendment can improve soil bulk density and reduce soil compaction [18, 19]. With the addition of biochar, soil bulk density would decrease, which would lessen soil compaction. According to the soil textural class, the optimal soil bulk density varies for different crop production [20]. Low soil bulk density has been shown by Zhang et al. [9] to enhance soil structure and make it easier for nutrients to be released and retained. The bulk density of the soil is decreased in part by biochar. After applying biochar from orchard pruning, Rombola et al. [21] research revealed that the soil bulk density fell from 1.44 0.10 to 1.38 0.06 g cm³. Biochar generated from crop residues, had a more significant influence on increasing soil bulk density and lowering soil compaction [18, 22]. Khan et al. [23] also discovered that applying maize straw biochar decreased the soil bulk density in sandy loam.

Through larger pores and aggregate stability, the application of biochar has also been shown to improve soil water retention [24, 25, 26]. Crop productivity has been significantly impacted by soil water retention [25]. According to the research, adding biochar affects soil water retention. This effect may be brought on by the biochar’s hydrophilic domains, high porosity, and high specific surface area. Oladele et al. [27] discovered that adding rice husk biochar at a rate of 12 t/ha improved soil water retention in sandy clay loam from 36.87 to 32.94%. According to Razzaghi et al. [25], biochar enhanced field capacity in coarse-textured and medium-textured soils by 51 and 13%, respectively. In addition, adding biochar increased the amount of water that was available to plants in coarse, medium, and fine-textured soils by 45, 21, and 14%, respectively. Furthermore, Hussaina et al. [28] found that a variety of variables, including the kind of raw materials, pyrolysis temperature, biochar particle size, soil type, and compaction condition, significantly influenced soil water retention.

3. Impact on soil chemical properties

Soils globally are mostly acidic and still experience the problem associated with acidity due to natural and anthropogenic activities [29]. Plant growth is dependent on many factors including the pH of the soil. An acidic soil affects the plant productivity and this is because in an acidity condition, aluminum fixes phosphorus making it less available for plant uptake [30]. Jeffery et al. [31] observed that the application of biochar significantly increased the pH of acidic soils such as alfisols, ultisols, and latosols [32]. Increasing the soil pH by biochar could be attributed to the following: (1) Functional groups such phenolic -COOH, OH, and alcoholic OH that are present on the surface of biochar and are capable of interacting with basic cations in soils based on varying reactivity levels to help raise soil pH [33]. (2) Biochar from different sources have different amount of ash which is alkaline. Ash has been reported to reduce soil acidity and increase the pH of the soil [34]. Biochar application directly to an acidic soil makes the alkaline components such as sodium oxides, hydroxides, potassium, carbonates, calcium and magnesium to be out in soluble forms into the soil. Thereafter, hydrogen ions and aluminum monomers present would react with these soluble substances to increase the soil pH [35, 36].

In order to determine cation exchange capacity (CEC), it is crucial to assess the adsorption capacity of soil and solid materials for cation exchange. In the process of reducing the nutrients leaching, CEC is also a relevant indicator of soil fertility in supplying sustainable nutrients for plants [37]. Increased soil cation exchange sites will result in an increase in soil CEC. The basic adsorption capacity of calcium, potassium, ammonium, and magnesium is higher in soils that have greater CEC, which enhances the use of nutrient in the soil and lowers nutrient loss [38]. Numerous research has investigated how biochar affects the CEC of soil. By applying maize straw biochar, Khan et al. [39] demonstrated that the cation exchange capacity increased from 12.9 to 15.6 cmol/kg, 17.4 cmol/kg, and 19.2 cmol kg with addition rates of 4 t/ha, 12 t/ha, and 36 t/ha, respectively. According to Hossain et al. [40], the addition of biochar resulted in a 20–40% increase in the total soil CEC and charge. The nutrients and alkaline cations in the soil will increase dramatically with even a tiny addition of charcoal. Additionally, according to Chintala et al. [41], both acidic and alkaline soils can benefit from biochar’s improved soil CEC due to the availability of more anions on its surface.

As soon as biochar is added to the soil, it starts to interact with the soil’s chemistry; this interaction then advances through dissolution (1 month), the development of reactive surfaces (1–6 months), and degeneration (>6 months) [12]. Along with improving soil nutrient status, specifically covering a long period of time, soil’s capacity to retain carbon and nitrogen may also minimize environmental degradation both now and in the future as well as its detrimental effects on animal and human health status [17]. Due to its surface properties and linked functional groups, biochar serves as a good adsorbent for nutrients and pollutants [42, 43]. As a result, biochar enhances nutrient retention in soil by boosting the soil’s capacity to collect and hold onto nutrients and agro chemicals, thus reducing their vaporization and leaking into groundwater. Biochar has the potential to provide vital nutrients to the soil solution over time through weathering, decomposition, and other processes. Due to its longer carbon half-life—possibly spanning thousands of years and also because of its durability, biochar can have a long-lasting effect on soil quality and carbon sequestration [44]. Due to its potential for decreased nutrient losses and increased fertilizer efficiency [45, 46, 47], biochar has been a strategic tool for agricultural and environmental purposes [12, 48]. In comparison to non-biochar soil, its incorporation into deficient soil improves soil fertility, crop development, and production [49] as shown in Figure 1. However, the source and characteristics of the applied biochar, as well as soil characteristics, environmental plant responses, biochar-soil interactions, and its capacity to absorb nutrients, all play a significant role in these factors [47, 51]. Depending on its inherent nutritional value, capacity for nutrient and water retention, and ability to reduce moisture stress, it may have a notable influence on crop growth and development both directly and indirectly [52]. It is occasionally possible to forecast poor crop performance for the first 30 days following cultivation on acidic soil (pH = 5.5), which may be caused by transient N retention in N-deficient soil [12]. While minimizing nutrient loss, it efficiently transfers nutrients to the soil [53]. In general, biochar can be applied to boost agricultural output due to its chemistry and characteristics, especially in soils with low fertility and soil degradation, where it can be extremely helpful. It has boosted crop output by enhancing soil base saturation [54, 55], retaining nutrients in the soil column where roots are situated, and enhancing nutrient use efficiency [56, 57].

Figure 1.

Biochar production and its application in agriculture [50].

4. Impact on the biological properties of the soil

According to studies by Beheshti et al. [58], and Gorovtsov et al. [59], Due to its alkaline pH and structure, which improves soil porosity, aggregation, and water-holding capacity and promotes soil nutrient bioavailability and microbial growth, biochar has been revealed to improve important soil biological properties. As soon as biochar is applied to the soil, it can interact with soil biological elements like plant roots and root microbes, even on soils that are contaminated with microplastics and metal(loids) [45, 59, 60]. This is true even though biochar is frequently thought of as an appropriate soil treatment for long-term purposes. According to many studies [21, 61], it is a major role in boosting soil microbe development, an increase in soil respiration, soil biomass carbon, and microbial diversity. In fact, it has been claimed that biochar increases crop systemic resistance and even aids in the management of soil diseases by enhancing the biological quality of the soil [62]. According to Warnock et al. [63], biochar has also been linked to increased soil fertility and improved soil carbon sequestration, both of which have been shown to benefit the mycorrhizal community in the soil. Moreover, soil that has been treated with biochar can offer a better environment for the growth and performance of beneficial soil microorganisms. Rhizobia inoculation significantly increased micronutrient fertility (Fe, Zn, Mn, and Cu), soil organic matter (SOM), and soil quality in lentil plants [64]. Depending on the pyrolytic temperature variation and feedstock, these benefits could fluctuate [8, 65]. The alkaline pH and soil aggregating ability of biochar, which is currently acknowledged to improve the soil biological environment [12] are the primary factors. With improved nutrient accessibility for plants and advantageous microbial activity, biochar can be utilized as a soil treatment to improve soil quality [6, 66]. When biochar is put into the soil, it immediately has a major impact on clay minerals and soil organic matter (SOM), which serve as nutrient pool and exchange sites [11]. Applications of biochar have shown across a variety of soil types that soils with enriched microbial diversity and activity are frequently more resistant to management practices [24, 60, 61, 67]. The decomposition of organic substrates to form SOM, biogeochemical nutrient cycling and mineralization, soil structure and aggregation, disease suppression, regulation of hormones that promote plant growth, increased soil water holding and availability to plants, neutralization of toxic compounds, and many other critical processes all depend on soil microbes. By encouraging soil microorganisms, biochar is essential for controlling soil health. The availability of nutrients needed by plants is increased by biological activity from soil microbes mediated with biochar, which reverses the fixation of nutrients through mineralization [58, 68]. Microbial diversity and community structure in soil are influenced by soil organic carbon, soil structure, and other biochemical soil variables. Recent advancements in technology have made biochar more effective for a range of environmental applications, including the treatment of soil pollution, targeted removal of metal(loid)s and contaminants from soil, solid waste management, and wastewater treatment [69]. This is because biochar can now be produced more efficiently and with the desired physical and chemical properties. The variety of microbial species found in soil provide a natural environment. Researchers can now delve deeply into the intricate mechanisms involved in establishing the interactions amongst microbes, soils, and plants [70] thanks to advancements in molecular studies. In soil, life can be found in three different domains, including prokaryotes, archaea, and fungi, animals, plants and protists. The addition of biochar changes the variety of soil microorganisms. Numerous studies supports adding biochar to soil to improve the microbial populations there, as seen in Table 1.

Feedstock	Type of soil	Impact of biochar	References
Chicken manure (500°C)	Sedimentary alfisol	Increase in activities of microorganisms in soil	Meier et al. [72]
Corn straw (500°C)	Sandy loam	Microbial biomass increased Increase in gram positive bacteria Increase in fungal population	Lu et al. [73]
Cotton straw (450°C)	Calcaric Fluvisol	Increase in enzyme activity, microbial activity, as well as microbial biomass	Liao et al. [74]
Date palm waste (300°C)	—	Sorption of heavy metals Increased the microbial biomass, soil respiration and soil organic matter	Al-Wabel et al. [75]
Gliricidia sepium wood (300°C, 900°C)	Serpentine soil	Increased overall soil enzymatic activity; Sorption of heavy metals	Bandara et al. [76]
Gliricidia sepium wood (900°C)	—	Microbial biomass increased Increase in bacterial and fungal count; Increase in plant growth-promoting bacteria population; Absorption of heavy metals	Herath et al. [77]
Glucose	Forest and arable soil	Increase in gram positive bacteria	Steinbeiss et al. [67]
Maize straw (400°C)	Silt loam	Arbuscular mycorrhizal fungi/ saprotrophic fungi ratio affected	Luo et al. [78]
Oak pellet (550°C)	Clay; Sandy loam	40–64% increase in phospholipid fatty acid biomass	Awad et al. [79]
Pinus radiata	Silt-loam	Beneficial for phosphate solubilizing bacteria and carbon degrading bacteria	Anderson et al. [80]
Pinus sp.; Dairy + bull manure (500°C)	Fertile Mollisol	Enhances the microbial biomass and microbial activity	Kolb et al. [81]
Rice straw (550°C)	Clay loam	Improved soil organic carbon mineralization; Slight increase in microbial biomass	Pan et al. [82]
Sawdust (550°C)	Sandy loam	Increase soil organic matter Increase in phospholipid fatty acid biomass	Gomez et al. [83]
Sugarcane bagasse (450°C)	Sandy loam	Reduction the bioavailability of heavy metals; Increase in Actinomycetes population (280%)	Nie et al. [84]
Pine cone and vegetable waste (200°C)	Sandy loam	Absorbed the heavy metals and increased microbial abundance	Igalavithana et al. [85]
Willow leaves (470°C)	Flinty clay loam	Nitrogen cycling is affected Bacteria +28%; Actinobacteria +62%; Gram-negative bacteria +27%	Prayogo et al. [86]
Willow wood Swine manure (350°C)	Temperature sandy loam	Microbial biomass increased Increase in gram-positive and gram-negative bacteria	Ameloot et al. [87]
Yeast	Arable and forest soil	Nutrients levels enhanced Fungal population increased	Steinbeiss et al. [67]

Table 1.

Biochar to soil to improve the microbial populations [71].

5. Biochar in remediation of metal, metalloids and pollutants

In contrast to remediation techniques such leaching, soil washing or excavation of heavy metals; biochar stabilizes heavy metals in the soil, emanating in reduced solubility and bioavailability of elements [7, 88]. Metal(loid) soil contamination is a problem that affects the entire world [89, 90]. The use of biochar as a soil treatment to enhance soil quality is one of its relevant research and application purposes. It is particularly suitable for poor-quality soils due to its distinct characteristics, such as its porosity, structure, particle size, sizes, pH range, specific surface area and water-holding capacity. Maximizing the carbon content is typically thought to be the most important component because biochar is frequently employed as a soil amendment in the context of agro-ecosystem operations [12, 45, 61]. Its physical, chemical, biological, and biological-chemical interactions (cation exchange capacity, elemental composition, electrical conductivity, pH), as well as its density, specific surface area, size, stability, pore size and distribution, shape, persistence, and other characteristics are all factors that affect the quality of biochar and its efficacy for agricultural and environmental applications [11, 68, 91]. By converting metals like Cd (II), Zn (II), and Pb (II) into oxide compounds, the formation of biochar at high temperatures (550–750°C) has demonstrated considerable benefits in phytoremediation [92]. However, the reactive surface area, pH, surface groups, ion-exchange capacity, and pore size distribution of biochar affect its capability for heavy metal adsorption [93]. One method used by biochar to lessen heavy metal contamination is to decrease the bioavailability of heavy metals in the soil [93]. Through diverse physical and chemical interactions and a strong adsorption mechanism, biochar serves to raise the pH of the soil and stabilize metals, potentially lowering the bioavailability and leachability of heavy metals in soils [89]. Because biochar has a higher specific surface area and a higher variable charge component, it has a better surface sorption capacity, which affects the soil’s capacity to retain water, nutrients, and organic molecules [89, 94]. When biochar is applied to soil, the surface oxygenation of the material alters the O²⁻containing functional groups (hydroxyl, carboxyl, carbonyl groups, and phenol) on the large internal surface area of the material, causing an induced negative charge and increasing the soil pH and cation exchange capacity (CEC) [95]. Through the release of cations from biochar, it has been demonstrated that the reduction in soil acidity greatly modifies the soil’s N ammonium and nitrate (NH₄⁺ and NO₃) and AB-DTPA extractable phosphorus and potassium contents [41]. Due to surface complexation with different functional groups, inter-cyclical complexation with free hydroxyl of mineral oxides, and simple physical adsorption leading to surface precipitation, biochar can help fix metals through sorption by exchange of cations (calcium (Ca²⁺), magnesium (Mg²⁺)). Additionally, metal(loid) stabilization may benefit from biochar that contains mineral components such phosphates and carbonates [96]. Currently, it has been revealed that soil-based biochar-nanocomposites or biochar-metal oxide modifications can further aid in the adsorption of pollutants. Concerns about its persistence in the environment, however, need to be more thoroughly examined [97].

Protons are added or removed from functional groups depending on the pH which gives biochar an electric charge. Tan et al. [98] and Xu et al. [99] claim that submerging biochar in a solution caused the surface of the biochar to become more negatively charged as the pH of the solution increased from 3 to 8. Through electrostatic interactions, these negatively charged functional groups bind to the cations of heavy metals. The fact that enhanced temperature biochar has a greater pH zero net charge (ZNC; the pH at which the biochar has zero net charge) explains why elevated temperature biochar has a lesser CEC [100]. This also implies that because the pH of a soil that has been altered with biochar is pH ZNC, there may be positive charge sites on the biochar that enable hazardous anionic metals to attach to the surface of the biochar [101]. Due to interactions, the electron-rich aromatic structure of biochar enables electrostatic attraction on metal cations with low electron density [102]. Cations connected to functional groups on the biochar surface can also interact with metals in soil solution through ion exchange or inner sphere interactions. Only when the pH and ionic strength of the soil solution are suitable for ion exchange can it occur. While high ionic strength solutions see reduced exchange as the level of competition in the solution increases, low pH solutions exhibit a reduction in ion exchange [101]. The ability of the biochar to adsorb metal cations via exchanging calcium, magnesium, potassium, and sodium equally increases as the CEC does [103]. According to research, applying biochar increases the availability of chromate (Cr) and arsenate (As), suggesting that combining phytoremediation with biochar application might be a good remediation strategy to effectively lower their mobility in soils.

Precipitation reaction is a crucial step in the detoxification of heavy metals using biochar. The majority of biochar is alkaline, which raises the pH of the soil solution and increases the density of hydroxyls on the surface of the biochar, which adsorbs aluminum or manganese [104]. This makes it possible for more hydroxyl-metal precipitates to form, which lowers the amount of heavy metals that can be found in soil solution. The mineral component of biochar, which includes CO₃²⁻, PO₄³⁻, SO₄²⁻, and SO₃²⁻, can also produce precipitates. By choosing the right feedstock and pyrolization temperature, sulphate and phosphate can be bonded by ligand exchange or precipitate on the biochar’s surface [102, 105, 106]. Due to an increase in aromatic carbon and the loss of O-containing functional groups, which were replaced with phosphorus (p) and Sulfur (s); P sorption increases as biochar is formed at higher temperatures. Precipitation can produce more stable bonds than surface complexation or bonding when these anions and carbonates are combined with metals like mercury (S specific), Zinc, lead, copper or cadmium.

The functional groups and structural makeup of biochars all influence their ability to transport electrons. According to Chacón et al. [107], phenols are the functional groups that are essential to biochar’s redox potential (reduction/oxidation). As was previously mentioned, the chemical structure of biochar changes to become highly aromatic as the pyrolysis temperature rises, increasing the _ electron availability. These enable speedy electron exchanges between the charcoal structure and the nearby metal [108]. Since mercury can get methylated if oxidation/reduction potential rises, the reducing potential may also be problematic [109]. Given that Iron (Fe) hydroxides are produced at redox conditions, this is related to Fe reduction [65]. This showed the possibility of a relationship between Iron and Mercury under long-theorized redox conditions [110].

Long-term experiments have shown that using biochar can successfully lower the bioavailability of heavy metals for uptake by plants. In rice fields, biochar treatments reduced cadmium (Cd) buildup in rice after one season, with the highest outcomes occurring at an application rate of 40 tons/ha over the course of three growing seasons, or two years [111]. One application of biochar resulted in an upward trend in accumulation throughout the course of the growing seasons, demonstrating that numerous treatments are likely required to retain healthy levels in crops [111]. Similar results were observed when 3 tons/ha of biochar were applied to fields growing Pak choi (Chinese cabbage), which decreased Cd and Pb concentrations below allowable levels for eating in their edible sections [84].

According to Wang et al. [65], the addition of 5% rice-straw biochar and 5% alkali-modified rice-straw biochar lowered the quantity of available Zn in soil by 28.96 and 36.86%, respectively. The addition of sphagnum biochar significantly decreased the mobility of heavy metals and the bioavailability of Pb, Cu, and Cd in contaminated soil, which was also reduced by 97.8, 100, and 77.2%, respectively [38]. This was accomplished through the coordination of metal electrons to C (double bond, length as m-dash) C (electron) bonds. The Cd buildup in brown rice decreased by 25.1 and 34.6%, respectively, when sulfur-modified biochar and sulfur–iron-modified biochar were introduced to the Cd-contaminated soil [112]. In general, biochar can alter the soil’s adsorption capacity and solid–liquid ratio, which can then have an impact on the bioavailability of heavy metals. This may have an impact on the types and distribution of heavy metals in soil. For the reasons indicated above, modified biochar also influences the soil’s pH, organic carbon content, and redox property, which further changes the bioavailability of heavy metals in the soil.

5.1 Reduced microorganism toxicity of contaminants

It is be possible to decrease the toxicity of soil contaminants on microbial communities by using biochar. The use of willow biochar produced at 700°C in heavy metal-contaminated soils could reduce the mortality of microorganisms [113, 114]. Through the immobilization of heavy metals such aluminum, cobalt, chromium, manganese, cadmium, and nickel on the pore of biochar [115, 116], it also improved the reproduction of Folsomia candida and reduced the leachate toxicity to Vibrio fischeri. This reduces soil contaminants and creates ideal circumstances for the development of both plants and microorganisms [117]. The application of rice straw biochar resulted in up to a 68% reduction in the levels cadmium, lead, of zinc, and copper. Bradyrhizobium japonicum, a bacterium that fixes nitrogen for plant growth, is no longer under heavy metal stress as a result [118]. Additionally, reducing the stress caused by the heavy metal on microorganisms enhances the synergy between biochar and soil microbial populations, which has a significant impact on the soil’s fertility.

5.2 Changing microbiological habitats

By enhancing the physical characteristics of the soil, biochar indirectly alters the microbial habitats. It reduces bulk density, improves soil aeration, and regulates microbial movement. Biochar boosts food availability to the microbial cells and promotes water retention [119]. Additionally, it guards against dry-wet cycles that are harmful to microbial activity and occur in the natural ecosystem [38]. Additionally, biochar alters the acidity level of soil. In contrast to chemical factors like carbon, electrical conductivity and nitrogen concentration, a small adjustment in pH (0.2–0.3 units) can have a significant impact on the soil microbiota. Bacteria are more prevalent in soil when heavy metals (like aluminum) are reduced and the pH is raised [120]. The strength and direction of numerous biochemical processes in soils are reflected by soil enzyme activity, which is the closest agent of organic matter degradation and an indicator of intense microbial activity [121]. In terms of microbial activity and community structure in soil, biochar has been quite important [9]. To mineralize phosphorus from nucleic acids, phospholipids and other ester phosphates, plants have the ability to manufacture and release soil phosphatases [121]. After 7 weeks, the treatment with biochar significantly increased acid phosphatase activity by 27.85% when compared to the control therapy. Thus, size of the biochar particles boosted the soil phosphatase activity [122]. According to Pandey et al. [123], the addition of biochar boosted dehydrogenase activity by 7.4–39% and urease activity by 27% when compared to the control group. According to Jia et al. [124] and Pandey et al. [123], applying various types and amounts of biochar activated soil enzymes (such as urea, invertase, and dehydrogenase). According to Song et al. [125], the addition of biochar increased protease, urease, alkaline phosphatase, catalase, and sucrase by 8.4, 13.9, 81.3, 21.7, and 150.5%, respectively. Recent research by Liao et al. [74] demonstrated that the temperature variations of the pyrolysis process may have some control over how biochar affects the activities of soil nutrient acquisition of enzymes. Generally speaking, reduced temperature biochar aids to activate soil enzymes. The 500°C-derived high-temperature biochar’s impact on soil enzyme activity, however, was minimal. Additionally, it was discovered that the properties of the soil and biochar only had a minor impact on how biochar affected enzyme activity. The characteristics of the biochar and the types of soil have a direct impact on the enzyme activity of the soil [74]. Therefore, the foundation of increasing soil enzyme activity is choosing the right biochar based on the qualities of the soil.

Therefore, using biochar in soil may be a viable way to improve nutrient usage effectiveness and, in turn, promote sustainable agricultural production. In order to increase the effectiveness of nutrient usage and ensure sustainable agricultural production, biochar may be a possible soil additive.

5.3 Future prospects of biochar in agro-environment

A lesser cost-effective, efficient way to increase the soil nutrient status, soil carbon sequestration soil quality, soil and water remediation and greenhouse gas reduction, biochar has significant promise in the agricultural and environmental industries. It is still essential to regard each biochar as distinct, characterize it, and test it for crop-soil management practices when using it in on crops because the positive depends on the type of biochar, soil conditions, management practices, etc. Technology advancements, notably in nanotechnology, have made it achievable to produce nano biochar with greatly increased specific surface area and reactive potential, which has significantly increased the carbonaceous material’s effectiveness in the field over a very short period of time. The smaller size of biochar, dropped to a nanoscale, could offer significant potential for environmental remediation due to the tremendous increase in specific surface area for adsorption and chemical reactivity of pollutants in the soil as well as water systems. Further research is needed to determine whether this type of nano-biochar will be effective for soil carbon sequestration and greenhouse gas mitigation, which are frequently fueled by more refractory carbon in biochar. Over time, significant advancement has been made in the use of biochar as a low-cost, carbon-neutral means of reducing pollution in soil and water systems by harnessing its unique adsorption capacity. This promise could be further realized with the development of technology, such as nanotechnology, by comprehending how specially created biochar interacts with metal(loid)s, microplastics, organic pollutants, etc., from the water, soil and gaseous medium. It may be essential to create a specific methodology and contaminant capturing/filtering system before using universal biochar to treat wastewater from municipal trash, water sources, and landfills. Biochar can help reduce environmental dangers, consequently lowering threats to human health, as the manufacturing and usage of dangerous chemicals is projected to increase globally. By estimating the carbon sequestration capacity of various biochars and how it interacts with net greenhouse gas emissions, the potential for use of biochar as a soil supplement could be significantly boosted [11, 12]. In the future, a feasible alternative may involve activating biochar with fertilizers to improve nutrient use efficiency and utilizing its capacity to mitigate climate change in a coordinated manner.

6. Conclusion

Almost any carbon (cellulose-lignin) feedstock, such as byproducts from industrial waste, agriculture, livestock farming forestry etc., can be converted into the highly calcitrant carbonaceous material known as biochar through pyrolysis under oxygen-limited conditions. As a result, biochar is a potentially affordable, carbon resource with a wide range of environmental and agricultural applications. The most important applications for biochar are as a soil amendment, which improves soil’s physical and biochemical properties, raises soil fertility, and increases soil production, especially over time. It may also increase water retention, soil aggregation, microbial activity and pH, thereby enhancing the general quality of the soil. Even yet, there is significant diversity based on the type of biochar and soil-crop environment parameters, with the effect of biochar being obviously apparent in a soil of coarse-texture. Chemical fertilizers and organic sources like manure and compost can be supplemented with biochar; however, the best application technique to obtain the most synergistic benefit is still unknown. The unique adsorption rate and chemical properties of biochar, which can assist to capture and immobilize contaminants like organic pollutants, metal(loid)s and hazardous emerging contaminants like microplastics, have the potential to significantly improve the soil and water quality. In more recent times, biochar has been improved through activation with nutrients, metals, etc., resulting in a grade of biochar suitable for particular uses, even permitting the treatment of pollutants in hazardous sites and landfills. Long-term research studies, the synthesis of cross-site mechanistic experiments using different systems and types of biochar, the assessment of the life cycle of biochar in soil–plant environments, the quantification of biochar persistence and soil carbon sequestration potential, the validation of net greenhouse gas emissions spanning different biochar-management systems, the economic analysis of biochar amendment and application, and the potential integration of carbon markets should be put into cognizance. Due to the presence of the functional group, the increased pH from the mineral content, and the greater volatile matter, biochars have higher CEC. These attributes aid in the adsorption and precipitation of metals, which lowers their availability in soils. Biochar has the potential to be effective in the remediation of organic and heavy metal pollutants. It might also be used in conjunction with bioremediation methods to potentially minimize heavy metals through plants that accumulate heavy metals. Before biochar can be produced and modified at a price that is affordable to farmers all over the world, more needs to be done to lower costs. This is crucial because altering biochar can have a significant impact on its ability to bind or stabilize pollutants. Biochar manufacturing is a product that many farmers ought to take into consideration as a remediation choice because it may turn waste streams into a beneficial resource. Given that cost is the primary issue for the majority of farmers, producers of biochar should look at alternatives to cut costs if they earnestly desire to record significant sales of the product and distributed extensively.

Conflict of interest

The authors declare no conflict of interest.