Biochar Functions in Soil Depending on Feedstock and Pyrolyzation Properties with Particular Emphasis on Biological Properties

Abstract

Biochar effects are strongly dependent on its properties. Biochar improves physical soil properties by decreasing bulk density and increasing medium and large aggregates, leading to faster and deeper water infiltration and root growth. Improvement of the chemical properties of soil is connected with pH neutralization of acidic soils, increase of cation exchange capacity and base saturation, providing a larger surface for sorption of toxicants and exchange of cations. Biochar increases the stocks of macro- and micronutrients in soil and remains sufficient for decades. Biochar effects on (micro)biological properties are mainly indirect, based on the improvements of habitat conditions for organisms, deeper root growth providing available C for larger soil volume, higher crop yield leading to more residues on and in the topsoil, better and deeper soil moisture, supply of all nutrients, and better aeration. Along with positive, negative effects of biochar while used as a soil conditioner are discussed in the review: presence of PAH, excessive amounts of K, Ca and Mg, declination of soil pH. In conclusion, despite the removal of C from the biological cycle by feedstock pyrolysis, the subsequent application of biochar into soil increases fertility and improves physical and chemical properties for root and microbial growth is a good amendment for low fertility soils. Proper use of biochar leads not only to an increase in crop yield but also to effective sequestration of carbon in the soil, which is important to consider when economically assessing its production. Further research should be aimed at assessing and developing methods for increasing the sequestration potential of biochar as fertilizer.

1. Introduction

In accordance with the International Biochar Initiative (IBI), biochar is defined as a solid material obtained by thermochemical carbonization in oxygen-limited conditions [1]. It is mostly produced during the pyrolysis process from organic matter at temperatures 300–700 °C at low access to oxygen [2]. Other pyrolysis products are liquid fuel and pyrolysis gas [3]. During the last decade, biochar has attracted attention in agroindustry and environmental sciences due to the prospects of its application as a soil conditioner and fertilizer, fodder additive, wastewater cleaning agent, renewable energy source and inexpensive sorbent used to remove heavy metals and organic pollutants. Biochar improves soil structure and fertility, promotes water holding capacity and is an important source of microelements and nutrients for plants, in stock and chicken farming [4]. Biochar application decreases methane and N₂O emission [5,6], stimulates the activity of soil microorganisms and plant germination. It is important that biochar shows advantages by amendments in almost all agricultural conditions, including soil type and climate [7]. Contrary to many organic fertilizers, biochar carbon (C) is quite stable and is mineralized very slowly with the release of CO₂ [8,9,10]. This makes biochar important in green sustainability policy, C sequestration, yield improvements and decrease of greenhouse gas emissions.

Biochar properties and hence the consequences of its application in agriculture depend on the biomass used for its production and the pyrolysis conditions [11]. Organic wastes including manure, plant residues, food wastes, sewage sludge, etc. have been used for soil fertilization since ancient times. In natural processes of biomass decomposition, about 90 to 95% of carbon is returned in the atmosphere within a few years (or earlier), whereas pyrolysis of bioresidues results in C sequestration and about half decrease CO₂ emissions [12]. Therefore, biomass pyrolysis dramatically affects the C cycle (Figure 1). A slower rate of biochar mineralization prolongs its effect on soil fertility and decreases demands in other amendments. Biochar can also affect nitrogen cycling by mitigating N₂O, another greenhouse gas also involved in atmospheric ozone decomposition, and by stimulating nitrification via indirect influence on the soil microbial community [13].

2. Biochar Production, Main Sources and Properties

2.1. Feedstock Materials

Biochar is commonly produced by pyrolysis of plant biomass, animal and food residues, forestry wastes, crop residues (nut shells, fruit pits, bagasse, straw [14,15,16,17]), algae [18,19], manures [11,20,21,22], biosolids and sewage sludge formed in wastewater treatment, etc. [23,24,25,26]. In the latter case, the content of heavy metals should be limited to avoid possible contamination of the soil. The elemental content of biochar (mainly, C, N, K, P, Ca and Mg) depends on the feedstock source and pyrolysis parameters (

Table 1. Classification of sources for biochar production.

Biomass Group	Biomass Sub-Groups
Plant biomass	Wood biomass (sawdust, lumps, stems, branches, foliage) Grass biomass (plant planting for biomass production: alfalfa, arundo, bamboo, banana, brassica, cane, cynara, miscanthus, switchgrass, timothy, etc.) Aquatic biomass (marine or freshwater algae; macroalgae)
Organic waste	Animal manure (chicken manure, cow manure, horse manure, pig manure, turkey poultry manure, etc.) Crop and food processing waste (straws, spoiled fruits, shells, husks, hulls, pits, pips, grains, spoiled seeds, coir, stalks, cobs, kernels, bagasse, spoiled food, fodder, pulps, cakes, etc.) Municipal wastes (Sewage sludge, organic fraction of municipal solid waste, paper-pulp sludge, waste papers, chip-board, fiberboard, plywood, wood pallets and boxes)

).

The use of animal manures often results in increased potassium content. Generally, mineral components of biochar vary from 1% (grasses) to about 20% (wastewater sludge). C content of the biomass used as biochar feedstock depends on the source, pre-treatment conditions, storage period, moisture content and other parameters, some of which can be partially considered prior to thermal treatment for unification of the pyrolysis parameters and product characteristics. The classification of biomass applied for biochar production is mainly based on its source, biological diversity and origin [27].

Plant residues, especially forest and grass residues, mainly contain cellulose, lignin and hemicellulose (lignocellulosic biomass (Dhyani and Bhaskar 2018) [28]). Cellulose is present in all plant cell walls. It consists of repeating D-glucose units interacting with each other by hydrogen bonds, providing mechanical strength and chemical stability. Hemicellulose is a less polymerized analog of cellulose containing glucose, galactose, mannose, xylose, arabinose and glucuronic acid units (50–200 monomers per molecule). Lignin is a cross-linked phenol polymer consisting of differently substituted phenylpropane units. Lignin forms the outer layer of cellulose fibers and binds hemicellulose and cellulose within the cell wall. Lignin content varies from 16 (soft wood) to 40% (hard wood) (

Table 2. Composition of lignocellulosic biomass of various origin.

Biomass Source	Composition, %					References
	Cellulose	Hemicellulose	Lignin	Extractives	Ash
Soft wood	41	24	28	2	0.4	[29]
	45–50	25–35	25–35	-	-	[30]
Hard wood	39	35	20	3	0.3	[29]
	40–55	24–40	18–25	-	-	[30]
Pine bark	34	16	34	14	2	[29]
Wheat straw	40	28	17	11	7	[31,32]
Rice husk	30	25	12	18	16	[33]
Hazelnut	41	27	26	-	3	[34]
Orange peels	14	6	2	-	1.5	[35]
Sugarcane bagasse	35	33	25	-	4	[36]
Rice straw	32	24	18	-	1.2	[37]
Banana	60–65	6–8	5–10	-	1.2	[38]
Newspaper	40–55	25–30	18–25	-	-	[39]
Corn cobs	45	35	15	-	-	[40]
Sponge gaurs fibers	67	17	15	-	-	[38]

).

In addition to the three main components, organic matter of biomass contains some extractives (alkaloids, essential oils, glycosides, pectin, phenolics, simple sugars, terpenes etc.) They can be removed from biomass by organic solvents without hydrolysis. Details of the biopolymers’ chemical structure and their physical properties as well as examples of the ash content of the biomass and ash are presented in review [41].

Organic matter of biosolids (more than 50% of dry matter) mostly contains hydrocarbons, amino acids and lipids but a rather small percentage of lignin and cellulose [26]. Lignin and cellulose content is higher in urban wastes but remains significantly below the level typical for wood or crop residues. Biosolids are also rich in nitrogen, mostly present in organic form (up to 6% of dry weight) and phosphorus (up to 3% of dry weight), but they can be partially lost in combustion and pyrolysis (the loss of P is less than the loss of N). Biosolids themselves and biochar produced from biosolids are characterized by a high ash content, which complicates the use of this parameter for calculations of the biochar yield.

The classification of biochar feedstocks mostly uses moisture content as one of the key features. Freshly cropped biomass, including agricultural plant and forest residues, sewage sludge, algae, animal manure, etc., typically contains more than 30% of water (

Table 3. Chemical composition of algae, sludge and manure biochar feedstocks.

Biomass Source	Chemical Content (wt.%)			Proximate Analysis, wt.%				Reference
	Proteins	Lipids	Carbohydrates	Moisture	Volatile Matter	Fixed Carbon	Ash
Chlorella	53.8	1.0	37.1	0	76.4	14.5	9.1	[47]
Cladophora glomerata	27.8	5.3	32.4	4.5	46.3	14.7	34.5	[48]
Sewage sludge	26.3	27.3	22.9	3.6	53.9	6.2	36.3	[49]
Sewage sludge	-	-	-	16.8	64.4	10.8	24.8	[50]
Chicken manure	31.6	2.1	34.5	-	-	-	15.9	[51]
Swine manure	22	9.1	39.1	-	-	-	15.1
Cow manure	18.1	8.7	52.6	-	-	-	12.0
Cattle manure	-	-	-	-	64.6	20.7	14.7	[22]
Chicken manure	-	-	-	-	64.8	14.9	20.2

). They are referred to as wet biomass. Agricultural wastes with a water content below 30% are classified as dry feedstock. Wet biomass can be pre-dried using technologies developed for drying other biomaterials. However, in most cases, such technologies are labor consuming and decrease the total economic effect of biochar application [42]. Biomass is classified on that specially harvested as bioenergy crops (bamboo, sorghum, willows, Miscanthus, switchgrass [43,44] and agricultural wastes [28]. The use of energy crops provides a high yield of the target product and minimal preliminary treatment is required. Thus, no pre-drying is commonly needed. The ash content of such a biomass can significantly vary depending on the harvesting time and planting density complicates the selection of optimal pyrolysis parameters and feedstock unification. Biomass from various wastes is more variable regarding its content. This type of feedstock involves wastes formed in agriculture, forestry, food production, sewage sludge from water treatment plants, animal manure, etc. Their conversion to biochar is always beneficial because it reduces expenses for their environmentally safe storage and/or processing. On the other hand, parts of such wastes that do not contain toxic metals and organic contaminants can be applied on land as soil amendments and organic fertilizers with no or minimal treatment. Thus, full processing of the plant biomass into biochar seems exhausting. It should also be taken into account that crop residue removal negatively affects the soil organic carbon pool, which should be reimbursed by agrochemical measures [45]. Preliminary treatment of biomass is one of the key factors affecting both the pyrolysis products and the repeatability of the technological parameters of biochar production. Moisture content, preliminary crushing or, vice versa, pressing of the feedstock are most frequently utilized on the preliminary stage of pyrolysis [46]. Appropriate protocols influence both the economy and preferable directions of biochar application.

2.2. Pyrolysis Conditions

The technological process of biomass pyrolysis starts with supplementary drying of the substrate if its moisture is higher than optimal. The following heating removes the natural volatile components present in the raw material. After that, decomposition of the biopolymers present in biomass results in consecutive increases in the C content and release of permanent gases (CO₂ and nitrogen oxides, ammonia and methane) and volatile organic compounds of decomposition. The latter can be condensed and then used as liquid fuel. The solid residue is biochar. The reaction pathways of the formation of gaseous and liquid pyrolysis products partially compete with each other and differ depending on the pyrolysis parameters (residence time, heating rate, pyrolysis temperature) and substrate properties. Thus, fast heating increases the yield of pyrolysis gases and fuel, whereas slow and rather long pyrolysis is more appropriate for predominant carbonization and maximum biochar yield.

Thermal treatment of biomass can be performed using several regimes, which are chosen depending on the type of feedstock, its pre-treatment, desired properties and future application of biochar. They are classified as slow, fast, flash and intermediate pyrolysis, dry torrefaction and hydrothermal carbonization, which are performed in the absence or limited access of oxygen. Pyrolysis can also be accelerated by microwave treatment. The characteristics of pyrolysis reported for some feedstock and regimes are presented in

Table 4. Biochar yield under various production conditions and feedstock used.

Pyrolysis Type	Feedstock	Pyrolysis Temperature, °C	Biochar Yield, %	Ref.
Slow	Rice straw	300	50	[52]
		500	39
		700	36.5
Slow	Switchgrass	400	48	[52]
		600	25
Slow	Canola straw	350	24	[53]
	Rice straw	350	33
	Soybean straw	350	32.5
	Pea straw	350	32
Slow	Poultry litter	300	60	[54]
		400	52
		500	48
		600	46
Slow	Magnolia leaves	300	62	[55]
	Apple wood	600	25
	Spotted gum wood	400	51
Slow	Black locust wood	300	42	[56]
		500	24
Slow	Pine chips	550	30	[57]
Slow	Sawdust	550	28	[58]
Fast	Pitch pine wood chips	300	61	[59]
Fast	Pine sawdust	400	55	[60]
		800	18
Torrefaction	Coffee ground	300	81	[53]
	Microalgae residues		56

as an example.

Long pyrolysis is a traditional technology of biochar production with heating at 300–700 °C by the rate of 5 to 20 °C/min for more than one hour. In traditional biochar production, carbonization can take place for more than one week. It produces a maximum yield of biochar against other technologies mentioned above (more than 50% of dry weight). Hard wood can be heated up to 1000 °C with the final carbonization level. Meanwhile, biomass from agricultural waste and other sources with rather mild temperature of ash melting is normally heated at not more than 700 °C [61,62].

Fast pyrolysis is mostly used for the production of liquid fuel: volatile products are condensed by rapid cooling to avoid polymerization of by-products. The biomass is heated for several seconds to pyrolysis temperatures. Gases contain various quantities of carbon oxides, methane and hydrogen [55]. Fast pyrolysis may result in incomplete conversion of biomass so that up to 9% of hydrocarbons (mostly in the form of cellulosic and hemicellulosic fractions) remain in the product. Their presence diminishes later C sequestration in soil. Raising the pyrolysis temperature decreases the content of this fraction but due to lower amounts of biochar obtained.

Flash pyrolysis [56] is performed by ignition of the press biomass at elevated pressure (1–2 MPa). The reactions provide heating of the feedstock up to 300 to 600 °C for about 30 min. The yield of biochar increases with temperature and pressure [57].

Dry torrefaction is a process in which biomass is heated in an inert atmosphere at 200 to 300 °C for the period of 30 min to 2–3 h. Up to 30% of biomass is lost, including a 10% decrease in energy content. Polysaccharides are depolymerized to solid residues with a low O/C ratio. Torrefaction assumes slow heating and can be categorized as mild pyrolysis. Torrefaction is commonly used for pre-treatment of biomass prior to its combustion or gasification. The product of dry torrefaction still contains significant amounts of volatile organic matter from the feedstock.

Gasification is partial combustion of the biomass at a high temperature (600 to 1200 °C) for several seconds [63]. As follows from the name of the process, gas consisting mostly of methane and hydrogen is formed. Up to 10% of the dry weight of the biomass is converted into biochar. Unlike other biochar sources, it can be contaminated with polyaromatic hydrocarbons and heavy metals so that it cannot be recommended for application as a soil amendment. Nevertheless, its application improves the physical properties of the soil and positively affects microbial activity [58].

Hydrothermal carbonization is performed at 180 to 260 °C with biomass dispersed in water under elevated pressure (2–6 MPa) for 5 to 240 min [64,65]. The char produced has a higher carbon content than the products of dry pyrolysis [59]. Its characteristics depend on the reaction temperature, pressure, residence time and biomass/water ratio. An increase in the temperature results in a higher yield of liquid products. At 250 °C, up to 70% (dry weight) of biochar are obtained. Preliminary treatment of the feedstock with mineral acids and salts increases the yield and decreases optimal temperature and pressure due to better solubilization of the biomass and suppression of hydrogen bonds between the biopolymer molecules [60,66].

Chemical reactions during biomass pyrolysis depend on substrate composition and heating conditions. Cellulose, hemicellulose and lignin comprising biomass undergo various conversion paths, including depolymerization, cross-linking and decomposition of the fragments. Hemicellulose decomposes at 200 to 260 °C, cellulose breaks at 240 to 350 °C and lignin starts decomposing at 280 to 500 °C [67]. Slow pyrolysis at rather low temperatures results in decomposition and carbonization of cellulose, whereas fast pyrolysis promotes volatilization and formation of levoglucosan [68], which is later decomposed by C-O, C-C scission and dehydration to low-molecular compounds [58,69]. In a similar manner, hemicellulose undergoes cleavage to 1,4-anhydro-D-xylopyranose yielding furfural and 2C-, 3C-fragments [70]. Lignin is involved in drying and fast and slow degradation steps result in the formation of a number of compounds, e.g., phenolics [63]. Regarding other elements, sodium and chlorine are released at a relatively low temperature, whereas calcium and magnesium can be bonded to organic species and removed at higher temperatures [71]. Alkali and alkali-earth metals exert autocatalytic effects on biomass pyrolysis due to the acceleration of secondary destruction of volatiles produced in pyrolysis and formation of more gases affecting biochar cracking [64]. Phosphorus, sulfur and nitrogen are mostly bonded in less complex organic compounds present in the plant cells and decomposed in slow pyrolysis at rather low temperatures. What becomes of various chemical elements in biomass during its pyrolysis and mechanisms of chemical reactions responsible for the formation of organic species, biofuel and biochar is described in detail in a review [65].

2.3. Improving Biochar Properties by Modification Approaches

Biochar application often assumes pre-treatment aimed at introduction of additional functional groups, increase of porosity and specific surface area [72]. Such a treatment is also desirable for the application of biochar for catalyst production because it increases the adsorptive capacity of potential catalytic sites, such as transient metal cations. Meanwhile, biochar application may have unexpected and undesired effects, e.g., formation and accumulation of toxic species (polyaromatic hydrocarbons). The general classification of biochar pre-treatment approaches is presented in Figure 2.

The introduction of additional functional groups starts from partial oxidation of biochar. Hydrogen peroxide [72], ozone [73], potassium permanganate [74] (and nitric acid [75] are used for this purpose. Oxidation products contain carboxyl, phenolic, peroxide fragments and lactones. Oxidation increases the hydrophilicity of the char surface. Similar results can be obtained by heating biochar in an inert atmosphere with low amounts of oxygen. The appropriate changes depend on the treatment temperature. Thus, post-treatment of chars obtained from carbohydrates and activated by heating to 300 °C in the presence of oxygen was investigated by FT-IR spectroscopy and X-ray fluorescence analysis [76]. Treatment below 450 °C promotes opening inner space without mesopore widening and results in preferable surface adsorption of inorganic components. Treatment temperature increase to 900 °C removes most oxygen-containing functional groups and stimulates the formation of more common aromatic structures.

Oxidation within the range of 450 to 900 °C makes mesopores wider and deeper. The contribution of the adsorption of inorganic species on initial organic matter decreases. Metal cations remain attached in pores and on the surface due to electrostatic interactions with aromatic π-electron systems. Meanwhile, full pore volume, surface area and capacity toward anions (phosphates) can decrease during high temperature pyrolysis [58,69]. High temperature post-treatment decreases the biochar mass by about 10% per each 100 °C. In turn, this increases the yield of mineral residue and the content of metal oxides and carbonates [77].

In addition to oxidation in the presence of small amounts of oxygen, micropore widening is achieved by controlled biochar gasification. For this purpose, it is treated with water vapor or carbon dioxide. This increases the internal volume of micropores, but the number of mesopores increases insignificantly. Thus, post-activation of biochar from coconut shells has been described with various rates of vapor and carbon dioxide supply with simultaneous microwave treatment [78]. The specific surface square was increased to 2000 m²/g, and micropore volume exceeded 80% of the total internal volume of the activated product. Simultaneous microwave treatment and water vapor/CO₂ treatment increased the product yield twofold [79].

Some catalysts, i.e., zinc chloride and phosphoric acid, make for porosity increase and are added to raw biomass prior to pyrolysis. Being absorbed by biomass, they prevent gumming in the first heating stages. Phosphoric acid hydrolysis glycoside bonds in hemicellulose and cellulose and breaks ether bonds in lignin. The reactions mentioned are often followed by degradation and dehydration of the cellulose structure and condensation of oligomeric by-products. In such a conversion, carbon oxides and methane are released. Phosphoric acid can also form esteric bonds with hydroxide groups of biomasses if temperatures do not exceed 200 °C. This promotes the cross-binding of polymeric chains [80]. As a result, carbon retention, sorption capacity and pore formation are synchronously improved [81]. Unlike phosphoric acid, zinc chloride acts as a dehydration agent. It decreases carbonization temperature for cellulose, hemicellulose and lignin and changes paths of their following chemical conversion in favor of gumming suppression and open pore formation. ZnCl₂ can then stimulate wood swelling due to its implementation in the biomass and depolymerization processes. Zinc chloride remains in melted conditions up to the end of pyrolysis (melting point below 700 °C) and hence inhibits C rearrangement in the structures to be obtained. Removal of ZnCl₂ from biochar leaves well-pronounced micropores. Unlike other chemical agents, zinc chloride maximizes the increase of the pore volume in biochar. Other additives, e.g., FeCl₃, mostly affect biomass graphitization and the formation of graphitic particles with a high specific surface area (up to 2000 m²/g) [82].

Biochar amination is the second most widespread method of biochar functionalization. It simplifies CO₂ capture and toxicant sorption [83,84]. Amination starts with preliminary biochar oxidation, followed by its ammonia addition [85,86]. Polyethyleneimine [87,88] is involved in similar reactions. In addition, amino groups are formed by nitration of biochar followed by reduction of nitro groups introduced with dithionite [62]. Aminated biochar finds applications in binding metal cations. In addition, amino groups change the hydrophilicity of the surface and improve biochar wettability, which is important for its field application. It should be mentioned that in addition to chemical modification, increased nitrogen content is achieved by an environmentally safer approach based on pyrolysis of biomass enriched with nitrogen-containing materials, e.g., chitin [89] and chitosan [90]. Such modifiers also increase sorption of metal cations [91] and are applied in the preparation of composite materials with the implementation of magnetic particles [92].

Biochar sulfurization (introduction of sulfonate groups) can be achieved by post-treatment of biochar with sulfuric acid, filtration and centrifugation of target products [93]. They are used for sorptional removal of toxic metals and as solid acids in soil remediation and wastewater treatment [79,94].

Most of the modified biochar particles have negative surface charges associated with a few carboxylic, carbonyl and sulfonate groups. Therefore, they can adsorb metal cations that are additionally bonded by complexation with amino (amide) groups. High ash content in the substrate increases the accumulation of metals, whereas higher pyrolysis temperatures affect this parameter in the opposite way due to partial destruction of acidic groups. Instead, the products of high temperature pyrolysis adsorb increased quantities of organic species, e.g., catechols or humic acids [80]. This is referred to as the general increase of the pore volume [95]. However, they do not effectively bind anions, e.g., nitrates, arsenates or phosphates. To avoid this limitation, metal oxides can be introduced into the biochar structure. The feedstock is soaked in a solution of metal nitrates or chlorides. Their following heating in the presence of atmospheric oxygen to about 300 °C releases nitrogen oxides (chlorine) so that metals are fixed in the biochar material in oxide form. Meanwhile, impregnation of metal oxides regularly decreases the biochar surface area due to partial filling of the pores [96]. Treatment of the biochar obtained from the corn comb by slow pyrolysis at 500 °C with Fe(III) nitrate resulted in 20 times higher accumulation of As [97]. Hybrid material obtained by pyrolysis of the mixture of naturally occurring hematite (γ-Al₂O₃) and pinewood biomass showed a much stronger ability to remove arsenic from wastewaters than biochar with no impregnation [98]. A similar efficiency was registered for biochar impregnated with gelatin containing ferrite magnetic particles [99]. Introduction of AlO(OH) [100], manganese oxides [101] and MgO [102] was also described for soil remediation and toxic metals removal from the sediments and wastewaters. It is interesting to mention that biochar containing MgO was obtained from tomato tissue biomass that accumulated Mg from irrigation waters [103].

In addition to metal oxides, chemical modification of biochar with clay minerals, e.g., kaolinite, montmorillonite and bentonite, has been described as increasing accumulation of ammonia and phosphates. The product obtained by mixing biomass with appropriate materials prior to pyrolysis is enriched with Al, Fe and Na [104,105]. Bentonite keeps its structure in the final product, whereas montmorillonite is dehydrated. Maximum sorption capacities of biochar for ammonia and phosphates exceeded 12.5 and 105 mg/g, respectively.

Biochars were also mixed with carbonaceous nanomaterials, e.g., graphene oxide [106] and carbon nanotubes [107]. They increase the porosity of the product, specific surface area and surface concentration of oxygenated functional groups. As a result, they accumulated metal cations better than the biochars themselves. Hybrid materials described have been utilized for the removal of Cr(VI) and Hg(II) from the wastes of sugar production.

2.4. Biochar Properties

2.4.1. Physical Properties

The physical properties of biochar are pre-determined by the processes taking place during carbonization. Biochar obtained in low temperature pyrolysis consists of graphitic layers. The distance between the layers, as well as their specific surface area, increases with the carbonization temperature [108]. Aromatic carbon atoms dominate the biochar structure obtained at 350 °C with rather small contribution of alkyl oxygen and C atoms. An increase in the pyrolysis temperature to 500 °C converts alkyl fragments into aryl groups with a low H/C ratio. Hydrothermal conversion results in the formation of roundish particles. The model study showed that they resulted from the preliminary formation of lignin-like structures with a high number of oxygen-containing functional groups (ether, quinone and pyron groups) distributed within hydrophobic internal layers covered by hydrophilic coats [46]. The nature and distribution of the surface functional groups were established using FTIR spectroscopy [109] and XRD analysis [110]. Biochar obtained at low temperatures contains a shell formed by furan-like pentatonic rings (60% of carbon atoms) bridged with sp2- (sp3-) C atoms [111]. At higher temperatures, such layers are converted into condensed aromatic rings to form graphitic particles. Based on solid-phase 13C NMR, FT-IR spectroscopy and GC-MS, the conversion starts at 270 °C and is accompanied by the evolution of volatile furan derivatives [112]. Similar behavior has been reported in other works utilizing various feedstocks and pyrolysis regimes [77,113,114].

The biochar particle morphology results from the chemical reactions during pyrolysis. At low temperature (at about 300 °C) free radicals appear [115]. The following heating destroys cellulose with the formation of anhydrosugars that are less reactive than radical products of the bond cleavage. Being volatile, such intermediates are condensed in the pores of biochar and then in secondary carbonization form a typical crystal lamellar structure [116]. The surface groups of biochar depend on the pyrolysis rate. Oxygen containing hydroxide and carboxylic groups are mostly present on the materials obtained in fast pyrolysis, whereas aromatic C-H bonds are mostly found on the surface of slow pyrolysis products [117]. Heating changes ratio of main components and structural properties (Figure 3).

The structure of the biochar particles changes slightly with the heating rate but is sensitive to the final pyrolysis temperature. With its increase, crystal fragments become bigger and their internal structure—more regular [118]. The variety of the structure fragments is higher in fast pyrolysis. As gases are released from the solid matrix, bulk density decreases. This results in a lower bulk density that also takes into account free space between the biochar particles. Thus, pyrolysis of woody biomass decreases density twofold against feedstock at the pyrolysis temperature of 350 °C, which is considered a lower limit of fastest changes of biochar porosity [119]. Similar changes in porosity were observed for grass biomass heated at 350 to 700 °C [120]. For the plant substrate rich in hemicellulose, the temperature promoting pore formation can be decreased due to the lower thermal stability of hemicellulose. The porosity changes caused by lignin decomposition at higher temperatures are more complex. On the one hand, decomposition of lignin can result in even higher porosity than that of hemicellulose carbonization. On the other hand, the shrinking of solid residue influences this parameter in the opposite direction [121].

It should be mentioned that the lower density of biochar against substrate is related to bulk density. The so-called true density describing solid phase disregarding pore space increases with residence time and pyrolysis temperature. Shrinking biomass during pyrolysis can also result in incomplete removal of volatiles, especially in low temperature pyrolysis (Figure 4) [122].

Changes in the specific surface area during pyrolysis follow porosity increase. Thus, it starts raising at 400–500 °C until 900 °C from about 10 to 100–500 m²/g depending on the feedstock and the pyrolysis conditions [123,124]. The following heating above 900 °C leaves surface area constant or slowly decreasing. This was attributed to changes in the particle structure resulting in pore widening or collapse of some pore walls. In addition, decreased area can be influenced by ash melting and fusing [125]. Similar consequences are expected from secondary reactions between char and volatiles remaining entrapped in the inner pore volume [126] and from crystallization of amorphous carbon into graphite [127]. Thus, the average pore size of the biochar particles obtained from sewage sludge increased from 4.7 nm (feedstock) to 8 and 28 nm at 500 and 600 °C and then decreased to 16 nm at 700 °C [128]. The surface area of the wood pyrolysis products obtained at 450 °C increased from 4 to 23 m²/g with the residence time varied from 10 to 60 min, respectively [19]. Biochar from rubber wood sawdust increased the surface area from 1.93 to 5.49 m²/g with the pyrolysis temperature raised from 300 to 700 °C [129].

Figure 4. Molecular structure of biochar from plant biomass across a charring temperature gradient; (A) Characterization of organic phases. (B) Char composition from gravimetric analysis [130]. With permission of the American Chemical Society.

Figure 4. Molecular structure of biochar from plant biomass across a charring temperature gradient; (A) Characterization of organic phases. (B) Char composition from gravimetric analysis [130]. With permission of the American Chemical Society.

In addition to porosity, the surface square depends on the particle size distribution [131]. The average size of the biochar particles regularly decreases with the pyrolysis temperature due to weakened macromolecular structure and formation of fragile particles likely to be broken [132]. Thus, pitch pine pyrolysis increased the fraction of particles below 500 μm from 70 to 95% (pyrolysis at 300 °C) [133].

2.4.2. Chemical Properties

The chemical composition of biochar is mostly determined by carbonization resulting from the loss of hydrogen and oxygen containing groups. The progress of carbonization is reflected by changes in the atomic rations. Regarding three main elements, i.e., carbon, oxygen and hydrogen, such changes are expressed by a diagram presented first by D. van Kleveren [134] (Figure 5). In natural carbonization there are various natural ways of carbonization. However, the release of oxygen is twofold faster than that of hydrogen until coal is formed. After that, the decrease in the H/C ratio becomes constant if the oxygen content is maintained at a low level. In technical conditions, the relative decrease of the O/C and H/C molar ratios can vary, but contrary to natural conditions, the appropriate rates do not change within the pyrolysis duration. The van-Kleveren diagrams are present in many reviews and original works devoted to biochar production. A summary of 212 experimental results obtained for various pyrolysis conditions and biomass origins is presented in [135]. Maximum decrease of the atomic ratios corresponds to a 250 to 350 °C interval in which the O/C and H/C ratios decrease twofold. The following removal of oxygen requires heating to 700 °C whereas full removal of hydrogen requires even higher heating. Van Kleveren diagrams reflect a general assessment of biochars and related products from the point of view of energy storage and energy losses in combustion.

Carbon, hydrogen, oxygen and minor quantities of nitrogen are the main components of biochar. Elemental composition follows changes in the atomic ratios corresponding to the appropriate temperature of pyrolysis and heating rate. In addition, moisture, silicon, phosphorus and metal residues should be considered. The particular content of biochar highly depends on the substrate used. Carbon content varies commonly from 45 to 60 wt.%, hydrogen from 2 to 5 wt.% and oxygen at the level of 10 to 20% [136]. Inorganic components (minerals) are present in a higher amount in plant resides and algae, and in a lower amount in woody biomass. Biochar obtained from plants contains more carbon than that from manure (

Table 5. Chemical properties of biochar produced from different feedstocks by pyrolysis (Nguyen et al., 2010) [137].

Feedstock	Oak Wood		Corn Stover		Poultry Litter
Pyrolysis temperature	350	600	350	600	350	600
pH	4.80	6.38	9.39	9.42	9.65	10.33
CEC, mmol/kg	294	76	419	252	1121	59
C, %	75	88	60	71	29	24
C/N	455	489	51	66	15	25
P, mg/kg	12	29	1890	2114	21,256	23,596
Fixed C	38	71	40	60	1.6	0.1
H/C	0.55	0.33	0.75	0.39	0.57	0.18
O/C	0.20	0.07	0.29	0.10	0.41	0.62

).

In most cases, carbonization of biomass promotes increase of the pH of the product to final pH = 8–10. The higher the pyrolysis temperature, the more the pH shift. In a similar manner, pH correlates with ash content. Thus, the use of woody biomass results in the formation of the product with a pH lower than that of manure carbonization products rich in mineral components. The consideration of the pH is complicated by the changes in acidity observed within the time after implementation of biochar in soil. As an example, the pH of the biochar obtained from oak chips decreased within a year of application to soil, whereas the coals from corn wastes increased from 6.7 to 8.1 [138]. Such relationships are related to the destruction of acidic functional groups at high temperatures of pyrolysis. Another factor influencing the chemical composition of biochar is particle size. A number of authors have found that different particle sizes of raw materials can lead to different heating during pyrolysis, so a smaller fraction could be more carbonated than a larger fraction and contain more C, as well as Ca, Mg and K ions [131,139].

3. Biochar Functions in Soil

Biochar is commonly used in rehabilitating infertile or degraded soils because it improves soil physicochemical characteristics, reduces greenhouse gas emissions, increases fertilizer application efficiency, increases yields, and absorbs organic pollutants [140]. Biochar affects physical and chemical soil properties directly, and biological properties mainly indirectly. The direct effects are mainly connected with feedstock sources and pyrolysis conditions.

3.1. Effects on Soil Physical Properties

The physical properties of biochars (microstructure of biochar particles) are crucial for their effects on soils. The biochar bulk density varies from 1.5 to 2 g/cm³ [117] and that of soil particles varies from 2.4 to 2.8 g/cm³. The application of 30 t/ha of woody biochar reduced the soil bulk density by 14% on a fallow land [141]. The bulk density decreased linearly with the biochar application rate in the range from 0 to 100% vol. from 2.62 to 1.60 g/cm³ [142]. In most cases, linear dependence of the biochar amendment and soil density decrease were observed with maximum effect for application of about 60 t/ha and no influence of small application rates (<10 t/ha) [143]. In coarse textured solids, reduction of the bulk density caused by biochar is higher than coalescence that in fine textured soils. The largest decrease, by 31%, reported in 2016 was found for sand [144]. This is explained by the mixing effect due to the rather high difference in the above parameters of the soils and chars used.

The mean size of the soil aggregates used for estimation of many important soil properties, e.g., water filtration, macropore development, soil particle cracking, etc., is also expressed as the mean size (diameter) of the aggregates present in amended soil. Biochar increases wet aggregate stability with no respect to the soil type and application rate due to the ability of organic matter to establish bonds between soil particles. The release of binding molecules as well as direct aggregation of biochar particles with organomineral complexes results in the formation of aggregates and their following coalescence. The effect is more pronounced in sandy soils [145,146]. Aggregate size increases have also been observed in grassland and volcanic soils with organic carbon levels of >10% [147,148]. Clay mineralogy is another factor altering the biochar influence on soil aggregates. Similar relationships were found for dry aggregation stability that requires a rather high application rate (20–25 t/ha) of biochar and was preferable on sandy soils [149,150].

Changes in soil bulk density affect soil porosity. This is mostly referred to as the soil C content, which varies with biochar amendment and hence influences soil porosity [151]. The porosity is normally increased due to the combination of various factors, e.g., reduced bulk density, increase of the aggregation, changes in the interaction with mineral components, and denser packing of the soil particles. For example, fine-textured soil (clay) is less affected by bio-char than coarse-textured soil (sandy) in terms of the degree of change in bulk density. To reliably reduce soil density, the application rate of biochar should vary in the range of 1–5% [152]. It has been shown that biochar can increase soil porosity by up to 19% and reduce bulk density by up to 35% [153]. To increase the overall porosity of the soil and reduce its bulk density, it is necessary to use a highly porous biochar, i.e., biochar obtained at low and medium temperatures and obtained from plant raw materials. Thus, biochar obtained by wheat bran pyrolysis at 800 °C increased soil porosity to a lower extent than that obtained at 1200 °C [154].

Tensile strength refers to the inherent capability of the soil to resist external forces that can cause fractures and ruptures. Biochar application on clayey soils decreases tensile strength because clays have viscoelasticity exceeding that of biochar. The tensile strength decreased from 64 to 32 kPa after 50 t/ha of biochar application and to 19 kPa for 100 t/ha [155]. In sandy soils, however, biochar added alone or together with organic fertilizers improves soil structure and increases tensile strength due to better aggregation [156].

3.2. Effects on Soil Chemical Properties

Great attention has been paid to studying the use of biochar for the remediation and restoration of polluted soils [157,158,159,160]. Its beneficial effects are most often attributed to immobilizing toxicants and reducing the bioavailability of toxins to soil organisms. An increase in the pyrolysis temperature increases the carbonization degree of biomasses and the surface of biochar particles, which contributes to an increase in their sorption capacity for organic pollutants and hence a decrease in their potential hazard to soils. In particular, a decrease in the bioaccumulation of polychlorinated biphenyls by soil organisms was found when activated carbons were introduced into soils, supported by an adequate change in the equilibrium concentration of the toxicant in the study of their extraction from water systems [161,162]. A similar effect of biochar on the equilibrium concentrations of organic toxicants was established for DDT [163] and polycyclic aromatic compounds [164]. Sorption of organic toxicants does not preclude their subsequent release from the bound form over time. Biochar also binds heavy metals in a carbon matrix. In particular, a relationship has been registered between a decrease in the mobility of copper, cadmium and nickel during the period of decomposition of biochar introduced into the soil and the content of organic matter in it [165]. Therefore, its production and application to soils can be considered an effective strategy in limiting the circulation of such elements in the environment. A fairly large number of examples of biochar use with a view to suppressing the phytotoxic properties of heavy metals have been described. For example, a regular decrease in the phytotoxic concentrations of cadmium and zinc was registered in a 60-day experiment with biochars introduced into silt masses [166,167]. The amounts of retained heavy metals correlate with the cation-exchange capacity of biochar, which provides evidence of the mechanism of their retention on the carboxylate groups of biochars. At the same time, an inverse relationship was found for copper, associated with an increase in soluble organic carbon in soil moisture when biochar from hardwood was introduced. This proves that the specific effects of biochar application on the bioavailability of toxic soil elements require verification for actual application conditions. There is a necessity to certify the maximum allowable levels of toxic elements in biochar [168]. The ability of biochar to remove arsenic from wastewater has been found [169]. Unlike metals, arsenic is present in soil in the form of oxo anions. Accordingly, the mobility of arsenic in soils does not decrease with pH increase, as in the case of copper, zinc, lead and cadmium, but increases. Its retention in soils is due to the presence of aluminum, iron and manganese oxides in them, which react with arsenic forms. Thus, there is a possibility of increasing the bioavailability of this element due to the fact that the introduction of biochar increases the pH and soluble carbon of soils. In laboratory tests aimed at assessing the influence of biochar on the arsenic content in wastewater moderately polluted with heavy metals, the effect of biochar introduced in an amount of 30 vol.% was not reliably detected [170]. Additional benefits in the removal of arsenic can be expected from biochar enriched with iron oxides, and if used in conjunction with compost, a simultaneous decrease in the bioavailability of other common heavy metals can be achieved [171]. Similar patterns were revealed for the binding of chromium (VI) compounds.

There are works devoted to the use of biochar produced from chicken manure with the aim of removing metals from the environment. Thus, its high efficiency with respect to copper (II) ions has been proven. The share of the removed metal increases with biochar mineralization and varies from 1.3 to 26 mg/g of the sorbent. The use of chicken manure as a raw material provided a higher capacity of biochars compared to coals produced from corn waste, pine wood, and eucalyptus [172]. A decrease in copper bioavailability was also observed when chicken manure biochar was applied to copper-contaminated sandy loamy soils. Biochar was prepared by slow pyrolysis at 500 °C for 2 h [173]. It was characterized by a high content of phosphorus (19.4 g/kg) and potassium (17.2 g/kg), pH 9.1 (water suspension 1:5) and a specific surface area of 11.5 m²/g. The interaction of biochars with copper was also controlled by changing the growth rate of O. picenis; the indicator species is a metallophile. In model experiments in seed germination in a greenhouse, biochar was applied in the amount of 5 and 10%. In addition to the effect on plants, a decrease in the concentration of copper in soil moisture and an increase in soil microbial activity are described. The manifestation of the phytotoxic properties of biochar with a high content of a number of other trace elements (phosphorus, boron, zinc, manganese) in relation to plants sensitive to their presence in the soil was registered [174,175]. There are other risks associated with the agronomic use of biochar. Thus, it was found that the introduction of 1% biochar made of eucalyptus chips halved the bioavailability of two insecticides (chlorpyrifos and carbofuran) [176]. Sorption of pesticides can help in reducing crop contamination, but it reduces the efficacy of their application against insect pests. In addition, the biochar itself may contain phytotoxic compounds [177]. The influence of biochar on the content of polycyclic aromatic hydrocarbons (PAHs) is the focus, as they are partially formed in the pyrolysis of organic matter, the precursor of biochar. Adsorption or other protection against microbiological degradation of PAHs limits their removal from polluted soils; saturation of biochar pores with other adsorbed substances reduces its effect on the removal of those compounds into groundwater. It is not entirely clear how the influence of biochar on the microbiological community and its protective effect on PAHs correlate, as both adsorption and microbiological activity depend in a complex way on soil pH after the application of biochars. Examples of agrochemical and ecotoxicological consequences of biochar application to soils are given in

Table 6. Agrochemical and Ecotoxicological Consequences of Biochar Application to Soils.

Biochar	Application	Effect		Ref.
		Positive	Negative
Wheat straw	0–40 t/ha + 300 kg N/ha	Increase in pH, C, N, decrease in bulk density, decrease in N2O emission	Increase in methane emission (34–41%) when applying 40 t/ha	[179]
Eucalyptus chips	0–90 g/kg of soil + rhizobium culture	Increase in biological nitrogen fixation, pH, bioavailability of soil B, Mo, R, Ca, P at 60 g/kg	Decrease in available nitrogen and yield when applying 90 g/kg of biochar	[138]
Paper production waste	10 t/ha + Nutricote fertilizer 12.5 g/250 g of soil (15% N, 4.7% P, 8.9% K)	Increase in organic carbon, pH, exchangeable Ca, and in total carbon	Decrease in the yields of wheat and radishes	[180]
Sawdust	20–60% of biochr	Increased sorption of atrazine and acechlor by 5% biochar	Increase in the yield of N2O and CH4 when applying 20% of biochar	[181]
Wood	13–52 t/ha	–	Decrease in the efficiency of herbicides when the dose of biochar increases	[182]
Eucalyptus wood	1% of coal	Decrease in bioavailability of insecticides, and in the accumulation of herbicides by plants by 10–25%	Increase in the resistance of herbicide residues in soil fertilized with biochar	[176]
Eucalyptus wood	0.5% of coal	Decreased bioavailability of chlorantraniliprole	Biochar influence on pesticide bioavailability depended on soil type	[183]
Corn cobs	Fertilizer from 2–20 g/kg of coal + N (0–100 mg/kg) + P (0–20 mg/kg), biochar 100 and 200 t/ha	–	Additional nitrogen application is required to increase the crop yield	[184]
Peanut husk	100 and 200 t/ha	Increased yield and drought tolerance Chenopodium quinoa	A positive effect is achieved only at the maximum application rate (200 t/ha)	[185]

[178].

In general, an increase in the pyrolysis temperature during the production of biochar increases the sorption efficiency in relation to organic pollutants. Presumably, this occurs as a result of an increase in the specific surface area and microporosity of coals.

3.3. Effects on Soil Biological Properties

Although the positive effect of biochar on soil fertility and agriculture ecosystems mainly refers to the pH changes in acidic soils and improved nutrient retention and aeration, remarkable influence of biochar amendments on microbial communities and soil animals has been reported [186,187,188]. Changes in microbiota composition and abundance also follow soil structure improvement and nutrient availability caused by biochar application (indirect biochar effect). All of these factors influence plant growth and crop quality and consequently, more input of rhizodeposits into the soil. The positive influence of biochar amendments can be counterbalanced by toxic metals contained in the pyrolysis products, especially those derived from sewage sludge or wastewater sediments. The consideration of the biochar effects on soil microorganisms reported in the literature mainly depends on what kind of effect is expected, i.e., biodiversity, abundance, or secondary effects on soil biota on crops and soil fertility. Thus, soil biota diversity is often believed to be a key factor for soil functions [189] and organic amendments derived from biochar are key factors affecting trophic relations in soil [190]. The second approach assumes indirect influence of biochar on nutrient availability and possible effects of various species adsorbed in biochar particles and released in the environment during biochar aging and mineralization. Microbial abundance always increases with biochar amendments with low dependence on the feedstock and soil type. The mycorrhizal response estimated by root colonization increased twice for a two-year exposition of 0.6–6 t/ha of biochar to Eucalyptus wood [191]. The protective effect of biochar particles, sorption of signaling compounds, detoxification of allelochemicals, and improvement of physical properties of soil were mentioned as possible reasons for the positive effect of biochar on extraradical mycelium [3]. In addition, biochar was reported to be able to stimulate spore germination [192]. An opposite effect of biochar amendment was attributed to symbiotic relations in conditions of high nutrient availability, undesirable pH shifts and toxic effects of metal cations or high salt content [193,194]. Some other effects of biochar on the microbial abundance affected by its application are summarized in

Table 7. Possible mechanism of biochar’s influence on microbial abundance in soil [188].

Mechanism of Influence	Rhizobia	Other Bacteria	Mycorrhizal Fungi	Other Fungi
Protection from grazers	0	(+)	(+)	(+)
Improved hydration	+	+	?	? or ±
Greater P, N Ca, Mg, K availability	+	+	-	-
Greater micronutrient availability	+	+	-	?
Higher pH	+	+	nc	nc
Lower pH	-	-	nc	nc or -
Sorption of signaling compounds	? or -	?	?	?
Greater N availability (also through sorption of phenolics and increased nitrification	-	+ or -	nc	nc
Sorption of microorganisms	nc	?	nc	nc
Biofilm formation	+	+	?	?
Sorption of inhibitory compounds	?	+	?	?
Sorption of dissolved organic matter	?	?	nc	?

‘+’—relative abundance increase, ‘-‘—abundance decrease ‘0′—no influence observed, ‘?’—influence not considered; ‘±’—multidirectional changes; ‘nc’—influence not calculated; parentheses—weak circumstantial evidence.

.

Fertilizers commonly reduce the influence of biochar application on microbial abundance. The effect is mostly pronounced for P, N-containing fertilizers that inhibit acceleration of root colonization observed with biochar added alone [195]. The plants did not rely on biological N fixation in the presence of fertilizers as much as in the presence of biochar. Non-symbiotic organisms can slightly increase their abundance in the same conditions due to the higher availability of nutrients from fertilizer. Biological nitrogen fixation can also be stimulated by introduction with soybean straw-derived biochar [196]. Microbial biomass increases with pH values caused by biochar in acidic soils. The effect is more pronounced for bacteria, whereas fungi are more tolerant to this factor and can even reduce their growth at high pH [197]. It should be taken into account that changes in the soil pH observed after biochar application can reflect secondary effects on microbial activity [192]. The pH effect significantly depends on the soil properties. Biochars added to soil protect microorganisms from the inhibitory effect of some species (catechols, flavonoids, phenolics) that are adsorbed on the pores and hence excluded from the microorganism metabolism. Such an effect was described for biochar obtained from corn stover [198]. Fast-pyrolysis biochar from wood promoted arbuscular colonization of asparagus due to limitations in access to some aromatic acids exerting an allelopathic effect.

The microbial response to the biochar addition is also sensitive to the adhesion of the cells on the particle surface. This formation of biofilms mostly affects bacterial but not fungal abundance. The attachment of the cells to the biochar particles can occur via hydrophobic attraction and electrostatic forces. The mineralization of the biochar, as well as higher salt content, accelerate adhesion. Inclusion of the cells into the pores is limited by their size, which should be at least 2–5 times bigger than the cell size. For Bacillus mucilaginous and Acinetobacter sp., it was estimated of 204 μm [150]. In addition to abundance, microbial community composition significantly differs with biochar addition against that of unmodified soils of the same mineralogy [151]. Viscosity decreases stimulate the development of plant roots and rhizobia because of the higher availability of nutrients. Additionally, invertebrates move easier through the amended soil, making pores and influencing predator/prey ratio. A similar influence on plant roots and fungi can be attributed to the changes in the soil bulk density caused by the rather small value of this parameter of biochar particles (typically between 0.09 and 0.5 g/cm³ against 1.5–2.1 g/cm³ for true density).

Transformation of soil nutrients. It has been established that nitrification is accelerated when biochar is added to forest soils, which is explained by the sorption of phenols that inhibit the process and an increase in the mass of ammonium-oxidizing bacteria [199]. In addition, an increase in the activity of alkaline phosphatase, aminopeptidase and N-acetylglucosamine oxidase was revealed upon the introduction of biochar [200], which may be associated with an increase in the production of organic nitrogen and phosphorus due to the accelerated growth of plant roots in the pores of biochar. Bradyrhizobiaceae (Rhodoblastus, Rhodopseudomonas, Bradyrhizobium and Nitrobacter) may take part in the process, as well as nitrogen utilizing Hyphomicrobiaceae (Rhodoplanes, Starkeya), nitrates and ammonia in nitrogen fixation or denitrification [201]. The same microorganisms can produce ethylene from fresh biochar, which in turn leads to a reduction in greenhouse gas (N₂O and CO₂) emissions [202]. To study nitrogen fixation by legumes (Phaseolus vulgaris L.) in soils treated with 0–90 g/kg biochar soil, the isotope dilution method (¹⁵NH₄)₂SO₄ was used [203]. At the maximum dose of biochar, the proportion of fixed nitrogen increased from 50% in the control to 72%. The increase in total nitrogen supplied from the atmosphere was 49% for the dose of 30 g/kg of biochar and 78% for 60 g/kg, a decrease was registered at the maximum dose of application. A possible reason for the increase in microbial nitrogen fixation was an increase in the availability of boron and molybdenum and, to a lesser extent, potassium, calcium and phosphate, an increase in pH, a decrease in the availability of nitrogen, and saturation of aluminum. At the same time, the input of soil nitrogen decreased by 14 and 17% at an application rate of 30 and 60 g/kg of biochar, respectively. Biochar has a positive effect on phosphorus-mobilizing mycorrhiza, as it protects mycelium, indirectly influencing the changes in the physicochemical characteristics of soils, and detoxifying allelopathic secretions [204,205]. There are also opposite observations, indicating the absence of the influence of biochar or its negative effect on mycelium. One explanation for this contradiction is the indirect effect of biochar due to changes in pH, saturation of the soil with oxygen, and changes in its porosity.

Microbial diversity in soils enriched with biochar has been studied using various methods, including total DNA analysis of the soil microbial community [206], colony culture and counting [207], substrate-induced respiration [208,209], microbial biomass [210,211], extraction of phospholipid fatty acids [212,213,214,215], contrasting, and direct examination of individual biochar particles [3]. The introduction of biochar increases microbial diversity in different ways for different groups of microorganisms. The two most common types of mycorrhizal fungi (arbuscular and ectomycorrhizal fungi) most often respond positively to the application of biochar (see review [3]). The response of mycorrhiza is usually assessed by root colonization. Thus, the rate of formation and the number of processes of ectomycorrhizae on larch seedlings in the presence of biochar in the soil increased by 20–160% [216]. Similar results were obtained on wheat in a two-year experiment with the introduction of biochar from eucalyptus wood (0.6–6.0 t/ha)—the acceleration of colonization was 5–20% relative to the control without the introduction of biochar [191]. It is less clear how the part of the mycelium located in the soil interacts with the biochar. The direct interaction of biochar and mycelium may be important. For example, the internal pores of the biochar particles can protect the extraradial mycelium from external influences, for example, from grazing animals or from soil overconsolidation [188].

The introduction of biochar at doses of 5 and 25 t/ha was studied when cultivating wheat for 10 weeks while simultaneously varying the dose of applied nitrogen [217]. It was shown that Cmic decreased when biochar was introduced, while Nmic remained practically unchanged. The experiment cannot be explained by the sorption of inorganic nitrogen on coals, since carbon dioxide emissions decreased in the presence of 5 t/ha of biochar, but not in the presence of 25 t/ha. The structure of the ammonium-oxidizing microbial community changed only when biochar was introduced together with a nitrogen source. The authors concluded that the introduction of biochar reduced the activity of the microbial community as a whole. An increase in the dose of mineral fertilizers reduces the positive effect of biochar on the rate of reproduction of microorganisms [218], depending on the nature of the fertilizer and the group of microorganisms. Thus, mycorrhizal infection is suppressed by phosphorus-containing fertilizers, regardless of the application of biochar, but it does not depend on the application of nitrogen fertilizers. The opposite situation was registered for Rhizobium [219]. The reason may be the different influences of external conditions on the symbiosis of microorganisms. Therefore, when nitrogen fertilizers are added, the plant may not need biological nitrogen fixation to the same extent as in the absence of top dressing. A similar reason may explain the different effects of increased carbon load in the rhizosphere during exudation. The microbial diversity of non-symbiotic microorganisms can increase with increased availability of nutrients, either as a result of their longer retention in soil enriched with biochar or due to their entry into soils together with biochar. In most cases, a similar improvement can be achieved by the direct application of nitrogen and phosphorus fertilizers without biochar.

An increase in the concentration of microelements, such as molybdenum or boron, can also cause an increase in biological nitrogen fixation by Rhizobium colonizing biochar [220]. This phenomenon was discovered during the study of the immobilization of toxic elements by biochar. Similarly, sorption reduces the toxic effect of antimicrobial compounds and elements by reducing the time of their action, especially when they first enter the soil [221].

An example of the indirect effect of biochar on soil microorganisms is a change in pH. An increase in microbial biomass (Cmic) from 20 to 180 μg/g and in ninhydrin nitrogen from 0.5 to 4.5 μg/g was registered with an increase in soil pH from 3.7 to 8.3 [222]. At the same time, bacteria and fungi react differently to changes in pH. Bacteria increase diversity with an increase in pH to 7 or more, while fungi do not change their biomass in the specified pH range and decrease biomass with further increase [197,223]. Therefore, biochar with pH varying within the specified range can affect soil biota differently depending on soil pH. In addition, even with the introduction of acidic samples, for example, hydrochar with pH 4, an increase in the pH of fertilized soils was observed, apparently due to secondary processes of microbial reduction of unidentified organic substrates and electron transfer mediators [192]. The said work indicates the importance of secondary processes in fertilized soils, which, in turn, are determined by the pH value of the soil before its cultivation, the direction and magnitude of the change in pH when biochar is added, etc. In addition, it should be taken into account that the pH on the surface of biochar particles, where a bacterial film is formed, may differ from that in the bulk of the soil [188].

In addition to changing the microbial biomass, the introduction of biochar and related changes in the conditions of microbiological community development change its structure up to the change of dominants. Such changes have been repeatedly recorded for Terra preta (artificial soil based on low-grade activated carbon, also called Brazilian black soil), soils enriched with coal from natural fires and soils with biochar and for soil fungi, bacteria and archaea [207,224,225,226]. As a rule, the diversity of the bacterial community increased up to 25% when biochar was applied to the soils of Terra preta but decreased with a similar tillage in soils after natural fires. Simultaneously, in these cases, the diversity of archaea and fungi decreased, which indicates the unequal effect of biochar on various components of soil communities. However, it should be noted that these studies were carried out at different times, from 0.5 and 2.5 years for soils after fires and with the introduction of biochar and up to hundreds and thousands of years for Terra preta soils.

The bacterial community in soils with a high content of biochar differs from that in soils possessing the same mineralogy without biochar in them [207,227]. This was also shown with the help of genetic fingerprinting of Terra Preta soils and unmodified forest soil in the Amazonian region. The first ones showed a larger number of unique taxonomic units. At the same time, all taxa present in the forest soil were found in Terra preta, while the latter contained unique units that were absent in the forest soil. The greatest difference between the microbiological communities of both soils was established for an evolutionary distance of 5%, which indicates the presence of differences at the genome and species levels. Studies [207] revealed a high taxonomic diversity in biochar-fertilized Terra Preta soils in four areas of the central Amazonian region compared to unmodified soils. The maximum difference is fixed at the family level. Similar discrepancy values of 80% were observed for soils enriched with biochar and agricultural soils differing in cultivation time, past and present crops, and other conditions [206]. For uncultivated soils, the diversity did not exceed 40%. Up to the present, the biochar of Terra preta soils, introduced hundreds and thousands of years ago, remains the main factor determining the development of the microbiological community, despite the dissimilarity of other factors, such as agricultural use, soil texture, mineralogy, nutrient content and pH. Although the application of fertilizers, especially for podzolic soils, leads to differences in the results of genetic fingerprinting, it is incomparably small with the effect of biochar additions, which at times drastically changed the microbiological community. An increase in the application of biochar to the soils of the temperate zone increased the divergence of the bacterial community composition in the rhizosphere and soil. Rhizospheric soils with the application of 12 and 30 g/ha of biochar were characterized by the greatest dissimilarity from the soil, with minimal or no additional fertilization (0 or 1 g/ha) [188]. It was concluded that the introduction of biochar into soils leads to the development of communities similar to rhizospheric ones, which were formed without the introduction of biochar.

Similar studies were carried out at the level of taxa. Thus, two new taxa of Acidobacteria [227], and later two potentially new taxa of α-Proteobacteria were identified in the soils of Terra preta. It was shown that Acidobacteria are widely represented in all studied soils and differ in their genetic profile from analogs included in databases by at least 2% of the code. Some genomes isolated from Brazilian soils enriched with biochar were grouped with 93% similarity with Verrucomicrobia, whose genome is mainly found in tropical rice husks, but is characterized by a gradual spread to other soils. Genomes assigned to Pseudomonas, Acidobacteria and Flexibacter sp. were found both in Terra preta soils and in control soils. In moderately rich soils with and without biochar additions, 70% of the isolated sequences are classified as Ascomycota, Basidiomycota or Zygomycota. However, the occurrence frequency of the main phylotype genes differed for soils enriched and not enriched with biochar. In the presence of biochar, the communities were characterized by less genetic diversity. Similarly, less diversity was found in the Archaean community in Terra Preta soils, particularly in the ammonium-oxidizing Chrenarcheota [225]. Probably, this effect indirectly reflects changes in soil pH caused by the introduction of biochar [228]. Soils enriched with biochar show multiple Zygomycota involved in the degradation of glucose and cellulose and forming Glomeromycota mycorrhiza. At the same time, the diversity of Basidiomycota decreases by a third compared to unenriched soils. Some Ascomycota are known to degrade lignin but can also use simpler organic compounds as a substrate. In this regard, the lack of available carbon in biochar can inhibit the colonization of this fungal species, while soluble carbon adsorbed on the surface of biochar particles will increase the proportion of Zygomycota that apparently find sufficient resources of oxidizable carbon.

Similarly, under the influence of biochar, the bacterial community also changes. In response to the introduction of biochar obtained in high temperature pyrolysis (pyrolysis products of oak wood and a mixture of herbs at 650 °C), bacterial diversity increases, including taxonomy, in contrast to the effect of biochar obtained at 250 °C [226]. An increase in the diversity of Actinobacteria and Gemmatimonadetes was found, in agreement with similar observations of Terra preta soils and forest soils naturally fertilized with fire coal [229]. Separately, the effect of biochar on pathogenic microorganisms should be mentioned. The application of biochar to soils can increase E. coli contamination of groundwater, especially since current approaches do not consider reducing microbiological diversity in soil moisture as a goal. The transfer of E. coli to soil and ground moisture depends on the biochar application rate and the pyrolysis temperature. Thus, high temperature biochar (700 °C) from poultry litter at a dose of 2% applied to sandy soils does not reduce the number of E. coli in the leachate; increasing the dose to 10% (not realistic in real land use for economic reasons) reduces the number of microorganisms by several orders of magnitude [230]. Soil treatment with the same biochar, produced at 350 °C, increases the abundance of E. coli itself, which should be taken into account when assessing the consequences of its use in agriculture.

Along with the conservation of carbon and the reduction of the greenhouse effect in general, the introduction of biochar into soils has a general soil-improving effect, mainly associated with agronomic and environmental factors. There are various assumptions about the nature of such an effect, but most of them associate biochar with improved storage of soil moisture and nutrients, improved soil structure and drainage [231]. There is evidence of a connection between the application of biochar and the state of the soil microbiological community. These factors also affect the yield of agricultural plants, sometimes indirectly, for example, through the acceleration of nitrogen fixation by free and symbiotic diazotrophs [232]. The ability of biochars to absorb pollutants should also be taken into account, thereby reducing their availability for biota during the restoration of damaged soils and the neutralization of production wastes [203]. At the same time, such processes can lead to secondary contamination of soils with the same pollutants in the future during the decomposition of biochars.

Biochar stability in soil, i.e., its resistance to degradation, leaching and chemical oxidation, depends on the aromatic structure, surface functional groups, and sorption characteristics for minerals and organic compounds [233]. Biochar destruction proceeds under the influence of biotic factors (microbial community) due to photooxidation and dispersion [234]. Observations of freshly prepared biochar during the year showed that a decrease in its mass is accompanied by an increase in the number of surface oxygen atoms in the composition of carboxyl and phenolic groups, a decrease in the positive charge of the surface and the acquisition of a negative charge by it [138]. The O/C ratio determines the stability of a biochar, as expressed by its half-life, from >1000 years (O/C < 0.2) to <100 years at O/C > 0.6 [181]. It is difficult to accurately determine the lifetime of biochar in the soil, since the inhomogeneity of samples and the tendency of biochar to modify depending on environmental conditions increase during exposure.

3.4. Mineralization of Biochar

Biochar has a high sequestration potential due to the conversion of easily accessible C from plant biomass, organic waste, etc., into a hard-to-reach form, due to the absence of a priming effect when used as a soil improver, due to non-energy-intensive production (the resulting pyrolysis fuel and pyrolysis gas ensures the maintenance of the process, without the use of additional fuel sources), as well as by increasing the frequency of application (every 3–10 years) compared to other organic fertilizers, such as compost or digestate [22,235,236,237,238,239,240,241,242]. Evaluation of retention time of biochar carbon in soils varies from hundreds to thousands of years, while biomass carbon is retained for several decades [243]. The introduction of biochar derived from biomass reduces the return of carbon to the atmosphere in the form of carbon dioxide. Thus, provided that the gases released in parallel, as well as the products released into the ecosphere during the production, transportation and storage of biochar, do not compensate for the positive effect, the introduction of biochar contributes to a decrease in the greenhouse effect [244]. The manifestation of such an effect depends on the degree of carbon conservation in the biochar, i.e., on the ratio of carbon content in biomass and the same value in biochar obtained from the specified amount of biomass. Slow pyrolysis gives a carbon conservation value of about 50%; higher values are typical for a less stable biochar with a stable existence time in soils of 4 to 29 years [245].

The rate of natural mineralization of coals under natural conditions is usually low. Its evaluation is difficult because the changes over a reasonable period of observations are too small and because of the difficulty of unambiguous identification of the chemical nature of the resulting products. One of the methods of such an assessment is the use of biomass enriched in the ¹⁴C carbon isotope in the production of biochar. In particular, it was experimentally possible to prove the mineralization of 6% biochar, obtained in this way from a chaff, to CO₂, during an 8.5-year experiment. This corresponds to the processing of 0.3% of the carbon introduced per year [246]. In another laboratory experiment with the carbonization products of ¹⁴C-labeled Pyrenean chaff, the rate of mineralization was estimated from the rate of ¹⁴CO₂ release after the introduction of coals into loess soils [247]. It amounted to no more than 0.5% of the introduced carbon per year, which, in terms of real conditions, gives a lifetime of coals in soils of at least 2000 years. At the same time, the joint application of glucose to intensify the co-metabolism of soil bacteria reduces this time by more than 10 times.

The literature provides a fairly wide range of data on the carbon footprint of biochar, from 0.04 tCO₂-eq to a net reduction of 3.9 tCO₂-eq per ton of raw material [153,248,249,250,251,252]. This wide range is obtained due to different pyrolysis feedstocks, different pyrolysis plants, and different system boundaries established during the biochar life cycle assessment. A similarly wide range is set for the price of biochar. According to calculations by Nematian [253] and co-authors, the cost of producing 1 ton of biochar in the USA is USD 449–1847, in European countries EUR 300–2000 [254].Calculations made for Russia showed that the cost of production of 1 ton of biochar is USD 40; at a cost of USD 110/t, biochar production becomes profitable, which is comparable to other soil fertilizers and immobilization agents. It is worth noting that the growing demand for biochar, according to the European Biochar Market Report 2022/2023 biochar production capacity continues to grow in 2022 by 52% to 53,000 t biochar [252]. Indeed, the cost of compost in European countries and the USA is about 10 times lower than that of biochar and amounts to USD20–50/t. However, it is important to consider that when producing 1 ton of biochar in European countries, up to 4 tons of carbon credits worth EUR 70–370/t can be returned [153,254,255]. In addition, despite the same application doses, the frequency of applying biochar is once every 2–5 years, in contrast to the annual application of compost. In connection with the above, the most rational thing is to calculate the carbon footprint of crop products grown using biochar, as well as to change its cost taking into account the carbon units obtained.

3.5. Biochar Influence on Soil Microorganisms

Biochar influences many soil processes, such as denitrification, methane oxidation [256,257], carbon mineralization [210,247] and the transformation of nutrients. The reasons for this are numerous. They include switching to other carbon sources, changing nutrient availability, sorption of inorganic and organic components, including enzymes, to biochar, soil moisture retention, and changes in infiltration or pore structure. Later, those that are directly or indirectly related to the microbial community of soils will be considered.

As mentioned above, the microbial community is able to actively respond to the introduction of biochar. Higher microbial diversity could potentially lead to greater mineralization or oxidation of the biochar itself, as shown for organic carbons of non-pyrolysis origin. Usually, these processes are stimulated by an increase in microbial biomass. However, some reports have indicated the opposite effect or a combination of decreased diversity and absolute respiration carbon (carbon turnover) [210,258]. This may be the consequence of the lower amount of available carbon, the high stability of biochar, or the adsorption of organic carbon on it, which could contribute to its slower degradation. Vice versa, the introduction of freshly prepared biochar from waste products usually increases both respiratory activity and community metabolism [208]. It is explained, inter alia, by a high content of nutrients, such as nitrogen and phosphorus, as well as a significant proportion of organic biochar, which, in particular, follows from a slight activation of respiration with increasing biochar application rates [259,260]. In the same work, a positive connection was described between the amount of volatile organic components in the biochar and carbon dioxide released during incubation. Both of these factors, i.e., an increase in the nutrient component and labile carbon, directly follow the introduction of biochar into soils, and their overall effect on biochar mineralization depends on the ratio of labile carbon and nutrient components both in the biochar itself and in the soil inorganic matter.

From the same standpoint, it would be logical to expect a multidirectional effect of biochar on the structure of the microbial community. The shift toward a greater variety of fungi after the introduction of biochar into the soil potentially indicates a greater mineralization of the biochar itself. Root fungi are known to promote the degradation of lignin in woody biomasses and coal [261,262]. Interestingly, among fungi, there is a shift in diversity toward taxa that prefer glucose as a source of carbon, and among bacteria, in the opposite direction. It is not entirely clear how the introduction of a much more oxidatively tolerant biochar could contribute to the development of such preferences, especially given the wide variety of other carbon sources available in soils (own organic matter, litter, etc.). It is possible that mineralization of biochar does not increase access to labile carbon but promotes the mineralization of available non-pyrolysis carbon. This statement is consistent with observations showing a connection between an increase in microbial biomass and a higher rate of soil carbon decomposition (so-called priming) in the presence of biochar. The fact that this increase usually remains below the initial higher salinity when freshly prepared biochar is added [209] suggests that there are other mechanisms leading to carbon loss, such as physical carbon removal, changes in nutrient content or pH. In addition, volatile carbon compounds present in biochar, along with similar smoke components, can stimulate microbial activity immediately after the application of biochar to soils, but then quickly mineralize [263,264]. Longer-term (more than a year) observations show that biochar reduces the rate of soil carbon mineralization [137,210,265]. A similar situation (more biomass but less soil respiration) was also registered in liquid waste treatment, when the biofilm on irrigated fields showed a higher rate of mineralization of soluble aromatic carbon than the same film on activated carbons with a larger specific surface area of the latter [266]. It is possible that carbon dioxide forms carbonates on the surface of the biochars due to the higher pH value of their surface. This explains the decrease in the amount of recorded CO₂ with a simultaneous increase in microbial biomass.

Changes in the composition and enzymatic activity of the microbial community upon the introduction of biochar into soils may be responsible for the lower mineralization of soil carbon. The activity of glycosidase and cellobiosidase decreased with the introduction of more than 12 t/ha of biochar. In addition to soil analysis, a similar change was recorded for the purified enzyme and biochar obtained by rapid pyrolysis from Panicum virgatum [200]. In such experiments, it is also necessary to take into account the localization of microorganism colonies on biochar particles. Their proximity to a carbon source increases the efficiency of carbon mineralization, even without additional enzyme production. An alternative explanation associated with the assumption of adsorption of enzymes on biochar seems less probable; for example, lipases form stable and highly effective adducts with activated carbons [267]. Biochar particles can generate regions of local enrichment in available carbon and thereby promote the growth of microbial colonies, as occurs in biological wastewater treatment [268]. In the latter case, immobilization of biochars also reduces the possible toxic effect of organic waste on microorganisms, which contributes to their higher metabolic activity [269]. Biochar can adsorb large amounts of soil organic carbon, which was demonstrated by studying microbial cultures and the processes of substance leaching from soil horizons [270]. Numerous evidence of strong adsorption of aromatic hydrocarbons on various kinds of coals and soot also proves this statement [271]. Although such processes, with the inclusion of microbial biomass and litter, are slower than the sorption of PAHs, they can significantly determine the influence of coals on the mineralization of soil carbon. The influence of biochar on multicellular soil organisms is of great interest from the point of view of monitoring the state of the microbiological community based on the following considerations: soil organisms are included in the general flow of soil matter and energy, they are present at the upper levels of the food chain, and their reactions are derived from the reactions of microorganisms to the introduction of biochar. Then, geophages, for example, earthworms, are important participants in the processes of biochar modification and its transfer to other soil horizons, which cannot but affect microorganisms. Finally, the reactions of soil organisms to toxic elements may reflect corresponding changes in the structure and abundance of the microbiological community.

The interaction of biochars with earthworms has been the most studied. They are able to absorb particles of coal, crushing them along the way and mixing them with the soil. In a number of experiments with microcosms, it was shown that worms prefer mixtures of soils with biochar over pure soils [180]. Perhaps they use coal particles to ground soil organic matter, or they are attracted to microorganisms and their metabolic products that accumulate in the pores of the biochar. Long-term plot experiments have shown that worms also contribute to the transportation of biochar particles within the soil layer but most likely not beyond it. At the same time, in similar experiments with PAH-contaminated biochar, a decrease in the mass of worms was observed in comparison with the inhabitants of the same soils without biochar [272]. The joint enrichment of soils with biochar and worms increased the concentrations of inorganic nitrogen, as well as the productivity of agricultural crops (for example, rice [273]). Data on the influence of biochar on nematodes are scarce. Soils treated with smoke from the production of activated carbons show a higher number of nematodes, which may be due not to the direct action of the components contained in the pyrolysis products but to their effect on competitive interaction in soils. The same can be said about arthropods, the change in the abundance of which in soils enriched with coals was established from the products of their vital activity.

3.6. Effect of Biochar on Crop Yields

Biochar application usually improves crop productivity by increasing the availability of nutrients, increasing the activity of microorganisms that determine the mobility of soil nutrients [274,275,276,277], and accelerating the development of the plant root system because of colonization of arbuscular mycorrhizal fungi and improved soil physical properties [278]. There is also a decrease in the loss of nutrients due to their reduced leaching from soils and an improvement in the physical characteristics of soils that contribute to water retention and oxygenation [279,280]. There is also data on the absence of the biochar effect or even its negative effect on crop yields. In

Table 8. Average yield changes as a percentage of control for various biochar applications.

Dose of Biochar, t/ha	The Mean Values and the Range Corresponding to the 95% Confidence Interval
135	−26–67
100	15–65
67.5	−16–45
65	−10–50
50	16–35
40	−8–38
25	10–38
22	−13–18
20	−15–40
16	−28–22
14	−14–28
11	−11–17
10	5–17
8	−5–17
6	−3–16
5.5	−40–50
5	−56–7
4	−32–22
3	−5–15
1.5	−7–12

, there is an example of generalization of the biochar effect on crop yields. Table 8 also contains information on the application rate and statistical parameters of the assessment [281,282].

When evaluating the contribution of biochar to soil fertility, it is necessary to take into account all aspects of a complex interrelated system that includes, in addition to biochar and soil, a specific agricultural crop, inorganic and organic fertilizers used in parallel, biochar application volumes, climatic and other environmental conditions (Table 8).

In

Table 9. Effect of biochar application on the productivity of various crops (according to Subedi et al., 2017) [282].

Biochar Source	Pyrolysis Conditions	Application Dose	Term	Crop	Effect	Ref.
Mixed wood	500 °C	30 and 60 t/ha	2 seasons	Hard wheat	+30% (grain)	[283]
Mixed wood	500 °C	10 t/ha	1 season	Hard wheat, corn	+10% (wheat), +(6–24)% corn	[284]
		30 and 60 t/ha	8.5 weeks	L. perenne	+(20–29)% (biomass)
		100 and 120 t/ha	8.5 weeks	L. perenne	−(10–20)% (biomass)
Corn heart, soft wood	400 °C	4 t/ha	2 seasons	Corn	+(233–322)%	[285]
Wheat straw	450 °C, enriched with minerals	670 kg/ha	15 weeks	Green pepper	+(16–16)% (fruits)	[286]
Eupatorium adenophorum	680–700 °C	750 kg/ha	17 weeks	Pumpkin	+(85–300)%	[287]
Eucalyptus	600 °C, activated	1.1 and 5.44 t/ha	1 harvest cycle	Sweet corn	-	[288]
Hard wood	500–575 °C (hydrothermal)	0–96 t/ha	1 harvest cycle	Corn	+(11–55)%	[289]
Spruce and pine chips	550–600 °C	5 and 10 t/ha	3 years	Horse beans, turnips and wheat	-	[290]
Willow	600 °C, decomposted	10 t/ha (biochar), 25 t/ha (biochar and compost)	1 harvest cycle for each crop	Bananas, papaya	−(18–24)% bananas, -papaya	[291]
Fruit tree branches	500 °C	22 and 44 t/ha	4 years	Vine	+(16–66)%	[292]
Hard wood	500 °C	8 t/ha	3 years	Vine	-	[293]
Mixed wood chips	450 °C	25 and 50 t/ha	3 years	Corn (year 1) and hay grass (years 2 and 3)	9 (corn), +(13–32)% (hay)	[294]

the results of testing various types of biochar in a field experiment are given. The effect of biochar on crop yield depends on soil type. For example, corn showed a 2–3-fold increase in yield when applied at 4 t/ha in acid sandy soils and only 30% to 40% when biochars were used in sandy-clay acid soils. In neutral clayey soils, no effect of biochar application was found.

In the study [295], a 50% increase in the yield of chaff was established on slightly acidic fatty loams and 44% on calcareous sandy soils. In Australian calcareous soils, a negative effect of biochar on the yield of lettuce and hay grass was registered [296]. Technological aspects complicate the dependence of yield on the type of soil, cultivated crops and the method of biochar preparation. Thus, in a greenhouse experiment, a decrease in the yield of corn in the first year of cultivation was registered when biochar from eucalyptus wood obtained in fast pyrolysis at 800 °C was introduced into acidic marl soils [297]. At the same time, biochar did not affect this value when applied to weakly acid silty loams. When using biochar obtained in slow pyrolysis at 350 °C, no increase in yield was observed for either of the two soil types. Different results were obtained in the second year of the experiment: after the introduction of biochar obtained in slow pyrolysis, the yield increased by 500% on slightly acidic loams and by 150% on acidic marl soils. The authors attributed the reasons for the decrease in the efficiency of using the fast pyrolysis product to the possible presence of PAHs and the antagonistic effect of excess potassium on calcium and magnesium, although reliable results for measuring these chemical compounds are not given in the work. However, an increase in yield in the second year of biochar application is recorded quite often due to an increase in the cation exchange capacity of soils, the availability of nutrients, the ability to retain water and a decrease in the mobility and toxicity of aluminum and manganese [296,298].

The direct or indirect effect of biochar on the growth of the root system and phytotoxic compounds indirectly affects crop yields. The acceleration of root culture development is due to the general improving effect of biochar on the soil structure, including its aeration and moisture saturation. Moreover, more than a hundred years ago, it was already believed that root development was accelerated due to the sorption removal of allelopathic compounds by activated carbons and similar products. At the same time, it should be taken into account that, most likely, the sorption removal of allelopaths and other phytotoxic compounds, including those released from litter and other plant residues, is complicated by the complex effect of biochar on the availability of nutrients, which leads to contradictory results in laboratory experiments [299]. In a number of studies, biochar was used as a substitute for peat in a nutrient medium for growing plants (soilless method). At a content of less than 30 wt%, biochar had a positive effect on plants, which was manifested in improved seed germination and seedling survival. In a number of studies, biochar was used as a substitute for peat in a substrate for growing plants (soilless method). Peat moss is conventionally used as a container substrate; however, this use causes several environmental concerns, such as peatland ecosystem destruction when peat is obtained, over-use of fertilizers since peat itself does not provide plants with essential nutrients, and carbon release since peat organic matter is quickly decomposed by plant-associated microbes. Biochar seems to have the potential to alleviate these concerns; however, the number of publications on peat substitution by biochar is still quite low as compared with the total number of publications on biochar used as a soil conditioner, suggesting that this study direction will be developed in the future. In most cases (depending on the initial substrate and pyrolysis process characteristics), biochar has a larger surface area, better pore size distribution, better rewetting characteristics, and higher content of N, P and other essential elements [300]. Examples of assessing the impact of biochar in the soilless method of cultivating a number of crops are shown in

Table 10. Study of biochar as a substitute for peat in a nutrient medium.

Crop	Dose of Positive Effect (% of Biochar)	Dose of Negative Effect (%of Biochar)	Ref.
Gaillardia (Gaillardia spp.)	25	50	[301]
Calathea (Calathea insignis)	20–35		[302]
Pelargonium zonale (Pellargonium zonale)	30	70	[303]
Kale (Brassica oleracea L. var. acephala)	1–5		[304]
Lettuce (L. saliva)	50–75		[305]
Sunflower (Helianthus annuus)	25–75	100	[306]
Tomatoes (Solanium lycopesicum)	5		[307]

.

It is necessary to discuss separately the effect of biochar on plant diseases [308]. Even 170 years ago, there was a decrease in rust damage in wheat and powdery mildew in other crops. Since then, the effect of biochar has been tested on 13 pathogens, of which 85% have responded to biochar application by reducing plant infection, 12% have not, and 3% have increased plant disease. Later [309], the list of potential pathogens was expanded to 15 (30 plant/pathogen pathosystems). In 60% of the cases (70% of pathosystems), the maximum biochar concentration of 0.5–1% did not have any effect or had a negative effect on plant disease. Moreover, increasing the dose of biochar to 3% did not affect or accelerate the course of the disease. The study covered 12 different environments and 5 different prepared biochars, differing in salinity, carbon content, alkalinity and other parameters. The negative effect of biochar was less pronounced for foliar infections compared to soil pathogens. Other examples of assessing the impact of biochar on pathogens and plant infections are given in the review [310]. Examples of the acceleration of pathogen effects on plants are given in

Table 11. Stimulation of plant diseases by biochar introduced into the nutrient medium.

Pathogen	Plant	Biochar Raw Material	Biochar Dosis	Ref.
Botrytis cinerea	C. annum	Citrus wood	3%	[311]
Fusarium oxysporum f.sp lycopersici	L. esculatum	Wastes of processing wood and green parts of trees	3%	[312]
Phytophthora cinnanomi	Quercus rubra	Wood	>5%	[313]
Pythium ultimum	C. annum, Ocimum basilicum	Spruce bark	50% (oб.)	[314]
Plasmodiophora brassica	Brassica rapa	Miscanthus	0.5%	[315]
Pratylenchus penetrans	Daucus carota	Pine (wood and bark), wheat husk	0.8%	[316]

.

Strengthening the influence of pathogens under the effect of biochar often coincides with its suppressive effect on the development of the plants themselves, which manifests itself at high application rates. Moreover, even in the case of a positive effect on the growth rate of plants, the risk of irreversible damage to plants in the event of infection is potentially high. The contrary is also true—infected plants are more sensitive than healthy ones to an excess of applied biochar. The reasons for this phenomenon are not fully understood, but similar dose-dependent effects were observed for other agrochemicals, for example, glyphosate, which acts as a growth regulator at low concentrations, and as a herbicide at high concentrations. Biochar may contain certain organic components that, individually or in combination, have hormone-like or phytotoxic effects. Ethylene released from some biochars can serve as an example of such an indirect effect. In small doses, it has an effect on plants as a growth promoter and increases resistance to diseases, and in large doses, it has an opposite effect.

3.7. Immobilization of Microorganisms on Biochar

The number of positive examples of the use of biochar with immobilized microorganisms in soil bioremediation is rather limited. Thus, accelerated decomposition of the pyrethroid cypermethrin in soils enriched with biochar with an immobilized consortium of Bacillus zhanjiangensis (TJTB48), Bacillus pseudofirmus (TJTB58), and Oceanobacillus kimchii (TJTB66) bacteria was described [317]. Bacteria were grown on a mineral substrate and then mixed in an aqueous suspension with biochar (concentrations of 0.5, 1 and 2%) for 24 h. The mixture was added to the surface layer of soil contaminated with pyrethroid. An increase in the rate of decomposition of cypermethrin was observed at a biochar load of 0.5% (time to reduce the concentration by 2 times 16.4 days).

Accelerated bioremediation of soils contaminated with PAHs under the action of bacteria immobilized on a biochar was described [318]. To do this, we used bacteria previously isolated by the ability to degrade polyaromatic hydrocarbons (Pseudomonas putida and the second one remains unidentified). The microorganisms were immobilized on pre-crushed biomass (wood chips, bamboo leaves, orange bark, pine needles) and its pyrolysis product at 100, 300, 400 and 700 °C under oxygen-deficient conditions. Immobilization was carried out by mixing the cell suspension with the carrier in the presence of sodium alginate. Next, the mixture was dried and subjected to gelation by the introduction of calcium chloride. The presence of bacteria increased the degree of sorption extraction of hydrocarbons from water, while the organic mass before pyrolysis insignificantly reduced the bioavailability of the absorbed toxicant. When microorganisms immobilized on an organic substrate and biochar were placed in soils in a 90-day experiment, the highest rate of decomposition of polyaromatic hydrocarbons was demonstrated by a sample in which the bacteria carrier was biochar prepared at 400 °C.

Corynebacterium variabile HRJ4, which is highly salt tolerant, was used to accelerate the oxidation of gasoline hydrocarbons. The bacterium was immobilized on biochar prepared from wood chips by pyrolysis at 250, 400, or 700 °C [319]. The pyrolysis product was mixed with a suspension of microorganisms in a ratio of 5:100 for 12 h and then stored at 4 °C. The immobilized bacteria showed the highest activity in the oxidation of aliphatic hydrocarbons C16, C18, C19, C26 and C26, as well as pyrene and naphthalene. Activity during 7-day incubation reached 79% of the relatively free non-immobilized bacteria. Biochar from corn cobs and pig manure was used as a carrier of the mutant strain B. subtilis B38 in the study of bioremediation of soils contaminated with heavy metals [320]. It has been established that the presence of microorganisms contributes to a decrease in the bioavailability of cationic and anionic forms of metals (cadmium, lead, chromium) and increases the biomass during the cultivation of lettuce three times more than in the absence of microorganisms. The introduction of phosphate-mobilizing bacteria into biochar was used to reduce the toxicity of lead in soils [242]. For this purpose, Pseudomonal chlororaphis in the culture liquid was mixed in a volume ratio of 1:10 with biochar prepared from cow dung by pyrolysis at 200, 300, 400 and 500 °C for 4 h. The effect of additives was assessed by the degree of lead extraction from contaminated soils.

4. Conclusions

It can be concluded that the interest of both the scientific community and enterprises in biochar is growing. The variety of substrates for the production of biochar and the scope of its application are increasing. The characteristics of biochar are very diverse; it is possible to achieve the optimal ones both through the selection of the optimal substrate for pyrolysis, pyrolysis conditions, and through pre- or post-treatment. The use of biochar as a soil fertilizer leads to changes in the physical, chemical and biological properties of soils. This increases bulk density, soil porosity, and wet aggregate stability, while clayey soils decrease tensile strength. Biochar increases the pH and soluble carbon of soils, immobilizes toxicants, and reduces the bioavailability of toxins to soil organisms. However, biochar can contain phytotoxic compounds, such as PAHs and heavy metals. The use of biochar as a soil fertilizer leads to an increase in the amount of macro- and microelements in the soil. The introduction of biochar into the soil leads to an increase in the number and diversity of the soil microbial community, with the greatest effect observed for the rhizobiome. All of these factors influence plant growth and crop quality and consequently, more input of rhizodeposits into the soil if the dose of biochar application was right chosen. In addition, the production of biochar is zero-emission technology; its use as a soil amendment leads to the sequestration of carbon in the soil and greater efficiency in the capture of carbon dioxide in plant biomass. We assume that further research will be aimed at optimizing the efficiency of the pyrolysis process and finding methods for increasing the sequestration potential of biochar.