Soil can be characterized as a natural, non-renewable, vital, multivariable, complex, regulatory resource, a significant driver of agriculture, a complex organic material [1], an environmental filter of nutrients [2], and a large pool of soil carbon [3] since its contribution to the ecosystem is necessary both for the dynamics and for the complex processes that take place in it [4]. Organic matter is one of the essential components that make up the soil ^[5][6][5,6] and contributes significantly to the assessment of soil conservation and quality [7].

The soil carbon pool is the largest terrestrial pool of organic carbon and plays a vital role in the global carbon cycle and in maintaining ecological balance [8]. Recently, soil management and the great biochar development scale have faced several new challenges ^[9][10][9,10] related to climate change due to increasing temperatures and anthropogenic factors. On the one hand, the continuous degradation of soils because of environmental degradation due to anthropogenic factors and the phenomenon of climate chains are presented as significant issues of environmental concern and are the primary concern of the global community, determining to a considerable extent the performance of productivity and, by extension, the quality, health, and properties of the soil.

On the other hand, increasing population trends, limitations of agricultural land for food production, water scarcity, loss of biodiversity, and maintaining food security in polluted soils ^[11][12][13][11,12,13] are some components that make the earth unsustainable and make it challenging to function [14]. It is imperative to maintain sustainable management and continuous monitoring of the change in soil properties to increase productivity and minimize or mitigate all those components that negatively affect it. According to the literature review, various types of research demonstrate the positive contribution of biochar in terms of its ability to maintain soil fertility and carbon sequestration for several years [15].

Its properties contribute to the soil’s physical, chemical, and biological properties and plant growth [16]. A sustainable soil resource management strategy is to incorporate biochar into the soil [17], whose use has been known for 2000 years [18], creating an agronomic regime with a positive impact on the environment ^[19][20][21][19,20,21]. Studies prove its primordial roots and its impressive utilization by the farmers of modern societies [22]. As a soil improver, it is increasingly gaining the trust of its users, which is reflected in many publications [23], and there is an increase in searches of the scientific literature with the word “biochar” ^[24][25][24,25]. Biochar is a porous material whose production is carried out through pyrolysis or hydrothermal treatment of raw biomass [13].

The thermal depolymerization of biomass at high temperatures without the participation of oxygen is called pyrolysis ^[26][27][26,27]. The conceptual approach to the term includes the diverse uses and applications of biochar in various sciences [28]. The use of biochar is an emerging practice for soil amendment that yields promising results [29], attracting worldwide interest, both in research and practical applications [30]. The type of raw material and the production conditions are largely inextricably linked to its physical and chemical properties [31]. These properties determine its physicochemical behavior when applied to the soil and in combination with the properties of the soil [22]. The longevity of the material and its persistence in the environment for a long time are often pointed out in the literature [23]. When it is incorporated into the soil, it is subject to a natural aging process. The natural aging of biochar in the soil is slow and lasts for many years; therefore, accelerated aging techniques are applied as alternative ways of approaching its assessment [32].

2. Properties and Characteristics of Biochar and Factors Affecting Its Performance

Biochar has a high percentage of carbon content and can act as a long-term carbon storage reservoir ^[33][34][35,95], with high stability and large-scale production potential ^[35][36][63,96]. It generally possesses a large surface area, a porous structure, and abundant surface functional groups ^[37][38][39,97]. It contains unstable, stable, volatile, and aromatic compounds due to the presence of carbon. Rechberger et al. ^[39][98] report that biochar is rich in carbon content, and most of it is aromatic in structure and difficult to degrade ^[40][41][99,100]. Hydrogen H, oxygen O, nitrogen N, sulfur S, and ash are present in smaller amounts ^[18][37][42][18,39,52]. Its structure is highly porous due to carboxyl esterification and aromaticity, with a large specific surface area and low solubility ^[43][34]. Biochar properties affected by pyrolysis temperature are surface form and structure ^[44][101] and adsorption capacity ^[45][102]. Initially, biochar consists of various functional groups, such as hydroxyl (-OH), carboxylate (-COOH), and aldehyde (-CHO), which are located on its surface ^[18][46][47][18,54,66]. Functional groups vary depending on the type of biochar, the temperature at which it was formed, and the interacting compounds in the soil ^[48][103]. During the pyrolysis process, due to increased water loss and dehydration of the biomass, the development of the porous surface of the biochar takes place. Thus, based on pore categorization, biochar is produced with tiny pores (2 nm), medium pores (2–50 nm), and large pores (>50 nm). Furthermore, the surface area of the biochar plays an essential role because the properties of adsorption and ion exchange are directly related to this property. In essence, it has an internal and an external surface. The first is the walls of the most profound and least open cracks and cavities (micropores). In contrast, the second includes all protrusions, larger cracks, and pores (mesopores and macropores). Macropores contribute less than micropores to the adsorption capacity ^[49][68]. Biochar ash is alkaline in nature, and therefore the pH of biochar reflects the ash content ^[50][104]. At the same time, its cation exchange capacity is related to its surface, the presence of carboxyl functional groups, the first matter of the biomass used, and the preparation temperature. The stability of the aromatic rings resulting from the increase in temperature turns the biochar into a solid material with high strength for centuries or millennia. Increasing production temperature leads to a decrease in aliphatic structures and the soluble fraction and an increase in the aromatic and insoluble fraction ^[51][105]. Therefore, biochar produced at a higher temperature may be more stable than that produced at a lower temperature. Many investigations agree that biochar can be preserved for many years in the soil, even for more than 1000 years ^[52][106]. This preservation is attributed to the durability of the biochar carbon and, in particular, to the aromatic carbon, which is more resistant to mineralization under abiotic and biotic stresses. In essence, C stabilization occurs in aromatic compounds, which certifies the fact of carbon inflow and sequestration in the soil when subjected to a pyrolysis process ^[53][107]. In addition, oxygen (O/C) and hydrogen (H/C) content is low, directly related to their durability in the soil. If the pyrolysis temperature is lower than 400 °C (T < 400 °C), the structure of the biochar resembles amorphous carbon. In comparison, if the temperature is higher than 500 °C (T > 500 °C), the structure refers to thermally reduced graphene oxides ^[54][55][108,109]. Research shows that biochar’s pore volume and surface area increase with increasing pyrolysis temperature ^[56][57][93,110]. At the same time, at a temperature >500 °C, the biochar is more resistant to the soil and contains a higher concentration of ash and pH. However, temperatures >700 °C are not acceptable for biochar yield but produce biochar with high stability [10]. Considering the pyrolysis process, which is an exothermic reaction ^[58][111], and the thermal behavior of the raw material ^[59][112], the control of the pyrolysis temperature can be considered a difficult task. Therefore, the reported temperature may be inaccurate and should be verified. The temperature has a significant impact on the adsorption capacity of biochar ^[60][113]. Regarding the effect of pyrolysis on the chemical characteristics of biochar, an increase in pyrolysis temperature also implies an increase in carbon concentration. However, increasing temperatures gradually decrease H and O concentrations ^[61][86]. Generally, at high pyrolysis temperatures (T = 400–700 °C), the feedstock is transformed into stable polycondensed aromatic structures containing high carbon ^[62][114]. On the other hand, when the pyrolytic process occurs at a low temperature (400 °C), a biochar product rich in C=O and C-H functional groups is produced. Consequently, the dominant organic compounds are aliphatic or less stable cellulose-like structures at low pyrolysis temperatures, which microbes can quickly degrade. In addition, high pyrolysis temperatures (T > 500 °C) create more hydrophobic biochar, have a larger surface area and pores, have a higher pH due to ash, and are more suitable for organic pollutants. Low temperatures (500 °C) lead to biochar having a smaller surface area and pores but more oxygen functional groups, making it more suitable for inorganic pollutants ^[49][68]. Generally, a higher pyrolysis temperature reduces the amount of solid product that can be made. Still, it improves its quality by giving it a more porous surface, a larger specific surface, more N and K, more C and aromatic substances, and fewer active O groups on its surface ^[63][115]. The residence time of biochar during the pyrolysis process significantly affects its porosity and specific surface area ^[64][116]. Biochar has a predominantly condensed aromatic structure highly resistant to microbial degradation ^[65][43]. The pH is either neutral or alkaline. Alkaline pH sometimes results from high-temperature biomass pyrolysis. However, at lower temperatures, high ionic-strength biochar is produced ^[65][43]. According to the literature review, biochar presents multifunctionality that lies in the fact of certain characteristics such as: alkalinity, stability, specific surface area, carbon content, mechanical strength, cation exchange capacity and nutrient retention ^{[66][67][68][69][70][71][72][73][74]}[117,118,119,120,121,122,123,124,125].

3. The Positive Effect of Biochar on the Ecosystem, with Emphasis on Soil

Aiming at sustainable development, biochar can play an essential role in the long term [18]. Therefore, it is a product gaining more and more research interest, as its use is intended to improve crop yield and soil properties ^[75][126]. However, using biochar in soil is not only important for carbon sequestration but also acts as a soil management practice by affecting its physical and chemical properties ^[17][76][17,127]. Soil degradation results in reduced organic matter due to continuous loss, so biochar is a solution for this constant loss of organic matter ^[33][35]. In particular, biochar contributes to the assurance and quality of the soil as an environmentally friendly material with a renewable nature without producing pollutants. In addition, its incorporation into the soil could lead to achieving sustainable agriculture ^[77][128]. There are three functions of biochar for which its use has been mainly promoted in recent years, which include mitigating climate change ^[78][37] by sequestering carbon ^[79][129], enhancing the yield of crops, and improving soil structure and functions through multiple beneficial actions. It, thus, plays a vital and active role in agricultural economic development and is an economical solution as an adsorbent material with many environmental applications ^[80][56]. One of the significant benefits of biochar is that it helps fight climate change by sequestering carbon dioxide from the atmosphere. It can also be used to rehabilitate problem soils. Biochar has many benefits for agriculture and environmental economics in the long term, so its incorporation into agricultural practices is highly recommended ^[81][130]. Biochar is considered a material of global application for the restoration and sustainability of soils due to its physicochemical characteristics and the extensive availability of raw materials ^[82][40]. It can promote soil fertility improvement and contribute to climate change mitigation ^[79][129]. Biochar as a soil conditioner is gaining more and more interest in research [17]. Three main functions have been discussed in recent years: its contribution to mitigating climate change, its contribution to carbon sequestration in the soil, improving crop yield, and improving soil structure ^[33][34][35,95]. Soil improvement with biochar mainly refers to the improvement of soil nutrient availability (e.g., nitrogen, potassium, and other macronutrients or micronutrients), moisture content, cation exchange capacity (CEC), organic content C, and reducing soil erosion due to improved structure ^[83][57]. The effect of biochar on plant growth depends mainly on factors such as the type of biochar, the degree of its incorporation into the soil, the depth of mixing with the soil, the availability of nutrients, the texture of the soil, and the species of cultivated plants ^[84][131]. In the last two decades, there has been substantial research interest in studying and further investigating the potential of biochar in terms of increasing productivity, improving soil characteristics, and mitigating climate change. In general, researchers report the beneficial properties of biochar on soil physical, chemical, and biological properties, as well as plant growth ^[78][37]. In particular, as a soil conditioner, it can improve long-term physical (bulk density, water holding capacity, permeability, etc.), chemical (nutrient retention, nutrient availability, etc.), and biological properties (microbial population, earthworms, enzyme activity, etc.) for the benefit of plant growth [26]. Consequently, its ability to adsorb pollutants is known. It has already been shown to be widely applicable in wastewater treatment applications, but to date, the mechanisms of pollutant adsorption still need to be fully understood [18]. Additionally, there are many studies on its use as a compound fertilizer due to its high porosity and large surface area ^[85][132]. The application of biochar to the environment is not harmful because it has been found to contain several polycyclic aromatic hydrocarbons that are environmentally friendly. However, the commercial use of biochar has not been widely accepted, even though biochar is only a waste ^[65][43].

4. Effect of Biochar on Soil’s Physical, Chemical, and Biological Properties

Biochar works in many ways when applied as a technique to improve soil quality. According to the literature review, many reports demonstrate the importance of biochar utilization in the agricultural ecosystem. The positive impact and beneficial effects of biochar incorporation on soil functions have now been demonstrated, as has its effect on soil physical, chemical, and biological properties. However, the factors influencing the long-term behavior of biochar in the environment still need to be better understood. A large body of literature supports the idea that soil amended with biochar has excellent potential to increase crop productivity due to the consequent improvement in soil structure, high nutrient use efficiency (NUE), aeration, porosity, and water holding capacity (WHC) ^[34][95]. Biochar exhibits a variety of functions when applied as a technique to improve soil quality. According to the literature review, various reports demonstrate the importance of biochar utilization in the agricultural ecosystem. The positive impact and beneficial effects of biochar incorporation on soil functions have now been demonstrated, as has its effect on soil’s physical, chemical, and biological properties. However, the factors influencing the long-term fate of biochar in the environment are still poorly understood. An abundant body of literature supports the idea that soil amended with biochar has great potential to increase crop productivity due to the consequent improvement in soil structure, high nutrient use efficiency (NUE), aeration, porosity, and water holding capacity (WHC) ^[34][95]. Regarding its effect on soil’s physical properties, biochar contributes to soil texture, grain size, bulk density, aeration, and moisture retention [27]. In addition, an increase in porosity, a decrease in bulk density, and an enhancement of soil aggregation are observed ^[86][87][133,134]. Many studies have shown that biochar affects soil’s water retention and hydrological functions. The results of these studies are variable due to different experimental conditions, including biochar and soil types. Few studies, however, have examined its effect on plant-available water (PAW) and water use efficiency (WUE) ^{[61][88][89][90][91][92][93]}[86,135,136,137,138,139,140]. There is an improvement in the movement of water in the soil and an increase in available moisture, mainly in sandy soils with a low content of organic matter, a moderate improvement effect in soils of medium texture, and possibly a reduction in moisture retention for clayey soils. High cation exchange capacity (CEC), high specific surface area, pore volume, and porosity can increase water-holding capacity and improve the effects created by saline soils. Biochar increases soil’s water-holding capacity, especially in arid soils considering climate change. According to studies, straw biochar formed at 300 °C had a water-holding capacity of 13 × 10⁻⁴ ml m⁻², but this decreased to 4.1 × 10⁻⁴ mL m⁻² when the carbonization temperature increased to 700 °C ^{[34][94][95][96][97][98][99][100][101][102][103][104][105][106][107][108]}[76,95,141,142,143,144,145,146,147,148,149,150,151,152,153,154]. As a multifactorial carbonaceous and porous material, it improves soil properties and fertility by enhancing the availability of nutrients ^{[109][110][111][112][113][114]}[155,156,157,158,159,160]. A stoichiometric change in the soil is observed with the simultaneous retention of nutrients ^{[74][115][116]}[125,161,162]. The enhancement of soil nutrient bioavailability is explained by reducing nutrient leaching and immobilizing toxic elements in contaminated soils ^[117][118][163,164]. In particular, the effect on the potential of nitrogen in the soil is variable since a decrease, an increase, and no change are observed. However, the production feedstock is vital in soil nutrient availability ^{[119][120][121][122]}[165,166,167,168]. Biochar is an essential means of long-term increase in organic C ^{[35][102][123][124]}[63,148,169,170]. Its incorporation into the soil increases the cation exchange capacity (CEC) and the degree of base saturation, thereby increasing the retention of nutrients ^[100][125][146,171]. According to research, the improvement of CEC reached up to 20% and electrical conductivity (EC) up to 124.6% ^{[34][100][126][127][128][129][130]}[95,146,172,173,174,175,176], increasing soil pH and reducing soil acidity by 31.9%. Significant crop production and nutritional value increases are observed when its application is in acidic and poor soils. Conversely, it can have controversial results in alkaline and fertile soils, causing a neutral or negative effect on plant tissue production ^{[68][89][127][131][132][133][134]}[119,136,173,177,178,179,180]. In terms of its contribution to soil’s biological properties shows growth in microbial activity and diversity ^{[135][136][137][138]}[181,182,183,184]. It immobilizes the microbial nitrogen (N) of the biomass and stabilizes the microbial nitrogen (N) of the soil ^{[128][139][140]}[174,185,186]. Its positive effects appear in the reduction in fertilizer leaching and improvement of soil aeration ^[100][141][146,187], in the improvement of stream flow resistance ^[142][188], in the decontamination of air and water soil ^{[143][144][145][146]}[189,190,191,192], and in the general improvement of soil aeration and hydrological soil functions ^{[147][148][149]}[193,194,195]. The effect of biochar on plant growth and crop yield depends on the crop type, soil type, biochar properties, rate, method, and frequency of its application. Crop yield responses are generally reported as positive. Biochar application has been reported to increase plant productivity by approximately 10% and up to 25% for aboveground biomass. Other research investigates biochar’s effect on increasing corn’s biomass due to increased soil pH and CEC. Increase grain and maize yields by 150% and 98% with 15 and 20 t/ha biochar, respectively, to improve soil’s physical and chemical properties. The effectiveness of biochar in improving plant productivity is variable, considering climate change, soil properties, crops, and experimental conditions. These differences could also be explained by the different pyrolysis processes and soil interactions with biotic and abiotic factors ^{[68][150][151][152][153][154]}[119,196,197,198,199,200]. A general increase in crop productivity and yield is observed. Biochar showed an increase in available plant water content of 33–45% in coarse-grained soils and 9–14% in clay soils. A tremendous increase was achieved by adding 30–70 Mg/ha. At the same time, there was an average increase of 27% in the rate of photosynthesis in C3 plants but no effect in C4 plants. Increased stomatal conductance, transpiration rate, and chlorophyll content were attributed to the combined effects of biochar, water availability, and nitrogen fertilization, sequestering soil carbon as a long-term carbon pool and reducing emissions of greenhouse gases (GHG) such as carbon dioxide (CO₂), nitrous oxide (N₂O), and methane (CH₄). However, the accuracy of its application remains controversial. Biochar application to paddy fields caused a 12% increase in CO₂ emissions but a decrease in NO₂ emissions. In a pasture ecosystem, biochar application showed no change in CO₂ or NO₂ emissions. Other research showed a CH₄ reduction in soybean crops after biochar addition due to increased soil pH and strong adsorption capacity and a 50% N₂O reduction in soybean plots on acidic soils. With the addition of biochar, there is a spatial reorganization of C within soil particles, but the mechanisms remain unclear ^{[101][125][148][155][156][157][158][159][160][161][162]}[147,171,194,201,202,203,204,205,206,207,208].

Mitigating Climate Change by Removing CO₂ from the Atmosphere

Biochar’s proposed climate change mitigation mechanisms are its molecular structure, dominated by aromatic carbons that make it more resistant to microbial decomposition, allowing it to remain in the soil for thousands of years and potentially limiting greenhouse gas emissions. The change in CO₂ emission rate can be related to soil temperature, soil type, pyrolysis temperature, and biochar application rate ^{[34][35][36][140][163][164][165][166][167][168][169][170][171][172][173][174]}[63,95,96,186,209,210,211,212,213,214,215,216,217,218,219,220]. Biochar provides a large capacity to adsorb heavy metals from the soil due to its large surface area, multi-layered porous structure, abundant surface functional groups, increased soil redox potential, pH, organic matter content, and cation exchange capacity ^{[97][175][176]}[143,221,222]. At the same time, it shows a reduction in the leaching of nutrients into the soil due to the high CEC of biochar ^[177][77]. The modern literature references the involvement of biochar in other areas as well. The use of biochar in the horticulture industry for fruit and vegetable production is considered ^[159][178][205,223]. In addition, it contributes to the restoration of forest ecosystems and urban soils due to its ability to adsorb and immobilize organic substances ^[179][224]. However, it exhibits combinatorial and synergistic action with other materials such as compost, manure, paper mill sludge, and biosolids ^{[180][181][182]}[225,226,227].