Biochar Characteristics and Application: Effects on Soil Ecosystem Services and Nutrient Dynamics for Enhanced Crop Yields

Abstract

Although intensive farming practices have greatly increased food production, they have undermined the soil ecosystem services on which agriculture depends. Biochar application in soils is increasingly gaining worldwide acceptance as a means of addressing these environmental challenges while enhancing agricultural productivity. Biochar offers dual benefits that support food security and ecological well-being through enhanced soil fertility and plant nutrition. These benefits include water retention, promotion of soil microbial functioning, carbon sequestration, and nutrient absorption, among others. In spite of these known benefits, many studies continue to emphasize the roles biochar plays in enhancing soil health and crop yields but often neglect the influence of biochar characteristics, which are key in optimizing these soil ecosystem services. Thus, it is important to understand how biochar characteristics influence soil in supporting, regulating, and provisioning ecosystem services. This review offers a comprehensive and integrative assessment on how biochar’s characteristics influence key soil ecosystem services rather than examining each service individually. The focus is on how biochar feedstock material and pyrolysis temperature determine the characteristics of generated biochar and how these characteristics influence biochar’s efficacy in supplying soil ecosystem services and nutrient dynamics for enhanced crop yields.

1. Introduction

Sustainable soil management is the first step towards its long-term supply of ecosystem services, nutrient dynamics, and crop yields [1]. Ecosystem services are the diverse benefits that the natural environment provides to humans [2]. These should be sustainably managed to support the well-being of the present and future generations [2,3]. In soil management, these ecosystem services include provisioning services such as production of food, fiber, and feed; regulating services that maintain ecological balance through regulating climate; and supporting services such as the cycling of nutrients and water [4]. Intensive farming has resulted in declining soil carbon levels, thereby compromising the ecological balance of soil and the soil ecosystem services it provides [5,6,7], a process that has accelerated in recent years [8,9]. Due to this, coupled with the changes associated with the changing climate such as heatwaves, droughts, intense rainstorms, flooding, and other modifications, it is increasingly important to restore and enhance soil ecosystem services in agricultural lands [10].

Incorporating carbon-rich materials like biochar could provide a long-term solution towards addressing these agroecosystem challenges [11]. While the main agronomic goal of biochar is to increase crop yields, its use can also enhance other soil ecosystem services and promote the concept of ecosystem multi-functionality. For example, it can simultaneously provide multiple services such as waste reduction, carbon sequestration, decreased pollution, and remediation of land [12], hence multifaceted solutions that benefit the environment, agriculture, and climate. When combined with sustainable biomass production, biochar can be carbon negative. Thus, it reduces atmospheric carbon dioxide and offers substantial prospects for mitigating climate change [12].

The performance of biochar is influenced by many factors, such as the material used to generate biochar, temperature during pyrolysis, rate of application, biochar properties, soil type, soil fertility, location, and climate [13,14,15,16,17,18,19]. For example, degraded and low-fertility soils tend to show greater crop yield when treated with biochar compared to highly fertile and fine-textured soils [16,17,18]. While some studies have shown that biochar made from dung, wood, and straw feedstocks improves the productivity and nutrient retention of sandy loam soils [14,20], others have observed no difference in crop production when biochar made from comparable feedstock was applied to a similar type of soil. Studies assessing effects of pyrolysis temperature also reveal contrasting results [21,22,23]. Daneshvar et al. [24] revealed that increasing pyrolysis temperatures enhance water retention in sandy loam soils while Wiersma and van der Ploeg [25] found that biochar produced under similar temperatures showed no significant impact on water retention capabilities. In other studies, an increase in the amount of applied biochar led to more substantial enhancements in plant growth [26]. Interestingly, contrasting results have been found in some other instances [27], which observed a decrease in the yield of crops at increased application rates.

These inconsistent outcomes could potentially be ascribed to the applied biochar characteristics. It should however be noted that biochar characteristics are strongly influenced by feedstock material and production conditions. To optimize the supply of biochar’s soil ecosystem services to agriculture, it is critical to understand how biochar production conditions influence biochar characteristics and their influence on biochar services. To address this knowledge gap, we reviewed the physicochemical properties of biochar as influenced by feedstock material and production conditions. We then discussed the influence of biochar on soil properties that are critical to enhanced crop yields. To fully address these, we focused on (1) the influence of feedstock materials and production conditions on biochar production; (2) how production conditions determine biochar’s characteristics; and (3) how these characteristics influence the supply of soil ecosystem services when biochar is applied to the soil. These three factors are key to understanding how these characteristics influence biochar’s efficacy in supplying soil ecosystem services for farming efficiency and plant growth after application to the soil.

2. Biochar Definition and Generation

Biochar is an organic material made by pyrolysis of different biomass materials such as wood, plant residues, animal waste, and municipal sewage (Figure 1) at temperatures between 300 and 1000 °C in partial or anaerobic conditions [28]. It was defined as a porous, carbon-rich solid by Xiong et al. [29] due to organic compounds undergoing thermochemical transformation in an oxygen-deficient environment. It is a finely textured, carbon-dense material with a porous structure that is produced when organic biomass undergoes a low-temperature, oxygen-free thermochemical conversion process called pyrolysis [30]. When added to the soil, this stabilized biomass alters the soil environment, thereby enhancing the supply of soil ecosystem services, nutrient dynamics, and crop yields. Carbon is the dominant element in biochar, but also contains essential macronutrients like nitrogen, phosphorus, sulfur, ash, hydrogen, and oxygen, and micronutrients like potassium, calcium, sodium, and magnesium [30], necessary for plant growth and productivity. The composition of biochar changes depending on pyrolysis temperature, with an increase in temperatures (300–800 °C) resulting in higher carbon content and lower levels of nitrogen and hydrogen [24].

Biochar is derived from a diverse range of biomass sources such as chipped wood, plant remains, organic material, and poultry manure [24,31]. To manage waste through production and utilization, agricultural, industrial, and urban/municipal wastes have all been widely utilized as feedstocks [32,33]. In the commercial sector, feedstocks utilized include organic wastes such as distillers’ grain, bagasse, paper sludge, wood chips, grass, and crop residues [33]. The resultant final carbon-rich, solid, and flammable substance known as biochar, consisting of various components including extracts, proteins, carbohydrates, minerals, cellulose, hemicellulose, and lignin [22,34], is generated through this pyrolysis thermochemical transformation [35]. This converted carbon is persistent and stable for tens of thousands of years [36,37] and therefore offers an opportunity to combat climate change. Additionally, its manufacturing process yields gases and oil as byproducts [38]. The conditions during pyrolysis, especially temperature and heating rate, have a significant impact on producing high-nutrient biochar [39,40]. Char yields are greatly affected by temperature, with higher temperatures leading to lower yields [41] while longer charring times at the same temperature result in lower yields [42].

2.1. Methods of Biochar Production

The biochar production method significantly influences its characteristics. Through a thermal process devoid of oxygen called pyrolysis, biomass is converted into biochar, bio-oil, and syngas [43,44]. Gasification under low-oxygen conditions can produce up to 10% char by weight, whereas pyrolysis typically generates about one-third of the raw material as char by weight. Pyrolysis is performed using specialized equipment and an inert gas such as nitrogen [45]. The process causes inherent biomass polymers including cellulose, hemicellulose, and lignin to undergo alterations, break down, fragment, and cross-link at different temperatures [34] as represented below.

Biomass (Solid) = Biochar + Liquid or oil (tars, water, etc.) + Volatile gases (CO₂, CO, H₂)

Pyrolysis typically yields three main products—solids, liquids, and gases—whose properties are dependent on the feedstock conditions and process design. The solid product biochar has an energy density range of between 20 and 30 MJ/kg (HHV) and a carbon content range of between 60 and 90% [46,47]. The bio-oil liquid product’s energy density ranges from 15 to 30 MJ/kg and is hydrophilic with rich oxygenated compounds [48,49]. This liquid byproduct can occur independently or mixed with water generated from the original feedstock or during the pyrolysis reaction [50]. Syngas, a gas product, consists of a combination of carbon dioxide (9–55%), carbon monoxide (16–51%), hydrogen (2–43%), methane (4–11%), and other hydrocarbons. Its energy content ranges from 8 to 15 MJ/kg, with variations subject to purification levels, inert gas presence, and compositional changes during processing [51]. Biochar can be produced using either rudimentary cost-effective methods with simple equipment or advanced industrial-scale processes in specialized facilities [52]. Common low-tech methods for small-scale production include mini biorefineries, basic campfires, and biochar ovens [48,53].

Pyrolysis is typically divided into two main types and several related technologies, depending on temperature and heating duration. The primary types are fast pyrolysis and slow pyrolysis. Fast pyrolysis involves rapid heating that takes place within seconds to minutes, while slow pyrolysis occurs over a prolonged time, extending from minutes to several hours [54,55]. While fast pyrolysis produces a greater amount of bio-oil and syngas with less biochar, slow pyrolysis results in a higher quantity of biochar but reduced amounts of bio-oil and syngas [56,57].

2.1.1. Fast Pyrolysis

Fast pyrolysis is designed to produce bio-oil in large quantities and operates at temperatures exceeding 500 °C with heating rates over 1000 °C per minute [58,59]. It involves rapidly heating materials to high temperatures (around 500 °C) and then swiftly removing the resulting vapors and micro-particles [60,61]. It thus yields higher amounts of liquid products with reduced amounts of char [60].

2.1.2. Slow Pyrolysis

Slow pyrolysis produces more biochar and operates over a longer duration (30 min to several hours) at temperature rates of 100 °C/min [62]. Temperatures ranging from 250 °C to 500 °C facilitate this reaction. The process of slow pyrolysis could be categorized into conventional and modern charcoal production methods. Slow pyrolysis generates not only biochar but also liquid and gas byproducts, which may or may not be collected and utilized [63,64]. The traditional charcoal process often involves direct biomass combustion (mostly wood) in pits, mounds, or kilns, which reduce char yield. The combustion byproducts, including smoke with particles, carbon dioxide, methane, hydrocarbons, amines, and other pollutants, result in a net positive radiative forcing effect [60,63]. Thus, conventional char production methods are not well suited for the carbon sequestration goals of biochar. Biochar yields from slow pyrolysis of biomass typically range from 24% to 77% [60]. Biochar production type selection is guided by the target use objectives. For example, biochar produced from wood with high stable carbon content sequesters carbon while animal manure biochar with high nitrogen enriches soil nutrients [64] and is better for increasing soil quality and boosting food production [65]. Therefore, consideration of the pyrolysis temperature and kind of feedstock is necessary to achieve the intended soil ecosystem service [15,66].

2.2. How Production Conditions Influence Produced Biochar’s Physicochemical Properties

Biochar properties are influenced by various production parameters, including pre-treatment methods, pyrolysis conditions, feedstock materials, and post-treatment processes [67]. These influence its suitability for application as a soil amendment [68]. Studies on biomass pyrolysis reveal that biochar production depends upon several factors such as type of biomass, moisture content, particle size, reaction conditions (reaction temperature, reaction time, heating rate), surrounding environment (carrier gas type, flow rate of carrier gas), and other factors (catalyst, reactor type) [69,70]. Thus, biochar’s role and efficacy in many areas depend on the biomass utilized to make it. Therefore, consideration of the pyrolysis temperature and kind of feedstock is necessary to achieve the intended soil ecosystem service [15,66]. Understanding the relationships between pyrolysis parameters and biochar characteristics is essential for tailoring biochar for specific applications.

2.2.1. Pre-Treatment Techniques

Pre-treatment is a crucial step in biochar production. It significantly influences its characteristics and performance. Pre-treatment methods are generally categorized into physical, chemical, and biological approaches [71].

Physical pre-treatments involve processes such as particle size reduction and drying. Reducing biomass particle size improves biochar yield [72]. For example, pine wood biomass pre-treated with a dilute acid solution demonstrated enhanced biochar properties [72]. Similarly, dewatered sludge is commonly dried, crushed, and sieved to manage moisture content before pyrolysis [70]. Chemical pre-treatment using acids, alkalis, or oxidants is employed to modify biomass feedstocks. These treatments enhance biochar’s surface area, pore structure, and functional groups, thereby improving its performance in pollutant removal and other applications. Finally, biological pre-treatment through biological processes, such as anaerobic digestion or biofuel production, produces biochar with improved surface area and adsorption properties. This method has been particularly effective in enhancing biochar’s stability and functionality [73].

2.2.2. Post-Treatment Techniques

Post-treatment is performed to further enhance biochar’s physical and chemical properties. Methods such as chemical activation, magnetic treatment, and ball milling are applied to improve biochar’s surface area, pore structure, and surface chemistry [74,75]. These post-treatment processes aim to enhance biochar’s adsorption capacity, catalytic performance, and overall environmental benefits.

2.2.3. Feedstock Materials

Biomass is a complex solid material composed of biological, organic, or inorganic substances derived from living or recently living organisms [76]. The type of feedstock significantly influences the content of organic carbon and mineral characteristics [15,21]. Biomass is generally categorized into two types: woody and non-woody biomass. Woody biomass refers to residues from trees and forestry operations [77]. It is characterized by low moisture content, low ash content, low volatile matter, high density, and high calorific value, making it a desirable feedstock for biochar production [78]. Non-woody biomass includes materials such as animal manure, agricultural residues, and industrial biowastes. Compared to woody biomass, non-woody biomass typically has higher moisture content, higher ash content, higher volatile matter, lower density, and lower calorific value [79].

The feedstock composition such as lignin, cellulose, hemicellulose, ash, and moisture affects biochar characteristics such as the amount of carbon, volatile matter, ash content, cation exchange capacity, and pH [42]. For instance, biochar generated from residual materials and animal dung feedstocks tends to have a higher cation exchange capacity but lower specific surface area, carbon content, and volatile matter compared to those made from wood biomass and crop residues [21,22]. Notably, wood biochar generally has a higher carbon content and lower levels of ash, nutrients (like nitrogen, phosphorus, potassium, and sulfur), and exchangeable cations compared to other types of biochar.

Moisture content is a critical factor influencing biomass suitability for biochar production. Biomass moisture can exist in different forms: liquid water, water vapor, or chemically adsorbed water retained within the biomass pores. Excessive moisture content reduces char yield and increases the energy demand required to achieve pyrolysis temperatures [80,81]. For biochar production, low-moisture biomass is generally preferred as it minimizes heat energy consumption and shortens the pyrolysis process. This improves process efficiency, making biochar production more economically viable compared to processing biomass with high moisture content. The water-holding capacity (WHC) of biochar increases as the pyrolysis temperature rises [75], resulting in biochar yield drops, while bio-oil yield rises [66,76]. This means that optimizing pyrolysis conditions is crucial to achieving the desired balance between biochar and bio-oil production.

The lignin, cellulose, and hemicellulose content: When present in the feedstock, these contents significantly affect biochar yield. Lignin, being a resistant polymer, promotes char production, while cellulose and hemicellulose decompose at lower temperatures, contributing to tar formation [82,83,84]. Feedstocks with higher lignin content, such as coconut husk, result in higher char production [85]. The variation in these components influences both carbon content and ash content of the resulting biochar [15,22].

Volatile matter and ash content play a critical role in biochar properties. Feedstocks with high ash content, such as manure and agricultural residues, produce biochars with higher ash content and lower carbon content [86]. For example, biochar from wood typically has lower ash content compared to non-wood biochars like those derived from poultry litter or cow manure [86]. This reduction in ash content makes wood-derived biochars more suitable for soil incorporation without increasing soil ash content [87].

The specific surface area (SSA): The SSA of biochar, an important property affecting its adsorption capacity, is influenced by feedstock type. Wood-based biochars generally have higher surface area (127 m²/g) compared to non-wood biochars, which have lower surface areas due to higher ash content [88]. The presence of lignin in feedstock materials contributes to higher surface areas by promoting the formation of pores during pyrolysis [89]. The SSA of biochars is generally lower when the feedstock contains high levels of inorganic components [22].

Cation exchange capacity (CEC) reflects biochar’s ability to retain and exchange cations, and varies with feedstock; for example, biochar from manure, such as poultry litter, has higher CEC due to higher ash content [90]. Poultry litter-derived biochars have a CEC of 48.4 cmol/kg compared to lower values from wood-derived biochars [91]. This high CEC enhances the biochar’s potential for improving soil fertility and nutrient retention.

pH of biochar: The pH of biochar is typically alkaline and is influenced by feedstock type and ash content. Non-woody feedstocks, such as poultry litter and sugarcane bagasse, often yield biochars with higher pH values due to the presence of alkaline salts in the ash [92,93]. Biochars derived from wood generally have a lower pH but may still be alkaline enough to enhance soil pH [94].

2.2.4. Pyrolysis Parameters

Pyrolysis Temperature: The pyrolysis temperature plays a crucial role in shaping the structure and physicochemical properties of biochar, influencing its function as a soil amendment. In the pre-pyrolysis stage (ambient temperature of 200 °C), moisture and light volatiles evaporate, causing bond breakage and the formation of hydroperoxides and carbonyl groups [22]. The second stage (200–500 °C) involves rapid devolatilization and decomposition of hemicelluloses and cellulose [95]. In the final stage (above 500 °C), lignin and other organic matter degrade with the formation of stronger chemical bonds [22]. As pyrolysis temperature rises, the components of biochar which are primarily cellulose, lignin, and hemicellulose [22] decompose progressively. Hemicellulose decomposes first, followed by cellulose, while lignin (being more recalcitrant) decomposes at a slower rate [26].

Temperature particularly affects the surface area and pH. pH changes result from the removal of functional groups including hydroxyl, carbonyl, and carboxyl groups [73]. Fu et al. [96] demonstrated that biochars derived from cornstalks, rice straw, cotton straw, and rice husks under fast pyrolysis showed increased surface area and total pore volume with rising temperatures, peaking at 900 °C before declining at 1000 °C. Similarly, Ighalo et al. [85] observed that coconut flesh waste biochars produced via slow pyrolysis between 350 °C and 600 °C exhibited increased carbon content and reduced hydrogen and oxygen as temperatures rose. This was attributed to moisture loss and the breakdown of volatile compounds. Low-temperature biochar (below 550 °C) is characterized by ash concentration and lower crystalline structure feedstock properties [42]. To produce biochar with enhanced nutrient levels, pyrolysis temperatures below 400 °C are recommended [15]. Alternative methods like co-pyrolysis and hydrothermal carbonization (HTC) can also be used to create biochar and bio-oil with unique properties [44].

Higher pyrolysis temperatures increase biochar surface area, carbonized fractions, pH, and volatile matter while decreasing cation exchange capacity CEC and the content of surface functional groups [15,22]. This is likely caused by the decomposition of organic materials and formation of micropores [97]. The destruction of aliphatic alkyls and ester groups, along with exposure of aromatic lignin cores, further contributes to an increase in surface area [98]. However, at temperatures higher than 600 °C, biochar becomes less hydrophilic and more hydrophobic [99]. Furthermore, higher pyrolysis temperatures release volatile compounds, thereby increasing pore formation and surface area [96]. For example, biochars produced from cottonseed hulls, poultry litter, and dairy manure exhibit low surface areas [22]. As temperature increases, surface functional groups, which are crucial for adsorption, are reduced, leading to a decrease in CEC [100]. These structural and chemical changes significantly affect biochar’s effectiveness as a soil amendment, with temperature being a key factor in determining its surface characteristics and functional group composition [15].

Pyrolysis temperature also influences biochar’s hydrophobicity and adsorption capacity. At temperatures higher than 650 °C, biochar becomes thermally stable and hydrophobic, enhancing its potential to adsorb organic and inorganic contaminants [22].

The pH of biochar is also strongly linked to pyrolysis temperature. Higher temperatures promote the formation of carbonates and alkaline elements, leading to increased pH values [94]. The release of alkali salts from organic materials during pyrolysis contributes to the alkalinity of biochar at temperatures above 300 °C [94]. Pyrolysis temperature thus significantly influences biochar’s physicochemical properties, which in turn impact its potential as a soil amendment and its role in environmental remediation [22].

Residence Time: Residence time refers to how long biomass remains in the reactor at a given temperature. Extending the residence time during pyrolysis has varying effects depending on the temperature. At a lower pyrolysis temperature (300 °C), prolonged residence time gradually reduces biochar yield while increasing its pH and iodine adsorption capacity. In contrast, extending the residence time at a higher pyrolysis temperature (600 °C) has minimal impact on biochar yield or pH but tends to decrease its iodine sorption capacity [101]. Patil et al. [102] found that increasing the residence time from 10 to 100 min at 500 °C during rapeseed stem pyrolysis slightly reduced yield, as prolonged exposure allowed more biomass to volatilize into biogas and bio-oil. Wang et al. [70] observed similar trends when testing biochars derived from cornstalks, rape straw, wheat stalks, and peanut shells at temperatures ranging from 300 °C to 700 °C with residence times of 1 to 4 h, where longer residence times and higher temperatures reduced yields but increased pH as acidic functional groups were degraded. Further experiments by Wang et al. [70] on bamboo wood, elmwood, rice straw, wheat straw, cornstalks, rice husks, and coconut shells revealed that extending residence times up to 16 h increased the Brunauer–Emmett–Teller (BET) surface area.

Heating Rate: The rate at which temperature increases affects biochar’s morphology and chemical structure. Slower heating rates (≤0.1 °C/s) enhance biochar yield and stability by promoting gradual heat conduction, while faster rates (>1000 °C/s) favor bio-oil production by accelerating volatile compound release [70]. Slow heating rates require the biomass to spend a much longer time to achieve 400 °C, which means that it would stay in the higher-temperature atmosphere for a longer time. Thus, the biomass would have a more complete transition to char and release more volatiles. At a higher heating rate, more bonds break before the rearrangement reactions happen [101]. Higher heat and faster pyrolysis rates enhance the creation of aromatic structures and cause greater decomposition of organic matter [68]. This results in biochar with reduced volatile matter and CEC but increased pH, ash content, carbon content, porosity, and specific surface area [69,70]. High-temperature and fast-pyrolysis biochar is more durable and hydrophobic, allowing it to resist water absorption in the soil for longer periods [71,72].

3. Biochar Characteristics and Their Influence on Delivery of Soil Ecosystem Services

Biochar’s characteristics are influenced by the pyrolysis conditions and feedstock material [42,103]. Higher heat and faster pyrolysis rates enhance the creation of aromatic structures and greater decomposition of organic matter [104], resulting in biochar with reduced volatile matter and CEC but increased pH, ash content, carbon content, porosity, and specific surface area [105,106]. High-temperature and fast-pyrolysis biochar is more durable and hydrophobic [107,108]. Temperature particularly affects the surface area and pH, with pH changes having to do with the removal of functional groups including hydroxyl, carbonyl, and carboxyl groups [109]. The type of feedstock significantly influences the content of organic carbon and mineral characteristics [15,21]. Low-temperature biochar (below 550 °C) is characterized by ash concentration and crystalline structure which are lower and more influenced by feedstock properties [42,67,110]. Thus, biochar yield can be regulated by optimizing feedstock and pyrolysis temperature [21,46,110]. The water-holding capacity (WHC) of biochar increases as the pyrolysis temperature rises [111], resulting in biochar yield dropping, while bio-oil yield rises [66,112]. This means that optimizing pyrolysis conditions is crucial to achieving the desired balance between biochar and bio-oil production. To produce biochar with enhanced nutrient levels, pyrolysis temperatures below 400 °C are recommended [15,66]. Alternative methods like co-pyrolysis and hydrothermal carbonization (HTC) can also be used to create biochar and bio-oil with unique properties [44], highlighting the importance of determining their properties.

3.1. Biochar Characterization Techniques

Biochar characterization involves determining its physical and chemical properties. This is crucial for predicting its suitability and effectiveness in various applications, such as soil amendment, carbon sequestration, and environmental remediation. It is essential for evaluating its potential in pollutant removal and other applications. Thus, it helps to predict biochar’s performance in the delivery of soil ecosystem services as well as its environmental impact [113]. Various techniques are employed to assess biochar characteristics, including analysis of its structure, surface functional groups, and elemental composition [114]. Common methods include scanning electron microscopy (SEM), Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), nuclear magnetic resonance (NMR), BET surface area analysis, and thermogravimetric analysis (TGA) [115]. Additionally, proximate and ultimate analysis, along with Raman spectroscopy, are widely used for detailed characterization [77].

3.1.1. Nuclear Magnetic Resonance (NMR) Spectroscopy

NMR spectroscopy analyzes the structural composition of biochar by examining the resonance frequencies of atomic nuclei [115]. It helps quantify carbon functional groups and assess the extent of aromaticity and the overall structure of biochar [68]. NMR also distinguishes between aliphatic and aromatic hydrocarbons, offering a means to evaluate biochar’s carbonization and stability [116]. However, ferromagnetic minerals in biochar can interfere with NMR signals, particularly in high-temperature pyrolysis biochar, which tends to have a low signal-to-noise ratio [117].

3.1.2. Thermal Analysis

Thermogravimetric analysis (TGA) is used to study biochar’s thermal behavior as temperature rises [118]. It helps to analyze the ignitability of biochar and its interactions with biomass. TGA provides insights into the heating process, with biochar typically heated from room temperature to 1000 °C [81].

3.1.3. Scanning Electron Microscopy (SEM)

SEM is widely used to examine the surface structure and pore size of biochar [81]. Combined with BET analysis, SEM reveals biochar’s surface area and microporous and mesoporous distributions, which are important for understanding its adsorption capacity. SEM can also be used to study the surface changes of biochar after pollutant adsorption, with Energy-Dispersive X-Ray Spectroscopy (EDX) helping to determine its elemental composition [119]. However, SEM-EDX is not suitable for studying organic pollutants [120].

3.1.4. X-Ray Diffraction (XRD)

XRD is essential for assessing the crystallinity and structure of biochar [81]. It provides insights into the amorphous nature of biochar at temperatures above 350 °C. XRD patterns reveal the crystalline structure of nanoparticles formed during pyrolysis [121], allowing for the rapid, non-destructive evaluation of biochar’s sorption potential.

3.1.5. Fourier-Transform Infrared Spectroscopy (FTIR)

FTIR is a technique used to analyze the functional groups on biochar’s surface by studying vibrational patterns [122]. As temperature increases, biochar undergoes significant changes in composition and structure, which FTIR can detect non-destructively [123]. The spectra show a loss of aromatic groups between 650 and 800 °C [124]. FTIR can be combined with diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS), where biochar is pelletized with potassium bromide (KBr) and analyzed via ATR-FTIR [123]. NMR spectroscopy can also be used to identify functional groups [125].

3.2. Biochar Characteristics Influencing Delivery of Soil Ecosystem Services

Key physicochemical characteristics of biochar include enhanced cation exchange capacity (CEC), low bulk density, increased surface area and porosity, neutral to elevated pH, and higher amounts of carbon. Additionally, it contains basic cations like Ca, K, and Mg as well as N, P, and other elements that are vital to plant growth and productivity [126,127]. The unique physical and chemical biochar characteristics resulting from feedstock material and production processes affect its efficacy in supplying soil ecosystem services [128]. These characteristics include bulk density, cation exchange capacity, particle size, pore structure and distribution, pH, surface area, and the quantity and diversity of diverse functional groups (Figure 2).

The biochar characteristics affect soil fertility, nutrient cycling, water-holding capacity and availability, soil structure and porosity, carbon sequestration and storage, soil biota and microbial activity, and environmental remediation and pollution mitigation in various ways [127,129]. Hence, to optimize biochar’s ability to supply these ecosystem services, its characteristics should be considered and matched to specific soil, environmental, and agricultural needs.

3.2.1. Specific Surface Area

Biochar exhibits an exceptionally high surface area, with a range of multiple hundred to more than a thousand square meters per gram [29]. The specific surface area is influenced by biochar generation processes such as heating rate, pyrolysis temperature, and availability of reactive agents like steam, CO₂, and O₂ [130]. Increasing pyrolysis temperatures form micropores which result in an enhanced biochar surface area [131]. During pyrolysis, volatile compounds are released, which contribute to the creation of porous material that increases the surface area of biochar [132]. However, extreme increases in temperature can result in biochar surface area decline [133].

The source of feedstock also affects the surface area, with biomass often providing a larger surface area compared to sewage sludge [28]. Further, biochar derived from lignocellulosic biomass generally features larger surface areas compared to biochar made from sludge or manure/litter [134]. The increased specific surface area plays a crucial role in nutrient cycling and farming efficiency by improving the soil’s CEC, water storage, and nutrient retention and release [131,132,133].

3.2.2. Porosity, Particle Size, and Pore Distribution

Biochar’s porosity, pore distribution, and particle size vary depending on the pyrolysis temperature and feedstock material [135]. Generally, a higher pyrolysis temperature increases biochar porosity even though some temperatures may unexpectedly decrease it. For example, Li et al. [136] reported that increasing temperature from 200 °C to 300 °C did not impact pore volume. This may suggest a temperature threshold for porosity changes. However, the same study revealed a slight increase (0.171 to 0.174) when the temperature rose from 300 °C to 400 °C, while further temperature increments to 500 °C, 600 °C, and 700 °C led to a fluctuating pattern of decrease, increase, and subsequent decrease in pore volume, respectively. The type and condition of the biomass feedstock play a role in biochar pore structure and distribution [137]. This variation can affect biochar’s effectiveness as a soil amendment, underscoring the importance of optimizing production conditions to maximize biochar’s benefits to soil health and ecosystem services.

The size, form, and grinding technique of the initial biomass influence the physical properties of the biochar [138]. The particle size of biochar offers vital soil services in determining the aggregation and stability of soil, water retention, and nutrient availability [87]. Studies have revealed that the size and shape of biochar particles profoundly determine the soil’s physicochemical and hydraulic properties [139]. Fine and small biochar particles can form aggregates with soil particles modifying soil structure and water retention by filling voids between soil and altering interpore structure [140], thereby enhancing soil aggregation and water retention [109,139,141]. Boosting macroaggregate formation improves soil characteristics and processes like water-holding capacity, mitigates water stress, reduces soil erosion, and improves plant growth [41,139,140]. Recognizing these effects will allow for optimized biochar application designs to boost soil nutrient use efficiency and plant growth.

Biochar’s impacts on soil characteristics are largely influenced by its pore structure, which includes macropores, mesopores, and micropores. Micropores affect the soil’s sorption and chemical properties, while macropores and mesopores enhance water-holding capacity and provide a suitable environment for soil microorganisms [136]. Additionally, biochar particle shape and size are essential in determining the soil’s physicochemical and hydraulic properties. Smaller particles can aggregate, affecting the movement and retention of water within the soil [135]. To optimize biochar’s benefits for soil ecosystem services, it is essential to consider its pore structure, shape, and particle size as these factors interact in complex ways.

3.2.3. Stability

Biochar’s stability is a crucial factor in determining its ability to store carbon over the long term [111]. There is no established method to measure biochar’s stability due to the challenges of predicting its durability over extended periods (ranging from centuries to millennia). This is because biochar’s degradation in the environment is determined by a complex array of biological, chemical, and physical processes. Moreover, biochar’s properties can vary significantly depending on its source, making it difficult to pinpoint a specific characteristic that ensures long-term stability [62]. As a result, researchers are exploring innovative approaches such as accelerated aging to better understand ways for assessing biochar’s stability.

3.2.4. Water-Holding Capacity

A beneficial characteristic of biochar that enhances plant development and yield, especially in dry and sandy soils, is its high water-holding capacity [135,138,142]. Enhancing soil water retention boosts microbial activity and lessens the effects of water shortage by raising soil moisture levels and lowering drought stress [143]. Biochar’s porosity, surface area, and hydrophilicity affect how much water it can hold [134,136]. Greater water absorption and retention are facilitated by higher surface area and porosity, while biochar’s hydrophilicity allows it to draw and hold onto water molecules [140].

3.2.5. pH

Biochar typically has a neutral to high pH that improves soil quality, available nutrients, and crop development, particularly in acidic and degraded soils. It is effective in raising soil pH and buffering acidity [144]. Increased pyrolysis temperatures and high ash content result in improved pH values for biochar [21,42,145]. However, some studies have noted that pH levels can decrease or become erratic with rising temperatures [146,147]. The feedstock also influences biochar pH. For example, lignin-rich materials produce biochar at around pH 8, whereas algae, waste biomass, and manure yield biochar with pH values near 9.5 [148,149]. Alkaline biochar can increase soil pH in acidic soils and limit their ability to transport cationic metals including copper, zinc, cadmium, and mercury [150,151].

3.2.6. Bulk Density

Biochar’s low bulk density is a beneficial property that improves soil structure and functionality [152]. By minimizing soil compaction and boosting porosity, biochar can enhance soil aeration, water infiltration, and drainage while also supporting healthy root growth and activities of microbes [127,152]. Biochar’s bulk density is shaped by biomass density and pyrolysis temperature. Lower biomass density and higher temperatures result in lower bulk densities [127,153]. This implies that biochar created at increased temperatures will have a lower bulk density, leading to greater improvements in the soil’s porosity and aeration [112,153].

The effectiveness of biochar characteristics in delivering ecosystem services is based on their physicochemical properties as influenced by the application rates, soil properties, climate conditions, and other factors like site-specific management practices, crop types, etc. [22,23]. The complex interplay between biochar’s properties and their temporal evolution is critical in determining the long-term efficacy of biochar in improving soil health and its ability to enhance ESs. For example, local climatic conditions characterized by temperature and rainfall influence the rate of biochar decomposition and how it impacts soil processes. In turn, the soil status (structure, texture, etc.) will play a significant role in nutrient and water retention/leaching, a very important determinant of crop yields.

The pyrolysis temperature, feedstock material, and application rate are primary factors, as these determine biochar’s porosity, surface chemistry, and nutrient content. A critical consideration is the potential for threshold effects, where the benefits of biochar application may only become apparent once certain thresholds of application rate or pyrolysis temperature are exceeded. For example, while low application rates may have minimal effects on soil fertility, higher concentrations may improve soil structure, enhance water retention, and boost nutrient availability, but could also lead to negative outcomes, such as nutrient leaching or soil acidity.

The effectiveness of biochar is also strongly influenced by site-specific factors, such as climate, soil type, and land management practices. Variations in environmental conditions across different agro-ecological zones play a significant role in modulating biochar’s efficacy. For example, in arid or semi-arid regions, biochar may improve soil water retention and reduce evaporation rates, while in temperate zones, its ability to enhance soil structure and nutrient cycling may be more pronounced. Similarly, soil type is a critical determinant of biochar’s impact, as clay-rich soils may benefit more from biochar’s ability to improve nutrient retention, while sandy soils may see greater improvements in water-holding capacity. Moreover, land management practices, such as tillage, cropping systems, and organic amendments, interact with biochar to either enhance or diminish its positive effects on soil fertility and productivity. Thus, a one-size-fits-all approach is not suitable for biochar application; site-specific factors must be carefully considered to optimize its performance and maximize its contribution to soil ecosystem services.

3.3. Optimizing Process Parameters in Biochar Pyrolysis

Optimizing process parameters such as feedstock flow rate, moisture content, reactor temperature and pressure, heating rate, and residence time is crucial for achieving desired product distributions. Variations in these factors can significantly impact product composition and pyrolysis efficiency [154]. Additionally, biomass properties such as chemical composition, moisture, ash content, and particle size influence the pyrolysis process, necessitating careful feedstock selection and preparation [155]. While laboratory and pilot experiments provide valuable insights into optimal parameters, they often require substantial time and financial investment [156]. Process simulation using software like Aspen Plus Version 14 offers efficient means to study and optimize pyrolysis processes because of its ability to model primary pyrolysis reactions without delving into complex reaction mechanisms. However, there is a scarcity of published works simulating biomass pyrolysis in Aspen Plus that align closely with experimental data [157,158]. Aspen Plus facilitates the simulation of chemical processes through interconnected unit operation blocks, such as reactors and heat exchangers. These blocks are linked by energy and mass streams, creating a flowchart that represents the process. The software’s built-in physical property database and sequential modular approach simplify model development and updates [157,159]. Simulating biomass pyrolysis in Aspen Plus typically involves three core stages: (i) defining the biomass composition, (ii) modeling the decomposition process, and (iii) simulating the pyrolysis reactor. Biomass pyrolysis models in Aspen Plus are typically categorized into thermodynamic equilibrium (TE), kinetic, and fixed data (FD) models. Each method has distinct characteristics, assumptions, and limitations. The choice between TE, kinetic, and FD models depends on the study’s objectives, available data, and the complexity of the pyrolysis system. While TE models offer simplicity and are ideal for identifying broad trends, kinetic models provide greater accuracy for dynamic and non-equilibrium processes. Meanwhile, FD models are practical for system-specific simulations but lack broader applicability. Understanding the strengths and limitations of each approach is crucial for selecting the most suitable method for biomass pyrolysis modeling.

3.3.1. Thermodynamic Equilibrium (TE) Models

The thermodynamic equilibrium (TE) approach, particularly the minimum Gibbs free energy (MGFE) method, is favored for its simplicity, particularly when exploring broad relationships between biomass composition, which relies on the ultimate and proximate analysis of the feedstock data that is widely available in the literature. The MGFE method calculates product distribution by minimizing the system’s Gibbs free energy, based on the feed composition, reaction temperature, and pressure [156]. TE models operate under key assumptions, including perfect mixing of all substances, constant temperature within the pyrolysis zone, and unlimited reaction time to ensure equilibrium conditions are achieved [156]. Additionally, because TE models do not require detailed pyrolyzer design data, they are easier to implement than kinetic models. However, TE models have limitations. Achieving true equilibrium is rarely feasible in practical pyrolysis systems, especially at moderate temperatures where reaction rates are slow. Moreover, in fast pyrolysis, where bio-oil production is prioritized, equilibrium conditions are intentionally avoided since they tend to promote the formation of unwanted byproducts. Despite these drawbacks, TE models remain popular due to their straightforward calculation process and minimal data requirements [156,159].

3.3.2. Kinetic Models

Kinetic models offer a more detailed and dynamic approach to biomass pyrolysis simulation. Unlike TE models, kinetic models focus on time-dependent reaction processes and are designed to capture the complex behavior of biomass decomposition. Biomass properties are defined based on biochemical analysis, providing detailed insights into the feedstock’s composition. The kinetic method relies heavily on reaction kinetics and hydrodynamic behavior inside the pyrolyzer. This allows kinetic models to track how product distribution and temperature profiles change over time under non-equilibrium conditions [160].

Kinetic models are well suited for simulating fast pyrolysis or systems operating under dynamic conditions. However, developing accurate kinetic models can be challenging due to the complexity of gas–solid interactions, particle behavior, and reaction pathways. Consequently, kinetic models often require extensive experimental data to define these intricate processes, making them more complex and less flexible than TE models [160,161].

3.3.3. Fixed Data (FD) Models

The fixed data (FD) approach offers a simplified modeling technique that integrates experimental or literature data directly into the simulation. Like TE models, FD models define biomass characteristics using ultimate and proximate analysis data. Unlike TE and kinetic models, the FD method treats the decomposition and reactor stages as fixed units. Data for these stages, such as biomass composition, operating conditions, and product yields, are obtained from previous experimental work or published studies. FD models are highly system-specific and best suited for replicating established pyrolysis setups. For instance, Zaini et al. [162] developed a biomass pyrolysis simulation model integrated with steam reforming and the water–gas shift process to produce hydrogen, biochar, and bio-oil. In this case, data for the pyrolysis unit, including biomass properties, temperature, pressure, and product yields, were sourced from an operating pilot plant in Envigas, Sweden. The hydrogen production system was then modeled using Aspen Plus to assess system performance under different conditions.

Due to their reliance on fixed datasets, FD models are limited in scope; they are only applicable to the specific system for which they were designed and cannot be easily adapted for different biomass types or pyrolysis conditions [158,160]. On the other hand, TE models, often based on the minimum Gibbs free energy principle, are popular due to their simplicity and reliance on readily available thermodynamic data. In spite of this, they may not accurately predict product distributions in non-equilibrium conditions [156]. Therefore, kinetic models offer more detailed predictions by considering reaction rates and mechanisms even though they require complex input data and their sensitivity to feedstock variations [160]. FD models use empirical data to define decomposition and reactor behavior, making them specific to particular systems and less adaptable to different feedstocks or configurations [163].

TE models assume that during pyrolysis, the system reaches a state where the rates of forward and reverse reactions are equal, leading to a balance of products and reactants. It involves calculating the distribution of products based on the minimization of Gibbs free energy, considering all possible reactions and their equilibria. TE models are useful for understanding the ultimate yields of various products under specific temperature and pressure conditions, assuming the system has sufficient time to reach equilibrium. Unfortunately, real pyrolysis processes often do not reach equilibrium due to rapid reaction rates and limited residence times, making this method less accurate for modeling actual pyrolysis behavior [158].

Kinetic models focus on the rates of individual chemical reactions occurring during pyrolysis, emphasizing the time-dependent evolution of products. They employ kinetic models, often derived from experimental data, to simulate the decomposition of biomass into various products over time. Kinetic models are essential for predicting the dynamic behavior of pyrolysis processes, including the influence of temperature, heating rate, and catalyst presence on product distribution [164].

3.3.4. Limitations of the Models

Kinetic models require detailed kinetic data and may be complex to parameterize accurately due to the multitude of reactions involved. The thermodynamic equilibrium method provides insights into the potential product distribution at equilibrium conditions, while the kinetic method offers a more accurate representation of the actual pyrolysis process by accounting for reaction rates and time-dependent changes. For modeling biomass pyrolysis, the kinetic method is generally preferred due to the rapid and non-equilibrium nature of the process.

Understanding the influence of process parameters on product yields is crucial for optimizing pyrolysis systems. Continued research and model development are essential to bridge the gap between simulation results and experimental observations, enhancing the efficiency and effectiveness of biomass pyrolysis processes [161].

4. Factors That Influence Biochar Action on Ecosystem Services and Their Evaluation

4.1. Biochar Factors That Influence Action on Ecosystem Functioning

4.1.1. Biochar Feedstock

The feedstock material selected for biochar production profoundly impacts its long-term stability, nutritional content, and carbon content [15,145]. Different feedstocks yield biochar with varying properties. For instance, biochar derived from manure typically has higher levels of phosphate and nitrogen compared to wood-based biochar [149]. Additionally, manure-based biochar tends to decompose faster and release less greenhouse gases [165]. Therefore, selecting a feedstock that meets the exact requirements of the crop and soil is essential to maximize biochar’s benefits [15].

4.1.2. Pyrolysis Temperature

Biochar’s physicochemical characteristics, including pH, surface area, and porosity, are significantly affected by the pyrolysis temperature [15,21,22]. Higher pyrolysis temperature produces biochar that has higher pH, greater surface area, and enhanced porosity compared to biochar created at lower temperatures [22,146,166]. This is because higher temperatures facilitate the formation of larger pores and surface areas which in turn elevate pH levels and improve adsorption capacity [166]. Therefore, adjusting the pyrolysis temperature to match specific soil and water conditions is crucial for producing biochar with desired properties [15,167].

4.1.3. Application Rate

The application amount of biochar significantly affects soil processes and properties. It influences crop productivity, biodiversity, water and nutrient availability [37,64,127,168]. Higher application rates can enhance the soil’s ability to store water and exchange cations [111,128]. However, it may also raise the risk of heavy metal accumulation and biochar toxicity [33,146]. To optimize its benefits, biochar should be applied based on the nature of the soil and environmental situation [111,169].

4.1.4. Biochar Properties

The physical and chemical characteristics of biochar including porosity, functional groups, and surface area are essential in their relationship with soil, water, nutrients, and microorganisms [131,168,169]. Biochar’s unique structure, characterized by a large surface area and extensive porosity, supports beneficial soil microorganisms [61,143]. To fully harness its benefits, biochar’s characteristics should be tailored to meet specific objectives for soil and crop sustainability [127].

4.1.5. Soil Properties

The suitability and performance of biochar are significantly influenced by the soil’s physicochemical properties [127,151]. Biochar is more beneficial in acidic, sandy, and nutrient-depleted soils where it improves pH, water retention, and nutrient availability compared to alkaline, clayey, and nutrient-rich soils [26,170]. Specifically, in sandy and acidic soils, biochar can improve nutrient availability and water retention and reduce soil acidity [142,146]. Hence, aligning soil characteristics with suitable biochar selection is essential to maximize its performance and supply of soil ecosystem services.

4.1.6. Climate

The long-term climatic conditions of a location, like temperature and precipitation, significantly influence the degradation rate, stability, and carbon storage potential of biochar [171,172]. Biochar decomposes more slowly and retains more carbon in cooler, wetter climates compared to warmer, drier ones, making it more stable and effective in lowering greenhouse gas emissions in the former [36,147]. Specifically, biochar’s half-life increases with lower temperatures and higher precipitation, enhancing carbon sequestration in wetter and colder regions [172], particularly in fine-textured soils with low carbon content in temperate regions [173]. While biochar can offset warming-induced GHG emissions in temperate agroecosystems, it may also increase N₂O emissions under elevated temperatures [174]. To maximize the optimal benefits of biochar, it is therefore crucial to align climate conditions with appropriate production and application methods [36].

4.1.7. Other Factors

Besides its characteristics and application methods, the benefits of biochar also depend on other factors such as crop type and quality, management practices, interactions with other soil amendments, and environmental and socioeconomic factors [130]. Biochar improves the overall quality and yield of crops that are vulnerable to nutrient and water stress [146]. However, biochar also has limitations, including increased biomass demand, potential competition with food production, and impacts on local livelihoods [175]. Hence, to maximize ecosystem benefits and minimize adverse effects, it is essential to consider these factors and to ensure they align with biochar use goals.

4.2. Evaluation of Biochar Influence on Soil Ecosystem Services

Spatial parameters, such as local climate and soil characteristics, along with temporal factors, are essential for identifying regions where biochar application can be prioritized, from local to continental scales. However, variations in local climate, soil properties, and biomass resource availability exist. Therefore, prioritization of biochar influence, through the development of biochar priority maps that incorporate both spatial and temporal considerations, can address key questions such as evaluating the influence of biochar on soil ecosystem services. For example, the influence of soil characteristics and their interaction with biochar and resulting soil ecosystem services, together with climate, on decomposition can be evaluated just like land characteristics (e.g., use and topography), which dictate the soil conditions and targeted soil ecosystem services. Temporally, the age of biochar, the season it is applied, and the length it will stay in the soil can be evaluated to determine its effectiveness. Karan et al. [176] propose a four-step framework to evaluate biochar’s influence based on distinct ES priorities. This approach integrates multiple criteria analysis (MCA) methods with GIS techniques to provide spatial insights on biochar deployment.

4.2.1. Defining Prioritization Narratives

Three prioritization narratives are identified to align biochar application with specific ES outcomes:

(a)

Improving Soil Quality: Focuses on regions where biochar can enhance soil structure, pH balance, and nutrient retention.

(b)

Crop Resilience: Targets areas where biochar’s moisture retention and stress mitigation effects can improve crop performance.

(c)

Reducing Nitrogen Leaching: Prioritizes regions where biochar can minimize nitrogen loss to groundwater.

4.2.2. Criteria Selection

Ensures that comprehensive prioritization, soil texture, pH, and soil organic matter (SOM) are selected as baseline criteria for all narratives. Studies have demonstrated that biochar application is particularly effective in acidic and sandy soils, reinforcing this criterion as a suitable indicator for biochar impact [177]. For specific narratives, additional criteria are incorporated. To reduce nitrogen leaching, nitrogen leaching data are integrated to identify vulnerable regions. To improve crop resilience, ground moisture data are added to capture areas prone to drought stress.

4.2.3. Criteria Prioritization via AHP

The Analytical Hierarchy Process (AHP) [178] is employed to assign numerical weights to the selected criteria through pairwise comparison. For instance, in the nitrogen leaching narrative, SOM receives a higher weight relative to pH and texture, based on the logic that nitrogen mineralization from organic matter influences leaching potential.

4.2.4. Weighted Linear Combination (WLC)

Here, a Biochar Use Indication Map (BUIM) is created using the WLC method [179], combining weighted criteria to assign priority scores on a scale of 1 (low priority) to 5 (high priority). Karan et al. [176] highlighted that spatial variability in soil properties significantly influences biochar’s capacity to deliver ESs in a Swedish arable land. In this study, 25% of the land was identified as medium to high priority (scores 3–5), aligning with regions that exhibited acidic and sandy soils. For improving crop resilience, 39% of the arable land received medium to high priority, particularly in areas prone to drought. For reducing nitrogen leaching, only 7% of the area showed high priority, emphasizing biochar’s limited but targeted effectiveness in reducing nitrogen loss.

Other studies have demonstrated biochar’s ES benefits in diverse environmental conditions. For soil quality improvement, Daunoras et al. [180] showed that biochar enhanced crop yields in acidic and sandy soils, particularly in temperate climates. Similarly, Tusar et al. [181] identified acidic soils with low SOM as prime candidates for biochar application. For crop resilience, biochar’s ability to improve drought resilience was reinforced by Jeffery et al. [182], who reported that biochar applications improved soil water retention in dryland agricultural systems. This effect is most pronounced in regions with low clay content, highlighting biochar’s potential in water-scarce environments and for nitrogen leaching reduction. Studies by Egyir et al. [183] demonstrated biochar’s ability to reduce nitrogen leaching in sandy soils with high rainfall exposure. However, these effects were site-specific, showing greater success when biochar was paired with low-N input systems.

Biochar’s influence on soil ecosystem services (ESs) varies across spatial and temporal scales. Field-scale studies have shown that biochar has immediate effects on soil health and nutrient availability, particularly in degraded soils with low SOM or pH imbalance [184]. At the landscape scale, GIS-based frameworks can identify spatial patterns and target biochar deployment to maximize ES benefits, such as mapping biochar application zones in Brazilian farmlands based on soil texture and SOM data (2002). Regional models have demonstrated that biochar’s influence can extend to broader soil ecosystem services like carbon sequestration and water retention, with regional hotspots showing reduced nitrogen leaching in nitrate-sensitive watersheds. Temporally, biochar’s effects can be observed in the short term, with improved moisture retention and soil conditioning within one growing season [185], and in the long term, with persistent carbon sequestration benefits lasting for decades [186]. Spatial correlation analysis has revealed synergies and trade-offs between ES priorities, with a strong correlation between improving soil quality and improving crop resilience, and a minimal correlation between reducing nitrogen leaching and other narratives [187].

4.3. Cascading Effects of Biochar Characteristics on Ecosystem Processes

The cascading effects of biochar characteristics on ecosystem processes can be understood by examining how its properties influence micro-, meso-, and macro-scale processes. These effects vary depending on biochar’s feedstock, production temperature, and post-production treatments. They also depend on the type of feedstock materials used, pyrolysis conditions, soil type, climate, and agricultural practices. For example, in highly fertile tropical soils, biochar may not show significant improvements in crop yields, while in degraded temperate soils, the benefits can be more pronounced [182]. Studies comparing biochar made from different feedstocks suggest that the origin of biochar influences its effects on soil processes. For example, biochar derived from agricultural residues often has higher nutrient content, while wood-based biochars tend to be more stable and effective at sequestering carbon [188]. Temperature and pyrolysis conditions play a critical role in determining biochar’s physical and chemical properties, which in turn affect its performance across different ecosystem processes. Biochar produced at higher temperatures tends to have lower volatility and higher stability, which is beneficial for carbon sequestration but may reduce its nutrient content [188].

Micro-Scale Ecosystem Processes (Soil Microbial Activity and Microbial Communities): Biochar’s characteristics, particularly its surface area, porosity, and chemical composition, significantly affect soil microbial activity and microbial community structure. Biochar with a high surface area and porous structure offers niches for microorganisms, thereby enhancing microbial colonization and activity. This can result in increased microbial biomass and diversity, as well as improved soil health [184]. The chemical properties of biochar, such as its pH and cation exchange capacity (CEC), can influence nutrient availability for microbes. Basic biochar (produced from wood or agricultural residues) may increase soil pH, fostering microbial communities that are adapted to slightly alkaline conditions, such as nitrifying bacteria [186]. Biochar’s ability to adsorb and stabilize SOC enhances microbial access to organic substrates, facilitating carbon cycling and nutrient transformation [70].

Meso-Scale Ecosystem Processes (Soil Fertility, Water Retention, and Greenhouse Gas Emissions): At the meso-scale, biochar impacts processes such as nutrient cycling, soil water retention, and greenhouse gas emissions. These are influenced by biochar’s physicochemical properties. The specific feedstock used to produce biochar (e.g., wood vs. agricultural waste) affects the nutrient content, such as nitrogen, phosphorus, and potassium. Biochar can act as a slow-release fertilizer by adsorbing and slowly releasing these nutrients, thereby enhancing soil fertility [189]. Biochar’s porous nature improves soil’s water-holding capacity, which can mitigate drought stress in soils with low water retention capacity. This is particularly beneficial in arid regions where water scarcity is a concern [182]. Biochar can influence greenhouse gas emissions, such as CO₂, CH₄, and N₂O [7]. Biochar’s presence in soil often reduces N₂O emissions by altering microbial processes involved in denitrification [190]. However, the effects on CH₄ and CO₂ emissions are less consistent, with studies showing both increases and decreases depending on the biochar’s characteristics and the soil environment. In some studies, biochar application decreases soil CH₄ and N₂O emissions [191], while others have reported no change or increases [192].

Macro-Scale Ecosystem Processes (Landscape Carbon Sequestration, Soil Erosion, and Biodiversity): At the macro-scale, biochar can have broad ecological impacts, including influencing carbon sequestration, landscape stability, and biodiversity. Biochar is a stable form of carbon that can persist in soils for centuries. Its incorporation into soil has the potential to sequester carbon on a landscape scale, reducing atmospheric CO₂; levels [189]. The stability of biochar depends on its feedstock and pyrolysis temperature; higher-temperature biochars tend to be more stable and thus more effective for long-term carbon storage [22]. The use of biochar in soil can improve soil structure by increasing aggregate stability. This reduces susceptibility to erosion, especially in sandy or degraded soils, enhancing soil retention and landscape stability [193]. The long-term effects of biochar on biodiversity are complex. On one hand, biochar can create more hospitable environments for beneficial soil organisms like earthworms and mycorrhizal fungi [1]. On the other hand, its impacts on plant communities can be site-specific, with biochar potentially altering plant growth due to nutrient imbalances or pH changes [194].

5. Effects of Biochar on Nutrient Dynamics for Enhanced Crop Yields

Scientific evidence indicates that biochar offers several benefits to agriculture [195] as depicted in Figure 3 below and discussed in this section.

5.1. Carbon Sequestration

Carbon sequestration is capturing and storing carbon dioxide for a long time, to prevent its release into the atmosphere [196]. It involves removing and accumulating carbon from the atmosphere and preventing its immediate release as carbon dioxide gas [197]. Carbon can be sequestered through natural processes such as photosynthesis, as well as through man-made technologies such as carbon capture and storage (CCS) [198]. Biochar has been demonstrated to store carbon in soil for extended periods, thereby mitigating climate change and enhancing soil organic carbon [13,199]. Biochar reduces atmospheric CO₂ levels [199] by enhancing the soil’s carbon storage capacity, decreasing soil emissions, and acting as a sink for the absorbed CO₂ [200]. Additionally, biochar reduces methane production and microbial respiration while boosting fertility and supporting plant development, which further supports carbon sequestration [201]. Its aromatic structure ensures long-term carbon storage and resistance to degradation [146,199]. Hence, biochar is a promising solution for mitigating global warming.

5.2. Effect on Greenhouse Gas Dynamics in Soil

Biochar influences the carbon cycle by lowering greenhouse gas emissions, sequestering organic carbon in soil, and converting labile biomass into stable biochar. Specifically, biochar decreases CO₂ emissions by promoting photosynthesis and plant growth while inhibiting soil respiration [200]. Biochar reduces methane gas emissions by inhibiting methanogenesis, increasing methane oxidation, and lowering soil moisture. It further reduces N₂O emissions by enhancing denitrification, reducing nitrification, and minimizing nitrate leaching [202]. Scientific evidence indicates that biochar significantly lowers greenhouse gas emissions. For example, Zhang et al. [172] reported average reductions of 23% for N₂O, 11% for CO₂, and 32% for CH₄. Moreover, Wang et al. [203] found a remarkable 54% average decrease in N₂O emissions with biochar use. These results demonstrate biochar’s potential to reduce greenhouse gas emissions and its contribution to climate change mitigation and improvement of air quality.

5.3. Effect on Soil Biology

Biochar significantly impacts the soil ecosystem thereby affecting the population dynamics of soil organisms [129]. Biochar creates favorable environments and provides resources for soil organisms, thereby enhancing soil biodiversity by promoting the growth and interactions of soil organisms. This is because of its ability to alter the soil’s chemical, biological, and physical properties, like moisture, porosity, temperature, pH, and nutrient availability [13,127,200]. For instance, biochar application increases soil microbial biomass by 37–69% and consistently increases organic matter, carbon, and nitrogen [141], important food sources for soil macro- and microorganisms [204]. In spite of this, biochar introduction can potentially harm soil ecosystems by introducing harmful substances or displacing native organisms [103].

5.4. Effects on Nutrient Dynamics

Biochar influences soil nutrient dynamics by acting as a source, sink, or conditioner of essential plant nutrients [205]. It supplies nutrients such as nitrogen, phosphorus, and potassium to crops dependent on the pyrolysis conditions and feedstock used [172]. By increasing the soil’s porosity, surface area, and charge, biochar enhances nutrient retention and reduces leaching and volatilization [206]. Further, biochar enhances nutrient availability and cycling by increasing soil pH, redox potential, and microbial functions [205]. Although biochar improves cation exchange capacity and organic carbon, its effects on soil nitrogen, phosphorus, and potassium vary [168].

5.4.1. Nutrient Retention

Biochar acts as a nutrient sink by reducing environmental losses via emissions and leaching [207]. Its application affects different soil characteristics including CEC, pH, water retention, bulk density, and biological activity, which in turn influence nutrient availability [13,153]. By lowering leaching and gaseous losses and making phosphorus more accessible, biochar improves soil nitrogen retention [172], while its impact on potassium is inconsistent [208]. This nutrient retention capability of biochar is ascribed to its large surface area, porosity and the existence of both polar and nonpolar locations on its surface [206]. Polar sites contribute to increased CEC which helps in retaining more nutrients and decreasing loss via leaching [21]. Additionally, biochar improves nutrient retention by raising organic matter levels and soil pH [128,209], while its ability for the slow release of fertilizing properties improves nitrogen retention and minimizes leaching [207,210]. Thus, biochar regulates nitrogen availability via both biological and chemical processes, thereby increasing nitrogen availability that promotes healthier and more robust plant growth [210]. It should however be noted that biochar performance is dependent on feedstock material, the pyrolysis process, and soil characteristics [65].

5.4.2. Nutrient Leaching

The interplay of biochar’s physicochemical characteristics, pyrolysis temperature, and soil conditions influences its ability to reduce nutrient loss through leaching [211]. For example, higher pyrolysis temperatures potentially lead to increased nutrient leaching [211], while its pore size, surface area, and chemical composition play a significant role in the storage and release of nutrients. In order to fully understand nutrient losses in relation to biochar’s impact, extended field investigations are necessary.

Nitrogen

Nitrogen management is crucial for crop growth and yield. Its loss from the soil via gaseous emissions and leaching of ammonia (NH₃) and nitrous oxide (N₂O) poses significant challenges [212]. Biochar’s effectiveness in mitigating NH₃ volatilization [213] and the reduction of denitrification [214] and overall N₂O emissions [215] has been demonstrated. Similarly, biochar adsorbs inorganic N ions which help in decreasing runoff and N leaching [207]. Its surface properties enhance ion adsorption, thereby reducing nutrient loss [149], lowering N₂O emissions [202,215], and decreasing NO₃⁻ leaching [216,217].

Phosphorus

Research by Yang et al. [218] and Ghodszad et al. [219] reveals the influence of biochar in enhancing P retention while minimizing phosphorus leaching through sorptive and adsorptive processes. In natural settings, phosphorus tends to highly bind to iron (III) oxides in soils, limiting its mobility [220], more so in magnetic biochars that exhibit higher phosphate adsorption capacities compared to their non-magnetic counterparts [221]. Enhanced sorption is attributed to multiple mechanisms, including electrostatic attraction, surface precipitation, and ligand exchange [221]. The substantial surface area and strong adsorption capability of biochar for ionized types of phosphorus allow it to reduce ortho-P leaching from soils with high nutrients and alter P availability [208].

Potassium

Biochar’s effect on potassium leaching is complex and influenced by several factors. Biochar made at increased temperatures (500 and 587 °C) tends to decrease potassium leaching in Typic Plinthudult soils. Some studies have observed temporary increases in K leaching in crop fields, which could potentially contaminate groundwater [222]. The application of wood biochar on acidic, low-fertility soils results in the leaching of potassium, calcium, and magnesium to a depth of 60 cm with concentrations diminishing with increasing depth [223]. Hardie et al. [224] reported a 65% increase in K leaching beyond the A1 horizon from high soluble potassium concentrations in biochar. Notably, increased production temperatures lead to reduced calcium leaching losses caused by biochar [211].

5.4.3. Nutrient Use Efficiency

Nutrient use efficiency (NUE) is the quantity of yield or biomass generated per unit of nutrient administered (soil amendment) and it is a crucial metric in agriculture [225]. NUE is influenced by various factors including plant, soil, and environmental conditions [226]. Biochar enhances NUE directly and indirectly by increasing nutrient uptake and reducing losses via emissions and leaching [138]. Arif et al. [227] established that adding wood biochar enhances phosphorus utilization efficiency in alkaline soils [227] and enhances NUE in green bean crops [228]. On the other hand, adding biochar to rice–wheat rotations increase nitrogen use efficiency by 20–53% and phosphorus use efficiency by 38–230% [141]. Biochar indirectly enhances nitrogen use efficiency by minimizing gas emissions [229], boosting soil organic carbon [227], and decreasing nutrient leaching [211]. In spite of this general assessment of biochar improving NUE, Haider et al. [230] revealed that mixed biochar made of polyaromatic hydrocarbons (PAHs) can lower NUE by making nitrogen less accessible to plants.

5.5. Effects of Biochar on Enhancing Crop Yield and Productivity

Biochar application reduces land use pressures and enhances crop yield, thereby boosting food security and farmer incomes. It enhances soil’s biological, physical, and chemical properties, resulting in greater agricultural productivity [130,168]. Biochar benefits plant productivity and yield by increasing nutrient availability, improving soil pH, enhancing cation exchange capacity, and providing better disease control mechanisms [144]. These effects vary and are dependent on crop and soil properties, application rate, and climatic conditions [231]. Utilizing biochar in farming increases crop production by 10% on average, with more pronounced effects in loose-textured and acidic soils [232].

Biochar promotes soil health by diversifying microbial populations, enabling a healthy habitat for microbial activities and stabilizing nitrogen for plant utilization [233], particularly for non-legume crops [234]. Moreover, biochar’s potassium content is readily absorbed by plants, further enhancing sustainable soil health and crop yields [109]. Its utilization improves crop yields and soil nutrient levels [214], thereby boosting biomass production [235] and highlighting biochar’s profound benefits for crop productivity and sustainable agriculture.

6. Conclusions and Future Perspectives

This study underscores biochar’s immense value in enhancing sustainable agriculture and ecological management through the supply of soil ecosystem services such as carbon sequestration, soil biodiversity, nutrient dynamics, and crop yields. Additionally, this study reveals that biochar characteristics are influenced by pyrolysis temperatures and feedstock material. Biochar’s characteristics such as water-holding capacity, pH, and surface area, among others, together with field conditions such as climate, soil type, and application rates play critical roles that determine its effectiveness. To maximize its advantages, careful consideration and tailored approaches are necessary.

Further field studies are required to fully understand the effect of feedstock material and pyrolysis conditions on the produced biochar and how these characteristics influence the services of biochar on enhancing soil health and crop yields. This is critical in producing the desired end product for different uses since biochar properties vary widely depending on feedstock and production conditions. Thus, it is important to know the biochar you will produce or purchase to make its application suit a wide range of needs. This calls for a comprehensive understanding of the cascading effects of biochar characteristics on micro-, meso- and macro-scale ecosystems processes and their evaluation.

Because the processes involved in biochar production, characterization, and evaluation are expensive, the development of cost-effective alternatives would be necessary to benefit farmers.

Finally, since biochar produced from similar feedstock material under different production conditions results in biochar with different conditions, it is difficult to compare their efficiencies. This calls for standardized characterization procedures, especially now that biochar has been accepted globally as an alternative soil amendment material.