Biochar as a Feedstock for Sustainable Fertilizers: Recent Advances and Perspectives

Abstract

The increase in the world population exerts significant pressure on expanding global agricultural production. To achieve this, the use of fertilizers is fundamental. However, highly soluble traditional chemical fertilizers can be easily leached and volatilized, causing environmental damage. Therefore, reducing the use of these fertilizers and developing new and smart fertilizers is crucial. Biochar, a solid and carbon-rich pyrolysis product, has been studied both as a standalone fertilizer and as a raw material for sustainable fertilizers. Recently, a wide variety of materials and techniques have been used in the production of biochar-based fertilizers (BBFs) and need to be grouped and critically evaluated. Thus, this study aimed to conduct a literature review on new biochar-based fertilizers, involving different routes for biochar-based fertilizer synthesis and their effects on various crops. Recent results indicate the growing interest in nanomaterials and microbial processes for producing new fertilizers. Most assessed studies use biochar to produce slow-release fertilizers. The results also indicate that these new biochar-based fertilizers increase crop yields and reduce the leaching and volatilization of nutrients in soil, demonstrating significant potential as an alternative to traditional fertilizers. Therefore, the agricultural use of biochar holds environmental importance by reducing the negative impacts caused by the use of highly soluble traditional fertilizers. However, long-term field experiments and the economic feasibility of BBF production routes must be carefully studied.

1. Introduction

The recent global crisis caused by COVID-19 has generated food shortages in several parts of the world. According to the Food and Agriculture Organization of the United Nations [1], 9.2% of the global population is chronically hungry. Furthermore, the population growth projection estimates 9.7 billion people globally by 2050 [2]. In this context, using fertilizers is essential to guarantee global food security. In this sense, it is necessary for government efforts to develop new technologies for the production of sustainable fertilizers.

Conventional fertilizers have a high concentration of nutrients, making them available to plants in the short term. Despite this, they can be easily lost through leaching and volatilization, which can cause environmental damage to soil and water. Furthermore, when misused, these fertilizers can acidify the soil [3,4]. When used excessively, conventional fertilizers can also deteriorate soil structure, reducing nutrient use efficiency and increasing greenhouse gas emissions [5,6]. Therefore, using biochar-based fertilizers (BBFs) is an alternative to reduce dependence on conventional fertilizers [7,8]. Biochar-based fertilizers typically have a slow release of nutrients, thus increasing use efficiency and reducing nutrient losses to the environment [4]. Further, BBFs in powder form can rapidly meet plant nutrient requirements. For instance, a BBF produced from P-enriched poultry litter provided a phosphorus supply to maize comparable to that of TSP soluble fertilizer, resulting in higher plant dry matter in fertilized treatments compared with the non-fertilized control, as indicated by an average relative agronomic effectiveness of 15% [9].

Biochar is a solid material, rich in carbon, produced from biomass pyrolysis from various sources, such as crop straw, wood, and sludge. Biochar generally has a significant specific surface area and high porosity [4,10]. Its physicochemical properties may vary depending on the types of biomass used and production conditions, such as temperature and duration of pyrolysis [11]. Biochar can potentially improve soil fertility, among other benefits [4]. However, depending on the raw material and pyrolysis conditions, the amount of nutrients made available by biochar may be limited to crop requirements [12], indicating the need for its combination with other fertilizers to obtain more nutritionally complete products. The simultaneous application of biochar with organic and inorganic fertilizers can increase fertilizer efficiency and productivity [4,13,14].

Pyrolysis is carried out under anoxic conditions, with temperatures varying from 300 to 800 °C [4]. At temperatures above 500 °C, biochar exhibits hydrophobic characteristics—large specific surface area and pore volume—which are useful for the adsorption of nutrients, accommodation of microorganisms, and retention of organic pollutants. At temperatures below 500 °C, the tendency is to form more oxygenated functional groups more suitable for immobilizing inorganic pollutants [15]. Thus, studies suggest that, for use in agriculture, the pyrolysis temperature should be between 300 and 500 °C, while, for carbon sequestration and other environmental uses, the temperature should be higher than 500 °C [16].

In addition to having direct effects as a fertilizer or a soil conditioner, biochar is also considered a feedstock for producing sustainable fertilizers. In this case, the presence of nutrients, high cation exchange capacity (CEC), high porosity, and surface properties are the main characteristics of biochar that are considered in producing sustainable fertilizers. The chemical composition and availability of feedstocks and additives help define biochar as a matrix for producing new fertilizers.

A large number of studies have been carried out over the last decade involving different biochar enrichment processes, contrasting raw materials and plant responses to BBFs. The great diversity of processes and terminology has caused some confusion for readers and users of BBFs. This review aims to organize and systematize the technological processes and additives used in the production of BBFs, and group the effects of these fertilizers on the soil and the plant.

Considering the growing interest in the production of biochar-based fertilizers (BBFs), the wide variety of BBF types, and the low availability of information on the production and agricultural use of BBFs, this review presents the routes and techniques used to produce BBFs, the type and quality of feedstocks, the synthesis of composites, and the blending of biochar with mineral fertilizers, in addition to highlighting the benefits of using these fertilizers for crops.

2. Methodology

This work was carried out by collecting information about BBFs and their effects on crops from peer-reviewed scientific articles on the Science Direct and Web of Science platforms (Main Collection), published in the last 25 years until 30 January 2025. The indexing terms were used to search in “title”, “abstract”, and “keywords” of the articles were as follows: biochar AND (“biochar-based fertilizer” OR fertilizer); biochar NEAR/5 (based OR enrich* OR rich*) AND fert*. After the selection and elimination of articles not specifically related to the topic, in the end, 120 articles were selected that fully met the following criteria: (I) articles published in peer-reviewed international journals, written in English, (II) specifically focused on biochar-based fertilizer, (III) description of the biochar raw material, and (IV) description of the technology used in biochar-based fertilizer. This study excluded articles explicitly focused on pure biochar without relation to BBFs. Studies with conflicting results were carefully assessed, and articles of low methodological quality were unconsidered. Furthermore, hydro-chars were not considered as feedstock for BBF production in the present work.

Interventional studies involving animals or humans and other studies that require ethical approval must list the authority that provides approval and the corresponding ethical approval code.

3. Results

3.1. Agri-Environmental Functions of Biochar: A Base for Fertilizer Production

From this bibliographic search, it was possible to group the main agronomic and environmental functions and applications of biochar (Figure 1). As a multifunctional material, biochar can be used for several agri-environmental purposes, including soil and groundwater remediation, wastewater treatment, carbon sequestration and reduction of greenhouse gas emissions, and improving soil health and ecological processes, in addition to being used as a fertilizer and soil acidity corrector [17,18,19,20,21].

Several studies demonstrated that biochar could improve soil fertility and facilitate plant growth by reducing soil density, increasing water retention and soil cation exchange capacity, and altering soil microbial communities [4,22]. Due to its morphological structure, high surface area, porous microstructure, presence of surface charge, presence of surface functional groups, and active carbon, biochar allows the gradual release of nutrients, avoiding losses due to leaching [4,23], volatilization, or surface runoff [23,24]. Thus, it improves nutrient use efficiency and reduces potential environmental threats usually seen when conventional fertilizers are used.

Results of a global meta-analysis indicated that, compared with unfertilized control, biochar could increase the contents of total C, organic C, microbial C, labile C, and fulvic acid C by 64.3, 84.3, 20.1, 22.9, and 42.1%, respectively [25]. This significant increase in the soil C content is one of the main advantages of using biochar as a soil amendment. Biochar application enhances soil C stocks, contributing significantly to C sequestration. In this sense, biochar can potentially sequester 1.8 Gt of CO₂ yr⁻¹ [26], representing an important strategy to mitigate soil C emissions.

Another environmental function of biochar is the immobilization of heavy metals in the soil. The pyrolysis conditions define the greater or lesser heavy metal retention capacity of the biochar. Biochar produced at higher temperatures (>400) has a greater capacity to retain heavy metals [27]. Two applications of 15 Mg ha⁻¹ of sewage sludge biochar in two consecutive corn harvests did not increase heavy metal concentrations in the soil or the plants [28]. Negative surface charges, alkalizing power, and high porosity explained biochar’s ability to immobilize heavy metals [27]. Furthermore, when applied to soil, the aging process of biochar can increase its ability to retain heavy metals, reducing the absorption of these metals by plants [28].

All of these agri-environmental functions of biochar previously described can be useful when BBFs are produced. A wide range of possibilities can be used to mix biochar with other raw materials to obtain sustainable fertilizers. Some of these possibilities are described below.

3.2. Biochar as a Feedstock for Sustainable Fertilizers

Pure biochar can be easily obtained and used. However, depending on the types of raw material and pyrolysis conditions, they may present limitations or nutrient imbalances when high crop productivity is desired [21], requiring high application rates and making their use by producers unfeasible. Given these limitations, several studies present biochar as a feedstock for obtaining sustainable fertilizers. Therefore, all of the functions of biochar described above in Figure 1 can be useful in fertilizers that use biochar in their composition. Biochar-based fertilizers can be obtained through different processes and technological routes. Generally, biochar with fertilizer can be mixed in pre- or post-pyrolysis [29], as described below.

3.2.1. Technological Routes and Techniques Involved in Biochar-Based Fertilizer Production

There are several techniques to enrich biochar with nutrients to produce BBFs. Mixing feedstocks to produce BBFs can be performed pre- and post-pyrolysis, or combining these processes (Figure 2). In post-pyrolysis, the mixtures can undergo additional treatments, including impregnation, enrichment, or encapsulation. BBFs can remain as a powder or be subjected to granulation or pelletization processes.

Pre-pyrolysis mixing can be carried out (i) by direct treatment or (ii) by pre-treatment [29]. In the direct treatment, nutrient-rich feedstocks are mixed and subsequently subjected to pyrolysis to produce BBFs. The main feedstocks used include sludges, compost, manures, algae, bone waste, mushroom substrate compost, and municipal and crop waste [30,31]. For instance, P-rich bone waste or K-rich banana peduncles can enrich biochar when mixed with a feedstock that is poor in these nutrients, thus producing BBF. Organic-based biomasses are usually used in direct treatment. Co-pyrolysis of fresh waste fruits and vegetables resulted in BBF that enhanced maize yield, growth, and community function comparable to organic fertilizer [32]. The great advantage of this procedure is the possibility of using locally available and low-cost organic waste.

On the other hand, the most significant limitation is the low concentration of one or more nutrients in BBF. Pre-treatment consists of mixing nutrient-poor organic biomass with nutrient-rich materials. In this case, mineral materials that supply one or more nutrients are used. Mineral materials typically used include conventional fertilizers (e.g., urea, simple superphosphate, mono ammonium phosphate, KCl, and K₂SO₄); minerals (zeolites, goethite, montmorillonite, diatomite, and gypsum); and waste from the mining industry. In the pre-pyrolysis procedure, there is greater control over the enrichment of BBF nutrients. In addition, pyrolysis can alter the mixture’s chemical and mineralogical composition, improving BBF properties such as nutrient availability. As an example of pre-treatment, BBFs produced by the mixture of poultry litter with phosphate and magnesium fertilizers raised soil pH after maize cultivation compared with triple superphosphate and, consequently, improved the soil’s ability to retain nutrients and reduced heavy metal toxicity and bioavailability [33]. Biochar-based fertilizers were also produced by co-pyrolyzing algae and hazelnut shell biomasses with triple superphosphate [31]. In this case, the slow-release performance of BBFs suggests their potential as promising alternatives to conventional phosphate fertilizers. Co-pyrolysis of rice straw with magnesium oxide (MgO) resulted in a novel biochar-based phosphate fertilizer that improved soil physicochemical properties, including increased available phosphorus, enhanced soil pH, higher electrical conductivity, and rapidly increased available potassium [30]. Moreover, co-pyrolysis with H₃PO₄ and MgO enhanced thermal stabilization, yield, and chemical oxidation, potentially forming thermally stable phosphorus complexes like C-O-PO₃ or (CO)₂PO₂ on the biochar surface [34].

In contrast, co-pyrolysis with triple superphosphate (TSP) increased surface area but could lower pH due to its high acidity [31]. Therefore, this type of enrichment is not interesting for acidic soils that predominate in tropical regions where P availability is decreased with acidic pH. Furthermore, combining biomass with Mg and P yielded BBF with delayed P release, wherein Mg functioned as a pH neutralizer while preserving P in a slowly dissolving form [35]. Mg compounds have been shown to elevate soil pH due to their inherent alkalinity [34,36]. A similar effect was observed with phosphate minerals, contributing to soil alkalinity. During dissolution, hydroxyapatite (Ca₁₀(PO₄)₆(OH)₂) consumed H⁺ ions, raising pH and releasing PO₄³⁻. These phosphate species then became protonated to form HPO₄²⁻ and H₂PO₄⁻, the primary forms of P available to plants [36]. In the Brazilian Amazon region, residues of açai palm seeds, a locally available material, were pyrolyzed with single superphosphate and triple superphosphate to produce BF, which reduced the soil arsenic contamination and increased phosphorus availability [37].

Post-pyrolysis is the most common procedure for producing BBFs. In this process, biochar is mixed or treated with materials to improve the final properties of the BBF. In this process, biochar can be combined with soluble fertilizers (e.g., urea, potassium chloride, single superphosphate, triple superphosphate, KCl, and K₂SO₄), minerals (e.g., zeolites), rock powder (including mining residues), and organic fertilizers (composted or not). Post-pyrolysis mixing can occur by enrichment, impregnation, encapsulation, or a combination of two or more processes. These post-pyrolysis procedures aim to increase the concentration and efficiency of nutrient use. Typically, efficiency is improved by reducing losses due to volatilization or leaching or even by lowering specific adsorption, which reduces the availability of some nutrients in the soil, such as phosphorus. In addition, pyrolysis treatment seeks to improve the release of nutrients over time. Fachini et al. [38] demonstrated that sewage sludge biochar fertilizer enriched with KCl presented a slow release of K into the soil, reducing leaching and increasing the efficiency of this nutrient by radish plants.

Co-composting of biochar with different types of organic waste can also be considered a post-pyrolysis technique for producing BBFs. In this sense, several types of combinations have been used. BBF was obtained by co-composting Acacia decurrens biochar, chicken litter, and cow dung manure at different ratios [39]. This BBF increased several soil properties such as cation exchange capacity, total organic carbon, available phosphorus, and Eragrotis teff (Zucc.) grain yield compared with organic and chemical fertilizers.

Biochar can also be fermented with organic materials to produce BBF. For instance, biochar made from maize straw, and pyrolyzed at a temperature of 500 °C for 40 min, was fermented with cattle manure, humic acids, and amino acids for 6 days [40]. The fermented BBF increased soil quality index, microbial biomass carbon, and beneficial microorganism abundance. Compared with chemical fertilizers, organic fertilizers can enhance CH₄ emissions, particularly in paddy soils, due to the availability of organic substrates for methanogenic microorganisms. However, the fermentation process transforms C components, reducing the proportion of unstable C forms, which may influence dissolved organic carbon (DOC) dynamics and microbial biomass carbon (MBC) availability [41]. Despite the promising results, the logistics for biochar fermentation and economic viability must be studied.

Biochar can also be impregnated with various solutions, such as organic and inorganic acids, oxides, biowaste, and microorganism solutions [42]. These solutions are used to improve BBF quality. For example, a solution of beneficial microorganisms was used to produce BBF (from straw rice biochar). It promoted greater agronomic efficiency and capacity to reduce Cd accumulation in rice grains [43]. Cd is translocated to the edible parts of the rice plant through natural physiological processes. Cd uptake begins at the root level through several metal transporters, such as OsNRAMP5 (natural resistance-associated macrophage protein 5) and OsHMA3 (heavy metal ATPase), which regulate Cd influx into root cells [44,45]. Cd is then translocated via the xylem-mediated root-to-shoot, facilitated by OsLCT1 (low-affinity cation transporter), which also plays a crucial role in Cd allocation to rice grains, likely facilitating intervascular Cd transfer within nodal tissues [46]. Biochar and BBFs mitigate Cd uptake by transforming it into less soluble forms in the soil [36]. Impregnating sewage sludge biochar with oxalic acid increased P release from biochar (produced at 300 °C) by 103-fold, compared with the control, from which P was extracted with water [47]. Similarly, BBF treated with acid and enriched with polyethyleneimine was the recommended slow-release Se fertilizer in red and brown soil [48].

Additionally, a variety of slow nutrient-release techniques have been explored in the production of enhanced-efficiency fertilizers. This includes the formation of aggregates, such as pellets or granules, and applying polymers, coatings, and binding materials, such as bentonite and different types of starch [38,49,50,51]. The agglutination of BBF establishes a physical barrier that slows the dissolution of fertilizers. Consequently, the formation of granules or pellets facilitates the controlled release of nutrients. Furthermore, coating layers have been shown to effectively regulate the liberation of nutrients, ensuring a sustained supply of essential elements to crops over the growing season [52,53]. These strategies contribute to optimizing the efficiency in the use of nutrients in agriculture and also help in the biofortification of foods, generating products with a high nutrient content, which can help combat human malnutrition [54].

A combination of pre- and post-pyrolysis was performed to obtain BBF with improved properties [55]. These authors mixed rice straw with NPK and hydrotalcite and performed co-pyrolysis to obtain BBF. In a second step, the obtained BBF was mixed with starch and urea in post-pyrolysis. The addition of the second step produced BBFs that increased the water-retaining ability of soil and the durability of nutrients in BBF with starch and hydrotalcite.

3.2.2. Materials Used for Biochar-Based Fertilizer Production

Several combinations have been used to enrich the final fertilizer in one or a few nutrients, as shown in

Table 1. Feedstocks used and blends obtained in the biochar-based fertilizer production.

Ingredient Blended	Crop/Crops Tested with BBF	Reference
Cow manure biochar + zinc (Zn)	Wheat (Triticum aestivum L.)	[54]
Wood chip biochar + N-containing solution	White cabbage (Brassica oleracea convar. Capitata var. Alba)	[57]
Wood chip biochar + NPK	B. rapa	[58]
Biochar from crop residues + matured chicken litter	Maize	[59]
Cotton straw biochar + K3PO4	Pepper seedlings	[60]
Rice straw biochar + ammonium molybdate solution	Brassica parachinensis	[61]
Cotton straw biochar + bentonite + Mg3(PO4)2	Pepper	[49]
Bamboo chip biochar + MgO solution + sepiolite	Maize	[62]
Wood-derived biochar + MgCl2 + KOH	Maize (Zea mays L.)	[63]
Sugarcane cake biochar + triple superphosphate or H3PO4, MgO, or CaO solution		[64]
Biochar from cow manure, vermicompost, and Lantana spp. + vermicompost tea	Moong bean (Vigna radiata)	[65]
Corn straw biochar + MgCl2 and ammonium nitrate solutions	Maize (Zea mays L.)	[66]
Betula pendula biochar + olive waste compost	Zea mays	[67]
Chicken litter biochar + rock phosphate + solubilizing bacteria	Maize	[56]
Biochar from chicken litter and coffee husks + H3PO4 + MgO	Megathyrsus maximus cv. Mombasa in sequence Zea mays in sequence Phaseolus vulgaris L. cv. BRSMG UAI	[68]
Cotton stalk biochar + K3PO4 and MgO		[69]
Rice husk biochar + urea hydrogen peroxide		[12]
Corn straw biochar + MgCl2	Corn	[70]
Wheat straw biochar + urea + bentonite + rock phosphate + Fe2O3 e FeSO4.7H2O	Rice seedlings	[71]
Chicken litter biochar + MgO + triple superphosphate or H3PO4	Urochloa brizantha grass	[72]
Crop residue biochar + micronutrient solution		[73]
Wood sawdust biochar + urea		[74]
Sawdust biochar + solution with N, P, K, Mg, S, and micronutrients	Corn	[75]
Chicken manure biochar + calcium bentonite		[76]
Coffee husk biochar + Araxá rock phosphate + Mg	Maize in sequence Brachiaria grass	[50]
Eucalyptus wood biochar + bentonite + pregelatinized corn flour + urea	Maize	[77]
Rice straw biochar + NPK + bentonite + humic acids	Rice	[52]
Sewage sludge biochar + calcium	Corn	[78]
Rice straw biochar + urea, NH4H2PO4 e K2SO4	Rice	[41]
Biochar + humus and minerals	Tobacco	[10]
Sewage sludge biochar + KCl	Radish	[38]
Corn straw biochar + K₂PO₄ solution	Spinacia oleracea	[79]
Bamboo biochar + gypsum + rock phosphate + boron (B) + K₂SO₄	Ginger	[80]
Biochar from rice straw and/or dry leaves + cow urine	Cabbage and kohlrabi	[81]
Rice husk and wood biochar + NPK + pig manure	Black locust seedlings	[82]
Corn waste biochar + rock phosphate		[83]
Barley straw biochar + N, P, K + Ca, Mg, Zn, K e NH4+	corn and wheat	[84]
Corn straw biochar + diammonium phosphate (DAP)	Chickpea (Cicer arietinum L.)	[85]
Biochar + NH4+ + polymer	Cotton seedling	[51]
Corn straw biochar + NPK	Pod pepper (Capsicum annuum var. frutescens L.)	[86]
Dairy cow manure biochar + sulfur (S) from biogas	Corn (Zea mays L.) in sequence soybeans [Glycine max (L.) Merr.]	[87]
Distillers grain biochar + urea, monoammonium phosphate and potassium sulfate	Tomato	[88]
Corn stalk biochar + tanned animal manure	Sugar beet (Beta vulgaris L.)	[89]
Rice straw biochar + urea		[90]
Apple wood and cotton straw biochar + sulfuric acid, hydrochloric acid, and nitric acid		[42]
Date palm biochar + HCl + chemical fertilizers (MAP and K2SO4)	Sweet pepper (capsicum)	[91]
Co-pyrolysis of algae and hazelnut shell biomasses with triple superphosphate		[31]
Co-pyrolysis of rice straw with magnesium oxide	Citrus seedlings (Shatang mandarin)	[30]
Co-pyrolysis of fresh waste fruits and vegetables	Maize (Zea mays L.)	[32]
Co-composting of Acacia decurrens biochar and chicken litter and cow dung manure	Dega Tef	[39]
Biochar from maize straw was fermented with cattle manure, humic, and amino acids	Maize	[40]
Açaí palm seeds pyrolyzed with single and triple superphosphate	Lettuce	[37]
Rice husk biochar + mineral fertilizer + microorganisms	Rice	[43]
Effective microorganisms, tobacco stalk biochar, and basal fertilizer	Tobacco (Nicotiana tabacum L.)	[92]
Corn straw, peanut shell, and sawdust + Fe(NO3)3⋅9H2O	Carrot and cabbage	[93]
Biosolids, cow manure, and chicken manure + Urea fertilizer and KH2PO4	Canola (Brassica napus L.)	[94]
Biogas slurry and biochar derived from lignocellulosic agro-residues	Cucumber (Cucumis sativus)	[95]
Eucalyptus wood chip biochar + HNO3 or 3M KOH +polyethyleneimine		[48]
Banana leaf sheath biochar + urea solution		[96]
Biochar + bentonite, biosolid, rice starch, and bioasphalt	Corn	[97]
Apple tree branch biochar + urea	Maize	[98]
Rice straw biochar + NPK + Hydrotalcite and starch	Tomato	[55]
Rice husk, peanut shell, and straw biochar + NPK	Wheat in sequence maize	[99]
Pine biochar + urea	Maize	[100]
Sugarcane filter cake biochar + MgO and H3PO4		[35]
Rice husk biochar + KH2PO4	Radish (Raphanus sativus)	[101]
Co-pyrolysis of eggshells and corn stalk		[102]
Sawdust biochar + ammonium sulfate		[103]
Lactic fermentation with fresh foliar tree and grass biomass	Beans (Theobroma cacao L. ssp.)	[104]
Rice husk biochar + nutrient solution (NPK)	Rice	[105]
Corn stover biochar + ammonium sulfate + polylactic acid		[53]
Rice straw biochar + humic acids and bentonite, cassava, or cornstarch	Rice	[41]
Oilseed rape straws biochar + urea and bentonite	Rape (Brassica napus L.)	[106]
Wheat straw biochar + urea, bentonite clay, rock phosphate, Fe2O3, and FeSO4.7H2O	Rice (Oryza sativa L., cv. Japonica)	[107]
Biochar from medicinal plant material, crop willow, and wood chips enriched with urea superphosphate (USP)	Wheat cv. Varius	[108]
Biochar from coffee husk and poultry litter enriched with H3PO4 and MgO		[34]
Biochar of maize straw, rice straw, and forest litter enriched with urea and coated by polyvinyl alcohol and polyvinylpyrrolidone		[109]
Sawdust biochar + NPK		[110]

. The raw materials mixed with biochar can come from different origins, including organic and inorganic sources. Manure, poultry litter, humic acids, vermicompost, and compost are normally used as organic sources. Despite the increasing use of mixtures with organic sources, mineral sources are still the most used and can provide a specific nutrient or group of nutrients. As a mineral source, conventional chemical fertilizers can be used with a high concentration of nutrients, nutrient-rich solutions, and various natural minerals in the form of rock powder. In the latter case, solubilizing microorganisms can be added to the mixture to increase the solubilization of nutrients present in the minerals [56].

Due to the high specific surface area and surface charges, biochar has a high capacity to adsorb ions. This is one of the main functions of biochar in the composition of fertilizers. In the case of mixing with nitrogen sources, biochar reduces losses of this nutrient through leaching and volatilization, thus increasing the efficiency of N use by plants [12,52,57,111].

Typically, conventional mineral fertilizers are employed in the production of slow-release BBFs. In this case, fertilizers with high solubility and therefore subject to leaching (e.g., KCl, K₂SO₄) or with high volatilization rate (e.g., urea) are mixed with biochar to produce slow-release BBFs. A fertilizer based on sewage sludge biochar enriched with potassium chloride (KCl) was developed by Fachini et al. [112]. In this study, BBF acted as a slow-release K fertilizer when applied to pure silica sand, reducing leaching losses compared with pure conventional fertilizer (KCl). The same fertilizers obtained by Fachini et al. [112] were also applied to clayey and sandy tropical soils and reduced K release in these soils [113]. In BBF produced from P-loaded biochar (500 °C) at a 2% application rate, P bioavailability measured by the Olsen method was 4.5 times higher in the BBF-amended soil (2647 mg kg⁻¹) than in the control soil (591 mg kg⁻¹) after 15 days. After 45 days, P levels remained 1.5 times higher in the BBF-treated soil (922 mg kg⁻¹) than in the control soil (626 mg kg⁻¹) [101]. The slow release of nutrients is the main characteristic of biochar-based fertilizers. This characteristic brings advantages to BBFs, such as increased nutrient use efficiency and reduced losses due to leaching and volatilization, thus reducing fertilization costs and the negative impacts of chemical fertilizers.

BBFs are often produced with nanomaterials, which show multiple functions in soil and exhibit specific chemical properties depending on the material used. Commonly employed materials include nanoparticles, ion-containing solutions, and iron oxides, which impregnate biochar to form nanocomposites [37,60,71]. The nanostructures formed on the biochar surface can increase its surface area and promote small pore formation. Consequently, nanomaterials reduce soil bulk density while enhancing water retention and infiltration [114]. Furthermore, specific nanocomposites can mitigate GHG emissions. For instance, BBF modified with MgO/Mg (OH)₂ nanoparticles has been shown to reduce CO₂ emissions, likely through MgCO₃ formation via chemisorption with soil CO₂. This reduction was more pronounced in MgO-modified biochar than in sepiolite–biochar [62].

Microorganisms have also been successfully used in the production of BBFs. In this case, microorganisms with diverse functions are applied, predominantly via solution, to increase the solubilization of nutrients present in the raw materials used to produce BBFs. Soil microbiota, including earthworms, play a crucial role in the soil ecosystem by accelerating the humification of organic matter. Additionally, whether in solid form or as leachate, vermicompost can be utilized in BBF production [65]. The application of acid-modified biochar-based bacterial fertilizers in soil resulted in maximum total phosphorus and soluble phosphorus concentrations reaching 37.21% and 16.65%, respectively, due to the synergistic interaction between biochar and phosphorus-solubilizing bacteria [42]. Furthermore, microorganisms have been shown to contribute to mitigating greenhouse gas emissions through various mechanisms. Aerobic microbes, in particular, have been observed to exhibit efficiency in the immobilization of ammonia, thereby reducing its volatilization [115]. Furthermore, BBF has been demonstrated to enhance microbial activity, modify the composition of the bacterial community to favor groups with high nutrient metabolic cycling capacity, and augment N nitrification by stimulating the expression of the ammonia monooxygenase (amoA) enzyme in ammonia-oxidizing bacteria [106], thus ensuring the soil N levels for crop growth.

3.3. Effects of Biochar-Based Fertilizers on Soil and Plants, and Environmental Implications

Figure 3 shows the main benefits of using BBFs for soil (carbon, nutrients, and microbiota), crops, and the environment. The increase in crop productivity is directly linked to the chemical properties of the soil, so BBFs positively alter the soil and, consequently, agricultural production [29]. In soil, the main function of BBF is the efficient supply of nutrients. Furthermore, unlike conventional fertilizers, BBF also contains stable C in its constitution, which can increase soil C stocks depending on the dose and frequency of application.

The high porosity of BBF can also serve to harbor microorganisms and increase microbial diversity in soils that have received these fertilizers. The environmental benefits promoted by pure biochar can also be seen in BBFs [23]. In addition to increasing C accumulation in the soil, BBF reduces nutrient leaching rates and greenhouse gas emissions from the soil [90], promoting environmental gains not normally observed in conventional fertilizers. BBFs have shown the ability to minimize leaching losses of nitrogen. BBF, produced from biochar and urea, reduced ammonium (NH₄-N) leaching in a moderately well-drained soil compared with this soil fertilized with urea alone. In a three-day soil leaching test, a 1:3 ratio of biochar to urea resulted in an average of 2.5 times more NH₄-N in the leachate of the urea-fertilized soil (1.04 mg) compared with the BBF-treated soil (0.42 mg) [100]. This BBF demonstrated a slow-release effect similar to soils fertilized with S-coated urea.

Therefore, using BBF is an effective solution with multiple gains (Figure 3), reinforcing the idea of a circular economy and sustainable development and becoming an important tool in organic waste management [21]. This review highlights a growing research trend in BBFs, with approximately 60% of studies published in the last three years. However, further research is needed to assess the production costs and environmental impacts of BBFs, particularly compared with current fertilizers, as this field remains underexplored in the literature (Figure 4).

To evaluate the efficiency of these new fertilizers, it is essential to carry out experiments that consider the soil–plant system. In the literature, the results of studies demonstrating the beneficial effects of BBFs on crop yields are consistent, especially after the enrichment of biochar with NPK fertilizers [14,38,77,98,112,116,117,118]. The effects of BBFs obtained from different raw materials on distinct crops are listed in Table 2.

Biochar used in BBF production is pyrolyzed in the 300–600 °C range. A wide diversity of biochar is used in the production of BBFs. The main raw materials include eucalyptus wood, urban waste (sewage sludge and urban green waste), crop residues (apple tree branches, rice straw, banana peels, coffee husks, reed straw, and corn cobs), and manure, in addition to combinations of organic and mineral raw materials (coffee husks, chicken litter, phosphoric acid, and magnesium oxide). This diversity of raw materials and pyrolysis temperature shows the possibility of using regionally available biomass to make BBFs.

As mentioned earlier, after pyrolysis, biochar is mixed (enriched) with various organic and inorganic materials [38,65]. BBFs can be enriched in plant macro- and micronutrients. Enrichment can be focused on a specific nutrient or a combination of nutrients. Urea, simple superphosphate, mono ammonium phosphate, potassium chloride, and the NPK mixture are the most used sources to enrich BBFs. Furthermore, enrichment materials have been demonstrated to facilitate the formation of stable bonds between chemical groups in biochar and elements. For instance, in the context of slow-release selenium (Se) fertilizer, the C = N chemical reactions of amide-bonding result in the formation of C-Se stable complexes, thereby gradually releasing Se as the BBF decomposes within the soil environment [48]. In addition to mineral fertilizers, organic sources such as vermicompost and algae are also used. Another strategy used in mixtures is the addition of beneficial microorganisms such as Bacillus subtilis, Bacillus megaterium, Bacillus mucilaginosus, and Pseudomonas spp. Microorganisms are used in mixtures to increase nutrient solubilization and mineralization processes from mineral and organic sources. It has been demonstrated that certain methylotrophic Bacillus sp. exhibit dual functionality in soil systems, thereby enhancing nutrient availability through reducing carbon compounds and, consequently, improving soil cation exchange capacity. In the context of tomato cultivation, the integration of these microorganisms with BBFs has been shown to enhance plant growth parameters, including yield and fruit quality [119].

Furthermore, specific microorganisms can control soilborne plant pathogens and stimulate the plant in physiological processes that improve nutrient absorption [29]. The effect of biochar in controlling soil phytopathogens such as Fusarium oxysporum and Macrophomina phaseolina has already been proven [120]. Despite the scarcity of studies, biochar-based fertilizers appear to have a similar effect as biochar, mainly by increasing the presence of beneficial soil microorganisms capable of controlling soil pathogens [121].

Soil microorganisms carry out essential processes in the soil. Most microbes are involved in nutrient cycling, promoting the decomposition of soil organic matter and the availability of plant nutrients [30]. Nitrogen-enriched BBF enhances nitrification by ammonia-oxidizing bacteria and accelerates the conversion of NH₄⁺ to NO₃⁻ [106]. In addition, phosphate-solubilizing bacteria such as Pseudomonas sp., Burkholderia fungorum, Acinetobacter sp., and Paenibacillus sp. have been shown to produce organic acids such as oxalic, quinic, D-malic, acetic, succinic, citric, and lactic acids. These acids allow the solubilization and availability of phosphorus from mineral sources such as phosphate rock [56]. Most BBF studies have focused on how the application alters native soil microbial communities rather than investigating BBF as a vehicle for introducing exogenous microorganisms into soil. The research reflects the significant technical barriers to developing microbial-loaded BBFs, including loss of viability of inoculated strains during storage and the need for specialized equipment [122]. This increases the cost of production compared with conventional BBFs.

Adding BBF promoted positive effects in 100% of the studies in Table 2. In general, the plant gains promoted by BBFs were expressed in a greater accumulation of nutrients in plants, better development of the root system, higher seed germination rate, and greater crop productivity. BBF from K-enriched sewage sludge promoted greater K accumulation in plant sap and greater productivity of radish tubers than conventional KCl fertilizer. Other vegetables were also favored with the application of BBF from different types of biochar. Increases in germination rate and root and shoot growth have been reported for cucumber, broccoli, and okra with BBF made from banana peel biochar. Increased development of tomato seedlings was also observed with the application of BBF made from reed biochar and liquid seaweed fertilizer [118].

Grain crops such as corn and beans have also benefited from the application of BBFs. In this case, the increase in grain productivity was obtained with BBF produced from biochar and conventional NPK fertilizer [123] and then biochar was mixed with vermicompost [65], showing the benefits of BBF obtained from organic and inorganic materials. The use of BBFs also increased the quality of the commercial product, as in the case of Camellia sinensis L. plants, where tea quality was observed [14]. A reduction in nitrate accumulation and an increase in nitrogen assimilation were also reported in Chinese cabbage (Brassica parachinensis) with BBF made from molybdenum-enriched rice straw biochar [61].

Date palm waste biochar modified with HCl and enriched with MAP and K₂SO₄ improved pepper growth under saline conditions, increasing shoot fresh weight by 9.41%, root fresh weight by 15.32%, shoot length by 5.22%, and root length by 12.57% [91]. In saline soils, the porous structure of biochar enhances water retention, thereby reducing the rate of rapid evaporation. Additionally, an observed increase in nutrient uptake in crops may be associated with synthesizing secondary metabolites, including flavonoids and phenolic acids. These compounds perform a crucial function as defense mechanisms, helping to mitigate stress-induced damage [91].

The application of BBF produced by co-pyrolysis of rice straw with magnesium oxide (MgO) significantly enhanced nitrogen, phosphorus, and potassium (NPK) uptake in citrus seedlings, as demonstrated by increased growth metrics, such as plant height, stem coarseness, and chlorophyll content measured by the SPAD index [30]. Few studies have been conducted under field conditions providing insights into real-world scenarios. In these cases, BBF enriched with N-P-K increased rice dry matter by 18.5–25.5% and grain yield by 6–11.8% compared with conventional fertilizers. The slower release of NH₄⁺, P₂O₅, and K₂O ensured prolonged nutrient availability, enhancing crop uptake efficiency [105]. A BBF composite of 51% biochar and 10% N, sidedressed at 80 kg N ha⁻¹, increased N use efficiency by 12% and corn productivity by 21% compared with conventional N fertilizer [77]. In addition, BBF enhanced some phyla such as Acidobacteriota and Ascomycota at 20–40 cm, while promoting Mortierellomycota and Chloroflexi at 0–20 cm. These microbial shifts positively influenced plant growth and soil quality, ultimately improving maize yield [40]. However, long-term data (>3 years) remain scarce alongside the positive results, making it difficult to assess the cumulative impacts of continuous BBF application to the soil. Moreover, economic aspects are still underexplored in the literature.

Despite the large number of studies demonstrating the positive effects of BBFs on the productivity of agricultural and forestry species, around 75% of BBF studies were carried out in pots and only 25% in the field (Table 2). These results demonstrate that studies in this area are still incipient and evaluations under real soil conditions need to be expanded to understand better the effects of these fertilizers on the soil–plant system.

Table 2. Effect of biochar-based fertilizers on crop development and productivity.

Feedstock	Temperature (°C)	Enrichment Material	Crop	Type of Study	Benefits	References
Sewage sludge	300	K (potassium chloride)	Radish	Pot	Greater tuber production; ~150% increase in tuber dry mass compared with conventional fertilizer, increased K concentration in plant sap	[38]
Rice straw	450	NPK (N: P2O5:K2O = 15:15:15); microorganisms (Bacillus subtilis, Bacillus megaterium, and Bacillus mucilaginosus)	Pak choi	Pot	Increase in plant growth, biomass, and height; ~5% increase in fresh mass compared with conventional fertilizer	[4]
Banana peel	600	NPK (urea, sodium phosphate monobasic dihydrate, and potassium sulfate)	Cucumber	Pot	Increased germination rate, root and shoot growth	[117]
			Broccoli
			Red okra
Mushroom substrate waste	500	NPK (urea, diammonium phosphate, and potassium sulfate)	Tea (Camellia sinensis L.)	Field	Increase in yield and weight of shoots, increase in shoot density; increased yield and quality of tea; ~20% increase in tea leaf yield compared with conventional fertilizer	[14]
Lantana Stem	350	Vermicompost	Mung bean (Vigna radiata L.)	Pot	Greater absorption of NPK by the crop, maximum germination rate, increase in growth and production parameters	[65]
Cow manure
Rice straw	600	Ammonium molybdate	Chinese cabbage (Brassica parachinensis)	Pot	Reduction in nitrate accumulation and increase in nitrogen assimilation, increase in growth, yield, and plant quality; ~126% increase compared with conventional fertilizer	[61]
Coffee husk, chicken litter, phosphoric acid, magnesium oxide	500	-	Mombaça grass, corn, beans	Pot	Similar or higher yields, both in the short and long term, for different crops	[68]
Eucalyptus wood	400	N (urea)	Corn	Field	Increased productivity and environmental performance	[77]
Reed straw	500	Liquid seaweed fertilizer	Tomato	Pot	Increased seedling growth in unfavorable soils	[118]
Corn cob	450	NPK (urea, triple superphosphate, potassium muriate) and microorganisms (Pseudomonas spp and Bacillus spp)	Corn	Field	Increased grain yield	[123]
Urban green waste	450–500	N (urea), bentonite, and sepiolite	Corn	Pot	Improved corn growth, increased biomass and root development; ~14% increase in fresh shoot mass compared with conventional fertilizer	[116]
Rice husk	400–500	Mineral fertilizer (NPK) and microorganisms	Rice	Pot	Improved nutrient and agronomic efficiencies compared with traditional fertilizers; ~29% increase in grain yield per pot compared with fertilized control	[43]
Palm agro-waste biomass	350	Mineral fertilizer (MAP and K2SO4) + HCl	Pepper	Growth chamber	Increased pepper growth indicators	[91]
Co-pyrolysis of fresh waste fruits and vegetables	450	Chemical fertilizers in different ratios	Maize	Field	Enhanced maize yield, growth, and community function are comparable to those achieved with organic fertilizer; ~36% increase over conventional fertilizer	[32]
Acacia decurrens	450	Co-composting of Acacia decurrens biochar and chicken litter and cow dung manure	Tef	Field	BBF increased several soil properties and plant parameters of Eragrotis teff (Zucc.); ~11% increase in crop height compared with conventional fertilizer	[39]
Tobacco stalk	500	Effective microorganisms, tobacco stalk biochar, and basal fertilizer	Tobacco	Field	Increased the soil water holding capacity and several other soil water parameters. Improved tobacco growth/yield components. Increased tobacco yield by ~24% compared with fertilized control	[92]
Corn straw, peanut shell, and sawdust	600	Fe(NO3)3⋅9H2O	Carrots and cabbages	Lab	Increased the growth of seedlings	[93]
Biosolids, cow manure, and chicken manure	300	Urea fertilizer and KH2PO4	Canola	Pot	Improved canola growth; ~73% increase in dry shoot biomass	[94]
Lignocellulosic agro-residues (quinoa straw, corn straw, rice husk, and sugarcane bagasse)	500	Biogas slurry	Cucumber		Nutrients released in a controlled manner; enhanced plant nutrition	[95]
Banana leaf sheath	500	Urea solution	-	Lab	Reduced leaching loss commonly observed in conventional chemical fertilizers	[96]
Sawdust	na	Bentonite, biosolid, rice starch, bio-asphalt, and polylactic acid solution	Corn	Pot	Extended N release duration, boosted yield, and reduced N loss in leachate; ~29% increase in seed weight	[97]
Apple tree branches	500	Urea	Maize	Pot	Slowed down the release of nitrogen; increased growth and yield of maize; ~8% increase in dry weight compared with conventional fertilizer	[98]
Rice straw	500	NPK + hydrotalcite and starch	Tomato	Pot	N–P–K utilization efficiencies of hydrotalcite and starch biochar were all higher than pure biochar	[55]
Rice husk, peanut shell, and straw	na	NPK	Wheat	Field	Reduced salt concentration in the root zone and increased the water use efficiency of wheat	[99]
Pine	500	Urea	Maize	Pot	Reduced urea–N release from the fertilizer pellets and increased the plant’s overall N uptake and soil N storage relative to traditional fertilizers	[100]
Sugarcane filter cake	600	MgO and H3PO4	-	Lab	P is potentially highly available to crops while having a reduced risk for P loss to surface waters	[35]
Rice husk	450, 500, 550, and 600	KH2PO4	-	Lab	BBF (500 °C) reduced the bioavailability of Pb and increased the bioavailability of P	[101]
Eggshells and corn stalk	800	Co-pyrolysis of eggshells and corn stalk	-	Lab	Exhibited excellent P-adsorbed capacity	[102]
Sawdust	na	Ammonium sulfate	-	Lab	Released N slowly	[103]
Landscaping wood	900	Lactic fermentation with fresh foliar tree and grass biomass	Cocoa	Pot	Improved the performance of T. cocoa plant growth and vigor beyond the levels achieved with either pure mineral fertilization or pure biochar; ~5% increase in aboveground biomass compared with conventional fertilizer	[104]
Rice husk	450	Nutrient solution (NPK)	Rice	Field	Slower nutrient release patterns	[105]
Corn stover	550	Ammonium sulfate + polylactic acid	-	Lab	Enhanced N release time and rate in both water and soil environments through the integration of biochar absorption and a PLA coating	[53]
Rice straw	400 and 500	Humic acids and bentonite, cassava, or cornstarch	Rice	Field	Decreased the N leaching losses	[41]
Oilseed rape straws	400	Urea and bentonite	Oilseed rape	Pot	Enhanced the yield (~ 16.6%) and nitrogen use efficiency (~ 58.79%) of rape by slowly releasing N	[106]
Wheat straw	400	Urea, bentonite clay, rock phosphate, Fe2O3, and FeSO4.7H2O	Rice	Growth bags	Increased plant biomass (by 67%), herbage N (by 40%), and P (by 46%) uptake by rice plants	[107]
Medicinal plant material, crop willow, and wood chips	na	Urea superphosphate (USP)	Wheat	Pot	The highest grain yield per plant was stated when wood chip biochar-coated USP fertilizer was applied	[108]
Coffee husk and poultry litter	500	H3PO4 and MgO	-	Lab	The addition of H3PO4–MgO increased the pH of the BBF, favoring its use in acid soils; however, it decreased the fixed carbon content during pyrolysis when compared with the addition of only H3PO4.	[34]
Maize straw, rice straw, and forest litter	500	Urea, polyvinyl alcohol, and polyvinylpyrrolidone	-	Lab	Exhibited an excellent release behavior of nutrient leaching	[109]
Sawdust	500	NPK	-	Lab	BBF had lower nutrient release and higher moisture retention and pH	[110]

Table 2. Effect of biochar-based fertilizers on crop development and productivity.

Feedstock	Temperature (°C)	Enrichment Material	Crop	Type of Study	Benefits	References
Sewage sludge	300	K (potassium chloride)	Radish	Pot	Greater tuber production; ~150% increase in tuber dry mass compared with conventional fertilizer, increased K concentration in plant sap	[38]
Rice straw	450	NPK (N: P2O5:K2O = 15:15:15); microorganisms (Bacillus subtilis, Bacillus megaterium, and Bacillus mucilaginosus)	Pak choi	Pot	Increase in plant growth, biomass, and height; ~5% increase in fresh mass compared with conventional fertilizer	[4]
Banana peel	600	NPK (urea, sodium phosphate monobasic dihydrate, and potassium sulfate)	Cucumber	Pot	Increased germination rate, root and shoot growth	[117]
			Broccoli
			Red okra
Mushroom substrate waste	500	NPK (urea, diammonium phosphate, and potassium sulfate)	Tea (Camellia sinensis L.)	Field	Increase in yield and weight of shoots, increase in shoot density; increased yield and quality of tea; ~20% increase in tea leaf yield compared with conventional fertilizer	[14]
Lantana Stem	350	Vermicompost	Mung bean (Vigna radiata L.)	Pot	Greater absorption of NPK by the crop, maximum germination rate, increase in growth and production parameters	[65]
Cow manure
Rice straw	600	Ammonium molybdate	Chinese cabbage (Brassica parachinensis)	Pot	Reduction in nitrate accumulation and increase in nitrogen assimilation, increase in growth, yield, and plant quality; ~126% increase compared with conventional fertilizer	[61]
Coffee husk, chicken litter, phosphoric acid, magnesium oxide	500	-	Mombaça grass, corn, beans	Pot	Similar or higher yields, both in the short and long term, for different crops	[68]
Eucalyptus wood	400	N (urea)	Corn	Field	Increased productivity and environmental performance	[77]
Reed straw	500	Liquid seaweed fertilizer	Tomato	Pot	Increased seedling growth in unfavorable soils	[118]
Corn cob	450	NPK (urea, triple superphosphate, potassium muriate) and microorganisms (Pseudomonas spp and Bacillus spp)	Corn	Field	Increased grain yield	[123]
Urban green waste	450–500	N (urea), bentonite, and sepiolite	Corn	Pot	Improved corn growth, increased biomass and root development; ~14% increase in fresh shoot mass compared with conventional fertilizer	[116]
Rice husk	400–500	Mineral fertilizer (NPK) and microorganisms	Rice	Pot	Improved nutrient and agronomic efficiencies compared with traditional fertilizers; ~29% increase in grain yield per pot compared with fertilized control	[43]
Palm agro-waste biomass	350	Mineral fertilizer (MAP and K2SO4) + HCl	Pepper	Growth chamber	Increased pepper growth indicators	[91]
Co-pyrolysis of fresh waste fruits and vegetables	450	Chemical fertilizers in different ratios	Maize	Field	Enhanced maize yield, growth, and community function are comparable to those achieved with organic fertilizer; ~36% increase over conventional fertilizer	[32]
Acacia decurrens	450	Co-composting of Acacia decurrens biochar and chicken litter and cow dung manure	Tef	Field	BBF increased several soil properties and plant parameters of Eragrotis teff (Zucc.); ~11% increase in crop height compared with conventional fertilizer	[39]
Tobacco stalk	500	Effective microorganisms, tobacco stalk biochar, and basal fertilizer	Tobacco	Field	Increased the soil water holding capacity and several other soil water parameters. Improved tobacco growth/yield components. Increased tobacco yield by ~24% compared with fertilized control	[92]
Corn straw, peanut shell, and sawdust	600	Fe(NO3)3⋅9H2O	Carrots and cabbages	Lab	Increased the growth of seedlings	[93]
Biosolids, cow manure, and chicken manure	300	Urea fertilizer and KH2PO4	Canola	Pot	Improved canola growth; ~73% increase in dry shoot biomass	[94]
Lignocellulosic agro-residues (quinoa straw, corn straw, rice husk, and sugarcane bagasse)	500	Biogas slurry	Cucumber		Nutrients released in a controlled manner; enhanced plant nutrition	[95]
Banana leaf sheath	500	Urea solution	-	Lab	Reduced leaching loss commonly observed in conventional chemical fertilizers	[96]
Sawdust	na	Bentonite, biosolid, rice starch, bio-asphalt, and polylactic acid solution	Corn	Pot	Extended N release duration, boosted yield, and reduced N loss in leachate; ~29% increase in seed weight	[97]
Apple tree branches	500	Urea	Maize	Pot	Slowed down the release of nitrogen; increased growth and yield of maize; ~8% increase in dry weight compared with conventional fertilizer	[98]
Rice straw	500	NPK + hydrotalcite and starch	Tomato	Pot	N–P–K utilization efficiencies of hydrotalcite and starch biochar were all higher than pure biochar	[55]
Rice husk, peanut shell, and straw	na	NPK	Wheat	Field	Reduced salt concentration in the root zone and increased the water use efficiency of wheat	[99]
Pine	500	Urea	Maize	Pot	Reduced urea–N release from the fertilizer pellets and increased the plant’s overall N uptake and soil N storage relative to traditional fertilizers	[100]
Sugarcane filter cake	600	MgO and H3PO4	-	Lab	P is potentially highly available to crops while having a reduced risk for P loss to surface waters	[35]
Rice husk	450, 500, 550, and 600	KH2PO4	-	Lab	BBF (500 °C) reduced the bioavailability of Pb and increased the bioavailability of P	[101]
Eggshells and corn stalk	800	Co-pyrolysis of eggshells and corn stalk	-	Lab	Exhibited excellent P-adsorbed capacity	[102]
Sawdust	na	Ammonium sulfate	-	Lab	Released N slowly	[103]
Landscaping wood	900	Lactic fermentation with fresh foliar tree and grass biomass	Cocoa	Pot	Improved the performance of T. cocoa plant growth and vigor beyond the levels achieved with either pure mineral fertilization or pure biochar; ~5% increase in aboveground biomass compared with conventional fertilizer	[104]
Rice husk	450	Nutrient solution (NPK)	Rice	Field	Slower nutrient release patterns	[105]
Corn stover	550	Ammonium sulfate + polylactic acid	-	Lab	Enhanced N release time and rate in both water and soil environments through the integration of biochar absorption and a PLA coating	[53]
Rice straw	400 and 500	Humic acids and bentonite, cassava, or cornstarch	Rice	Field	Decreased the N leaching losses	[41]
Oilseed rape straws	400	Urea and bentonite	Oilseed rape	Pot	Enhanced the yield (~ 16.6%) and nitrogen use efficiency (~ 58.79%) of rape by slowly releasing N	[106]
Wheat straw	400	Urea, bentonite clay, rock phosphate, Fe2O3, and FeSO4.7H2O	Rice	Growth bags	Increased plant biomass (by 67%), herbage N (by 40%), and P (by 46%) uptake by rice plants	[107]
Medicinal plant material, crop willow, and wood chips	na	Urea superphosphate (USP)	Wheat	Pot	The highest grain yield per plant was stated when wood chip biochar-coated USP fertilizer was applied	[108]
Coffee husk and poultry litter	500	H3PO4 and MgO	-	Lab	The addition of H3PO4–MgO increased the pH of the BBF, favoring its use in acid soils; however, it decreased the fixed carbon content during pyrolysis when compared with the addition of only H3PO4.	[34]
Maize straw, rice straw, and forest litter	500	Urea, polyvinyl alcohol, and polyvinylpyrrolidone	-	Lab	Exhibited an excellent release behavior of nutrient leaching	[109]
Sawdust	500	NPK	-	Lab	BBF had lower nutrient release and higher moisture retention and pH	[110]

4. Conclusions and Future Perspectives

This study confirms the potential of biochar as a feedstock for producing sustainable fertilizers capable of replacing conventional fertilizers. Due to the low efficiency of conventional fertilizers, high application rates are typically required. This practice is expensive and has an environmental risk. Developing new biochar-based fertilizers aims to improve crop nutrient use efficiency and reduce environmental losses. There are still challenges regarding the synthesis of new fertilizers, evaluation of the release, and bioavailability of nutrients and long-term evaluation under field conditions considering the soil-plant system.

Furthermore, the choice of raw material must follow strict safety criteria to avoid environmental contamination since pyrolysis can generate undesirable compounds. Furthermore, there is still a challenge related to analyzing the economic feasibility of large-scale production of biochar-based fertilizers. Despite this, studies have shown that biochar has great potential for developing new sustainable fertilizers.