1 Introduction

For most agricultural systems, nitrogen (N) is the most limiting nutrient for productivity, thus compromising economic sustainability. To address this limitation, synthetic N fertilizers are applied to about 135 million Ha of agricultural land. However, these synthetic N inputs also have a range of environmental consequences including the contamination of groundwater resources (Wang et al. 2019), surface water contamination leading to eutrophication (Ayele and Atlabachew 2021), damage to reefs (Lapointe et al. 2019), gaseous losses contributing to acid rainfall (Penuelas et al. 2020) and greenhouse gas emissions (Puga et al. 2020). It has been reported that about 50 million tonnes of reactive N are released per year into the environment (Bodirsky et al. 2014).

Nitrogen use efficiency (NUE) has been broadly defined as the amount of fertilizer N that is utilized by plants in both the current and subsequent seasons (Hirose 2011). Delivering increased NUE is of paramount importance to ensure food security for a diminished area of arable land (Zhang et al. 2015) while minimizing off-site environmental impacts.

One of the key strategies to achieve greater NUE is to improve the synchrony between N supply and crop demand (Fageria and Baligar 2005). Slow-release N fertilizer has been shown to achieve this outcome (Wen et al. 2017). Some novel enhanced efficiency fertilizers such as nano structured slow-release fertilizer (Gurusamy et al. 2017), slow-release fertilizer hydrogel (Ramli 2019), lignin-based controlled release fertilizer (Chen et al. 2020a, b), brown coal based slow-release N fertilizer (Saha et al. 2021), and carbon-based slow-release fertilizer (Rashid et al. 2021), have shown benefits to crop production with lower impacts on the environment.

While the production and environmental benefits of enhanced efficiency N fertilizers (EENFs) are well demonstrated, the economic efficiency should also be taken into consideration. There is a paucity of information on the economic costs or benefits of EENFs, however, Khakbazan et al. (2013) have shown that the controlled release urea products typically have a lower net revenue of production compared to conventional urea application. To address this consideration, a body of literature is emerging on the use of biochar as a cost-effective and environmentally friendly carrier for N fertilizer. Biochar is a stable carbon (C)-rich material that is highly porous with a large surface area (Bolan et al. 2022; Chen et al. 2022b), possessing many functional groups (Zhang et al. 2021a, 2022; Basak et al. 2022). Therefore, biochar has become a versatile material for removing contaminants from water (Niazi et al. 2018; Yin et al. 2021; Chen et al. 2022a) and air (Zhang et al. 2019, 2021b), enhancing soil productivity (Li et al. 2018; Sun et al. 2019; Chen et al. 2022c), mitigating climate change (Lu et al. 2019; Song et al. 2019), and remediating soils contaminated with potentially toxic elements (Lu et al. 2014; Pan et al. 2021; Yang et al. 2021, 2022) and organic pollutants (He et al. 2019; Qin et al. 2018; Nie et al. 2021). The biochar properties, such as high porosity and adsorption capacity, have also been suggested as favorable in the development of an N-carrier for sustainable agriculture (Chen et al. 2019a, b; Sun et al. 2021). The use of biochar-based N fertilizers (BBNFs) has been considered among the most effective strategy for reducing N loss and improving NUE (Liu et al. 2019; Chen et al. 2019a, b; Dong et al. 2020). For example, Jia et al. (2021) showed an increase in NUE by about 20% using BBNFs, when compared with urea. However, the preparation of BBNFs and their role in soil improvement and remediation, as well as their impact on plant growth and crop yield, have not been critically reviewed. Thus, we aim to highlight the recent progress on the preparation of BBNFs and the mechanisms by which they improve NUE and crop production, and their role in crop production.

2 Preparation of BBNFs

Biochar-based N fertilizers can be formulated by either using a biochar/polymer coating of synthetic fertilizer granules, or through utilizing the structure of biochar where fertilizer is either adsorbed or reacted via mixing, solubilization of the fertilizer or melting of urea into the structure. The product can then be physically altered to facilitate easier material handling, via processing such as extrusion and pelletization. Detailed production processes of the BBNFs are summarized in Table 1. Promising technologies mainly comprise physicochemical methods, which include physical mixing, coating, microwave, graft co-polymerization and infiltration which are shown in Fig. 1.

Table 1 Production methods for BBNFs
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Fig. 1
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Schematic showing methods used for preparations of BBNFs

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2.1 Blending of biochar with fertilizers

Perhaps the simplest and most widely used approach for the preparation of BBNFs involves the mixing of biochar with fertilizers. It can also include additional physical preparation techniques of the blended materials to facilitate easier application, including extrusion or pelletization. This method also allows simple adjustment of the formulation of BBNFs to address specific crop/pasture nutrient requirements. Various organic and inorganic N fertilizer sources have been used to prepare BBNFs, including poultry litter (Steiner et al. 2010), urea (Shi et al. 2020), (NH4)2HPO4 (Dong et al. 2020), vinasse (Amaro et al. 2016), and swine manure compost (Zhou et al. 2021).

During the physical mixing process, clay minerals such as bentonite (Yao et al. 2015), sepiolite (Shi et al. 2020) and kaolin (Chen et al. 2018a) are added to help the pelletization of BBNFs and enhance the direct binding of N fertilizer to biochar. With the addition of bentonite, the mechanical strength of BBNFs can be increased and the granular structure can be protected from collapse due to the association between N–H of fertilizer and Al–OH of bentonite (Shi et al. 2020). The use of bentonite can also promote beneficial microorganisms when added to a clay loam soil (Yao et al. 2015) and contribute to higher water-holding capacity (WHC%) and water-retention (Wen et al. 2017). Moreover, Wen et al. (2017) found that lamellar bentonite could be exfoliated to form a complicated physical network, which can further prolong the diffusion path of N release from the BBNF. The interaction between Mg-OH of sepiolite and N–H of N fertilizer was also found to be effective in reducing N leaching from BBNFs (Shi et al. 2020). Kaolin has also been used as a binding agent during pelletization of BBNFs (Chen et al. 2018a). Kaolinite in the soil can enhance the stability of biochar by reducing chemical oxidation and biological degradation, which promotes long-term C sequestration (Yang et al. 2018). Biochar can also be activated to increase its surface area or surface functional groups. In the preparation of a BBNF product, barley straw-based biochar was physically activated using CO2 and then mixed with N, P, K and micro-nutrients resulting in lower N losses from a sandy clay loam soil (by up to 63%) compared to either urea or NH4NO3 (González-Cencerrado et al. 2020).

2.2 Coating of fertilizers with biochar

Biochar coating of chemical fertilizers is a promising approach in the development of controlled release fertilizers, having been shown to limit N loss pathways and associated environmental problems (Wang et al. 2015; Naz and Sulaiman 2016). Coating of chemical N fertilizers with polymer films composited with biochar has been shown to further improve N release characteristics. Waterborne polymer (polyacrylate) (Zhou et al. 2015), waterborne copolymer (made of PVA (polyvinyl alcohol) and polyvinylpyrrolidone (PVP)) (Chen et al. 2018b), the water-retention polymer (acrylic acid and 2-acrylamide-2-methylpropanesulfonic acid) (Wen et al. 2017), and biodegradable polymers (such as starch, and ethyl cellulose) (González et al. 2015) were applied to synthesize biochar-based polymer coating for BBNFs.

Considering the low-cost and minimal environmental impact, waterborne polymers (i.e., water-soluble) have been developed as coatings to produce BBNFs. For example, controlled release of nutrients over 12 months was achieved with the inclusion of biochar in a polyacrylate coating and the product had no significant impact on the dominant soil bacterial community (Zhou et al. 2015). Chen et al. (2018b) developed BBNFs using biochar-based waterborne copolymers to coat urea, resulting in a slow-release formulation that released 65% of the total N over 22 days. Due to the cross-linking networks between biochar and copolymers, BBNFs with biochar-based waterborne copolymer films showed low water absorbency and were more effective in retaining N than copolymers (Chen et al. 2018b). In another coating technology, biochar was mixed with humic acid, bentonite and modified starch (Dong et al. 2020). The SEM and FT-IR observations revealed that an effective dense layer was formed that slowed N release from the granule.

Water supply plays a key role in agricultural production, and thus polymers that improve soil water retention and water holding capacity can play a role in improving the functionality of BBNFs. Acrylic acid (AA), 2-acrylamide-2-methylpropanesulfonic acid (AMPS), bentonite and NH4+-loaded biochar were utilized to synthesize BBNFs. SEM analysis verified that biochar embedded in BBNFs could hold a substantial quantity of water due to its highly porous structure. The results showed BBNFs at 2% (w/w) application rate increase the water holding capacity of soil from 29 to 61%, compared to the treatment without BBNFs. Moreover, the soil without the samples lost all its absorbed water after 12 days, whereas BBNFs still retained 10.6% of the soil moisture on the 30th day (Wen et al. 2017). It should be noted, however, that most of the reported biochar-based BBNFs can only marginally change soil water retention. Therefore, the development of biochar-based BBNFs with high water retention represents a significant opportunity.

Another formulation of BBNF was produced whereby biochar was impregnated by urea in a batch reactor (150 ± 5 °C) at atmospheric pressure and then encapsulated using biodegradable polymers including sodium alginate (SA), cellulose acetate (CA) and ethyl cellulose (EC) (González et al. 2015). For soil columns planted with wheat, leaching of urea-N was greatest with urea (4.1 mg/kg dry soil) while BBNF lowered this to 2.4 mg/kg (dry soil). The BBNF products were shown to retard urea hydrolysis, another mechanism that may explain the higher NUE.

2.3 Exploitation of sorptive properties of biochar

Porosity and reactive surfaces contribute to the adsorptive nature of biochar, which have been exploited to develop controlled release fertilizer formulations. Various articles describe processes such as solid liquid adsorption and infiltration to develop these products.

For example, Khan et al. (2008) fabricated BBNF by placing charcoal in a rotary vacuum evaporator with N fertilizer solution and then rotating for 24 h at 100 °C. This resulted in a BBNF with a significantly retarded release of N into soil leachate. A similar BBNF preparation method using biochar pyrolyzed at 200 °C and (NH4)2SO4 showed that more than 90% of the NH4+ was retained after a 21-day desorption experiment (Cai et al. 2016). Biochar made from rice husks was put into urea-hydrogen peroxide (UHP) solution to prepare BBNFs via an adsorption method. As compared to urea, more C–O and C=O groups could be obtained in the products due to the oxidation of carbonized surfaces by H2O2, contributing to the improved slow-release of N (Chen et al. 2018a). A BBNF (An et al. 2020) was made by incorporating biochar into semi-interpenetrating polymer networks through graft co-polymerization with superabsorbent hydrogels. The product exhibited a high water-retention capacity of 73% after 25 days, which was far greater than fertilizer without the incorporation of biochar. It showed the incorporation of biochar can significantly improve the pore structure of BBNFs and create more cross-linking points.

In another approach, molten urea (about 155 °C) was used as both a binder and N source to synthesize BBNFs (Xiang et al. 2020). The molten urea was shown to enter the micropores of biochar and completely react with the amino and carbonyl groups on the surface of biochar, leading to a BBNF that reduced loss of N from volatilization or leaching from soil (Xiang et al. 2020).

Another methodology reacted urea, biochar, bentonite and polyvinyl alcohol in a sealed reactor at the temperature close to the melting point of urea (132–135 °C) using hydrothermal carbonization (Liu et al. 2019). The cumulative release amount of N from this BBNF was 61% within 28 days when incubated in water and 54% within 98 days when incubated in a silt loam soil. The release rate of N from BBNF was shown to be controlled via adsorption to bentonite, hydrogen bonding and electrostatic interactions of the N–C=O and oxygen-containing functional groups (Liu et al. 2019).

While the physical properties of biochar have been utilized to prepare BBNFs, the surface functional groups play an important role via sorption and chemical interaction. For example, the reaction between rice husk-derived biochar, NH3 and H3PO4 has been utilized to produce BBNFs, which, as compared with (NH4)3PO4, showed a much slower release rate of N (NH4+ and NO3−) in sandy soil (El Sharkawi et al. 2018). Aromatic C, C–O and O–H functional groups, demonstrated by FTIR, can be conducive to adsorption and consequently react with NH3 and phosphate (El Sharkawi et al. 2018).

2.4 Influencing factors for N release

Factors such as biochar properties, component ratios, organic N content and types of N precursor can make important contributions to the properties of BBNFs.

The surface functional groups and doses of biochar used for BBNFs play important roles in the resulting structure and properties of BBNFs. For example, urea particles encapsulated by rice biochar-based copolymer with fewer OH groups showed a more compact and denser structure than maize biochar and forest litter biochar, and thus exhibited a sustained slow-release capability (66% of N released from product over 22 days) (Chen et al. 2018a). Biochar pyrolyzed at low temperatures had a low specific surface area and was shown to be unsuitable in the preparation of BBNFs as it did not deliver N as slow release (Jia et al. 2021). Pyrolysis conditions may also control the properties of BBNFs. For example, Ye et al. (2019) found that BBNF produced under a H2O (g) and CO2 atmosphere preserved the N derived from the feedstock more effectively than under an N2 atmosphere.

The type of N fertilizer added in the BBNFs also affects the slow-release property of BBNFs. Organic N released by BBNFs was much lower (39–56%) than the treatment using urea alone (Shi et al. 2020). When NH4HCO3 was used to prepare a BBNF, it was shown to have limited stability, decomposing at a temperature above a range of 36–60 °C (Lee et al. 2013).

In summary, BBNFs prepared using simple physical mixing possesses have advantages of being low cost with a simple production process enabling batch-to-batch manipulation of the fertilizer ratios. BBNFs using biochar-based coating of chemical fertilizer are more complex to manufacture, but can deliver higher rates of fertilizer per weight of product. Both methods have been well studied and have generally delivered products with improved NUE, lower losses of N to the environment and in some cases, have improved the water retention properties of soil. Other production methods such as solid–liquid adsorption, hydrothermal production and chemical reaction are less well studied but show promise in the development of BBNFs. A detailed summary of the preparation methods of BBNFs is presented in Table 1.

2.5 Slow release of N from BBNFs

Slow-release BBNF formulations are designed to synchronize N fertilizer supply in soil more effectively with plant N demand (Jia et al. 2021). By providing this control, the large mineral N pools that can occur following mineral fertilizer application are avoided, thus minimizing N loss pathways including denitrification (Puga et al. 2020), leaching (Cen et al. 2021) and volatilization (Shi et al. 2020). The kinetics and mechanisms of the slow release of N from BBNFs are discussed below.

2.5.1 Kinetic models of N release and slow-release mechanisms of BBNFs

The cumulative N release from BBNFs has been examined using pseudo-first-order and pseudo-second-order kinetics models. The results showed that both models provided good fits to the data, with high level of confidence (Fig. 2). Interestingly, the cumulative N release data can be even better described by the pseudo-second-order kinetics model. This may be attributed to the fact that pseudo-first-order kinetics was controlled by diffusion, while pseudo-second-order kinetics was dominated by chemical adsorption (Fang et al. 2018). This suggests that the slow release of N from BBNFs is likely to be a combined influence of diffusion and chemical adsorption processes.

Fig. 2
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N release rate comparing urea to BBNFs across published studies. U represents urea alone and B + U represents urea in combination with biochar

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The slow-release properties of BBNFs can be further analyzed by comparing the constant ‘k2’ of the fitted pseudo-second-order kinetics model. The k2 of BBNFs can be reduced by 59% (Chen et al. 2018b) and approximately 92% (Jia et al. 2020) as compared to that of urea or chemical N fertilizer, demonstrating the slow release of BBNFs. The k2 of BBNFs prepared using the coating method was greater than that prepared by the adsorption method, which indicates that the adsorption method could exhibit better slow-release properties (Wen et al. 2017). However, the variability of k2 across studies, resulting from widely differing soil and climatic conditions, greatly limits its practical use for predicting N release properties.

The most commonly used source of N in the production of BBNFs is urea, which needs to be hydrolyzed via the urease enzyme in soil before it becomes plant available (Shi et al. 2020). Comparing the hydrolysis of urea or urea-based BBNF, Shi et al. (2020) showed that the DOC concentration using urea fertilizer decreased after day 3, while the DOC remained stable for over 10 days with BBNF. This suggests an interaction between organic matter and mineral additives in BBNFs, thereby protecting urea from rapid hydrolysis. González et al. (2015) also observed that urea hydrolysis could be retarded with the use of polymeric coating materials, thus leading to slower N release.

Wen et al. (2017) found that hydrogen bonding and electrostatic interactions governed the slow release of NH4+ in BBNFs. When pyrolysis temperatures for biochar manufacture were in the range of 200 to 400 °C, incomplete carbonization resulted in more oxygen functional groups (Cai et al. 2016), which could provide sorption sites for NH4+ and contribute to the slower release of N when this biochar was used to manufacture the BBNF. Furthermore, diffusion processes may also play an important role in the N slow release of BBNFs. Sealing BBNFs with a bio-oil coating can also retard the diffusion of N, leading to less N released when initially applied to soil (Ye et al. 2019).

3 BBNFs for soil improvement

Fertilizer loss results in water pollution, air pollution, soil quality degradation and other potential negative impacts. The retention of nutrients in soil can be prolonged by modifying soil properties using biochar products (Table 2). For example, soil physical properties such as porosity, water storage, bulk density and chemical properties (e.g., cation and anion exchange capacity, pH, and organic matter) can be modified by the application of BBNFs.

Table 2 Effects on N loss from BBNFs used in field studies
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3.1 N conversion process for soil

In recent years, increasing attention has been paid to the influence of BBNFs on N cycling for both agronomic and environmental outcomes (Table 2). A schematic for the changes to N cycling following BBNF application is shown in Fig. 3.

Fig. 3
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The modification of N cycling processes in soil following BBNFs application

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The sustainable development of agriculture depends on increasing NUE and consequently reducing N losses from different sources, such as NO3− leaching, NH3 volatilization, and N2O emissions. BBNFs could decrease the loss of N as compared with conventional chemical fertilizers (Puga et al. 2020) thus providing a range of environmental benefits. While biochar amendment to soil can reduce N loss from leaching (Yoo et al. 2014; Hossain et al. 2020), volatilization (Nguyen et al. 2017), or denitrification (Cayuela et al. 2015).

Leaching is a primary loss pathway for N (González-Cencerrado et al. 2020). Among all the solid nitrogenous fertilizers, urea contains the highest N content of 46% and is therefore widely used. However, urea is highly soluble and as such can move easily with water movement. Jia et al. (2021) reported that soil N loss could be reduced by about 30% after replacing urea with BBNFs prepared by coating biochar on the urea particles. Cen et al. (2021) used polylactic acid (PLA) to coat the particles to produce BBNFs. Over a 25-day experiment evaluating this BBNF, soil analysis indicated that up to 66% N from BBNFs remained in the soil available for crop uptake, but N remaining in the soil from (NH4)2SO4 was just 41%. In another study, the peak of NH4+ leaching loss was delayed from day 17 with urea to day 25 with BBNFs (Jia et al. 2021), likely due to the delayed hydrolysis of urea in the BBNFs (Shi et al. 2020). Fertilized under the same level, the cumulative leaching loss of NH4+ was significantly decreased (39% to 50%) by the application of BBNFs (Jia et al. 2021). Dong et al. (2020) found that the maximum concentration of NH4+-N could be reduced by 55% in percolation water after replacing compound fertilizers with BBNF. As biochar can have cationic sorption sites, it is able to adsorb NH4+, therefore reducing NH4+ losses in soils with pH below 6.3 (Mandal et al. 2018).

Being a soluble anion, NO3− is highly susceptible to leaching loss in agricultural soils. In general, BBNFs, in which biochar is negatively charged, possess little adsorption capacity for NO3−. NO3− leaching accounted for 83–84% of total loss of N via leaching from BBNFs (Jia et al. 2021), however, leaching losses of NO3− were still 20% lower than conventional fertilizers. Wang et al. (2015) also observed that NO3− leaching loss was the predominant pathway of N loss and contributed to 52–96% of total N leached from BBNFs. But compared with urea and NH4NO3, NO3− leaching loss was reduced by over 60% with BBNFs (González-Cencerrado et al. 2020). Jia et al. (2021) reported that the cumulative leaching loss of NO3− with BBNFs was 25% lower than urea. The large surface area of biochar and abundant oxygen functional groups such as carboxylic, hydroxyl and lactone groups on the surface of BBNFs play a dominant role in adsorbing NH4+ and NO3−, ultimately reducing the N loss (Zheng et al. 2017).

NH3 emissions from agriculture account for up to 90% of the global NH3 emissions (Brentrup et al. 2000)  and NH3 is a contaminant in aquatic systems (Park et al. 2019), is an indirect greenhouse gas and contributes to ozone generation in the near surface atmosphere (Ravishankara et al. 2009; Ma et al. 2021). The volatilization of NH3 was found to be significantly inhibited (50%) by using BBNFs (González-Cencerrado et al. 2020). BBNFs with 51% biochar and 10% N showed the lowest NH3 dissolution rate (Puga et al. 2020). However, due to the prolonged NH3 volatilization period (from 23 to 26 days), the cumulative NH3 discharged by BBNFs may exceed that of urea (Jia et al. 2021). However, due to the higher overall NUE of BBNFs, lower N applications are required, meaning NH3 volatilization can be reduced to below that of urea (Jia et al. 2021). Shi et al. (2020) found that the NH3 volatilization loss from BBNFs was similar to that of urea, indicating that the alkaline pH microenvironment around urea adsorbed into BBNF may be similar to that around hydrolyzing urea granules in the soil (Puga et al. 2020).

NH3 volatilization can also be affected by soil pH, temperature and fertilization level (González-Cencerrado et al. 2020). The alkaline pH of BBNFs is usually the result of the alkaline nature of biochar and may potentially lead to greater NH3 volatilization (Puga et al. 2020). NH3 volatilization can be lowered due to the cationic charge associated with BBNFs (Puga et al. 2020). Furthermore, Puga et al. (2019) showed that acidification of biochar, prior to the coating of urea granules, further lowered NH3 volatilization from BBNFs.

The potential for NH3 volatilization from BBNFs is dependent upon the production conditions of the BBNFs. In order to determine the volatilization losses associated with the preparation of BBNFs, Cen et al. (2021) mixed biochar with dissolved (NH4)2SO4. They found NH4+ ions were attached on surfaces and trapped within mesopores and micropores of biochar due to chemisorption of polar and nonpolar compounds, including hydrophobic bonding, π–π electron donor–acceptor interactions resulting from fused aromatic C structures, and weak unconventional H-bonds. The dissolved NH4+ ions are released to the interface between biochar and the coating layer via diffusion, under concentration or pressure gradient, or a combination of these as the driving force. This resulted in a slower NH3 release rate.

BBNFs have also been shown to lower N2O loss from soil. Puga et al. (2020) found that cumulative N2O emissions from BBNFs when compared with urea were decreased by 8% while Qian et al. (2014) showed a decrease of 40%. The lowered N2O emissions may be due to a decreased population of denitrifiers in soils under BBNF treatment compared to urea treatment (Liao et al. 2020). Organic N associated with biochar particles would not be as accessible to denitrifying bacterial as inorganic N (Harter et al. 2014; Oladele et al. 2019). Also, adsorption of mineralized N on biochar surfaces could retard N utilization and transformation by microbes that produce N2O (Wang et al. 2013). However, data on the mitigation of N2O losses from soil with BBNFs are still limited.

3.2 Improved soil properties with BBNFs

3.2.1 Physical properties

Owing to the low bulk density of biochar used in the manufacture of BBNFs, their porous structure and large specific surface area, BBNFs can decrease the bulk density of soil, leading to improved aeration and permeability (Chen et al. 2018b; Zhang et al. 2018; Puga et al. 2020). The reduction of bulk density could also affect the water-holding capacity of soils. For instance, Liu et al. (2019) found that BBNFs (1 g N, 200 g of dry soil) could directly increase the water holding ratio from 40 to 70% of the soil moisture over a period of 30 days. In this study, BBNF was applied at 2% w/w and much of the additional water holding capacity was attributed to the content of bentonite in the fertilizer formulation. Cen et al. (2021) used the water loss (%) of soil samples to represent water retention capacity, and they observed that BBNFs had significantly lower water loss than that of all other samples until day 3. Hydrogel formation through the utilization of polymers in BBNF manufacture has been shown to slow the release rate of N due to the physical barrier of hydrogel (Wen et al. 2017). The water-holding capacity and water-retention period of soils with BBNFs prepared by adsorption and the use of polymers increased and lasted 12–18 days and 30 days, respectively, as compared with unamended soil (Wen et al. 2017). It should be noted, however, that the BBNFs were applied at 2% w/w which is significantly greater than conventional fertilizer application doses. These effects on soil water holding capacity are likely to be relatively minor when lower doses of BBNFs are utilized.

3.2.2 Chemical properties

3.2.2.1 Soil pH

pH is one of the most important chemical properties of soils. BBNFs, containing N and mineral elements such as K, Ca and Mg, can exchange H+ in soil and thus increase soil pH. Jia et al. (2021) found that soil pH following amendment with BBNFs (pH range 6.03–6.10) was slightly higher than soil fertilized with urea (5.89–5.95). Zheng et al. (2017) also found, this time in an alkaline soil, that soil pH increased slightly from 7.89 to 8.06 under BBNF treatment. A large elevation in pH (to pH 9.0) on another alkaline soil with BBNF application was observed by Puga et al. (2020). BBNFs that are alkaline could therefore be unfavorable for use in alkaline soils due to the increased volatilization of NH3 and development of toxicities. Lee et al. (2013) found that mixing biochar 50/50 by weight with NH4HCO3 reduced the pH of biochar material from 9.85 to 7.89, since NH4HCO3 acts as a pH buffer. The effect of BBNFs on soil pH may be affected by soil type. For example, in some studies, the effect of BBNFs on the pH of calcareous soils was negligible (Zheng et al. 2017; Shi et al. 2020). In contrast, the application of BBNFs obtained from orange peel, residual wood and water-treatment sludge impregnated with anaerobic digestion slurry reduced lettuce production due to an excessive increase in soil pH (Oh et al. 2014).

3.2.2.2 CEC

Cation exchange capacity (CEC) is a very important indicator of soil fertility (Keller et al. 2001). The application of BBNFs could improve soil CEC, which is conducive to retaining cationic nutrients in the soil (Lee et al. 2013). The charge carried by biochar can convert some stable state elements into active states (Domene et al. 2014), for example, maize-cob-based BBNF increased the exchangeable cations from 16 to 24 cmol/kg (Zheng et al. 2017). However, Zheng et al. (2017) pointed out that there was no significant difference between any of the BBNF and fertilizer treatments with regards to soil CEC. Therefore, the beneficial effects of BBNFs on soil CEC should be further investigated under conditions of continual BBNF application.

3.2.2.3 SOC

Numerous studies have shown that one of the promising approaches to enhance soil organic carbon (SOC) is via BBNF application. The C content of BBNFs (when applied as an N fertilizer at 225 kg N/ha) can result in a dramatic increase in SOC of 16% compared to inorganic N fertilizers, and thus contribute to higher crop yield (Zheng et al. 2017). Liao et al. (2020) reported that the application of BBNFs increased SOC by 20% compared to urea. The long-term SOC content of the soil would also increase with the input of BBNFs, resulting from the recalcitrant C provided by biochar (Zheng et al. 2017; Chen et al. 2020a, b). In addition, Puga et al. (2020) reported methane fluxes did not exceed 5.71 g C/m2/d when using BBNFs, because BBNFs have predominantly stable C and are resistant to decomposition (Grutzmacher et al. 2018). However, some factors can reduce the concentration of SOC, such as excessive application of fertilizers. Even BBNFs containing low organic C can decrease SOC if applied at high rates (Qian et al. 2014). However, when used at fertilizer rates, the effects on SOC levels from the addition of BBNFs into soil are likely to be very small (Shi et al. 2020).

3.2.3 Modification of N cycling processes

Microorganisms perform a vital role in the soil N cycle, which can be significantly affected by biochar (Zhu et al. 2017; Siedt et al. 2021). The pores of BBNFs can absorb a large amount of water and nutrients, which can provide resources for microorganisms (Shi et al. 2019). BBNFs can also influence the physical and chemical properties of soil, thus also modifying soil processes (Amaro et al. 2016). During the harvest stages, higher urease activity (about 10%) was observed with BBNFs compared with urea (Liao et al. 2020). The relative abundance of nitrifiers such as Acidothermaceae and Sphingomonadaceae increased under BBNF treatment (Liao et al. 2020). Dong et al. (2020) reported that BBNF amendment could increase the relative abundance of nitrifying bacteria such as Nitrospira, Nitrospora and Nitrobacter, and the relative abundance of denitrification bacteria such as Burkholderia, Cupriavidus and Bradyrhizobium. The lower leaching or other loss pathways of N from BBNFs may be closely related to soil N cycling microorganisms. However, the exploration of the influence of BBNFs on soil bacterial communities and soil properties is still at its early stages.

4 Promoting plant growth and crop yield by BBNFs

4.1 BBNFs for NUE improvement

Increasing nutrient use efficiency continues to be a major challenge for agriculture globally, especially N, which is a critical factor limiting agricultural productivity in China (Cui et al. 2010). Zhang et al. (2021b) compared 13 N budget datasets and found NUE declined from 59 to 46% over the last six decades. This is partly due to the increased rate of N fertilizer application which inherently lowers NUE. With BBNF addition in a 30-day pot experiment, compared to NH4Cl, the total NUE of cotton plants increased from 36 to 64% and N leaching rates decreased by 74% (Wen et al. 2017). Liao et al. (2020) found that BBNFs had the highest N agronomic efficiency (NAE) and NUE, with a ~ 28% higher NAE and ~ 59% NUE than urea across a 7-month pot study. In a field experiment in which maize was planted in clay loam soil urea had a NUE of 28%, while BBNF increased NUE to 43% (Puga et al. 2020). The improved NUE was related to lower N losses from leaching of mineral N or gaseous loss of N2O (Puga et al. 2020).

Conventional N fertilizers used in crop production typically have use efficiencies of only 30 to 40%. A primary reason for the low NUE is of the lack of synchrony between fertilizer N availability and crop N demand. Cen et al. (2021) found that 60% of the N content of BBNFs was retained in the soil.

Under the same fertilization level, BBNFs showed the highest N uptake in oilseed rape compared to either urea or biochar combined with urea (Jia et al. 2021). Using an 15N tracer technique, higher N uptake during the crop growth including bolting, flowering and harvest was achieved using BBNFs compared to urea (Liao et al. 2020). Peng et al. (2021) found that using BBNF could increase N retention (74–80%) in a brown soil, promoting the N uptake of maize from 84 to 106 kg N/ha. Shi et al. (2020) observed more inorganic N such as NH4+ and NO3− was produced by BBNFs due to the slower mineralization of organic N as well as the adsorption of inorganic N by biochar, resulting in greater crop N uptake. Liao et al. (2020) reported that BBNFs enhanced NUE for rape growth by increasing soil NO3− content via the slow-release properties of BBNFs, increased nitrification via AOB activity and reduced denitrification therefore lowering N loss.

4.2 BBNFs and crop yield

The synchrony between N supply and crop demand can be improved by the slow-release characteristics provided by BBNFs. Many studies have been reported for a wide variety of crop yield improvement as a result of BBNFs application to soils (Table 3). Maize grain yield increased by 10% (Zheng et al. 2017) or 21% (Puga et al. 2020) with BBNFs as compared with that with matching doses of chemical fertilizers. The fresh weight of Japanese mustard spinach plants increased by 33% after using BBNFs to replace NH4H2PO4 (El Sharkawi et al. 2018). Compared to commercial chemical fertilizers, the yield of green pepper after BBNF application increased from 9 t/ha to about 11 t/ha (Yao et al. 2015). Even with a 20% reduction of fertilization application, the biomass of vegetable (oilseed rape) obtained was higher (about 10%) under BBNF treatment compared to urea treatment (Jia et al. 2021). When maize yield was normalized by mass of N fertilizers (N agronomic efficiency, AEN), the improvement observed with BBNFs was further increased to above 40% (Zheng et al. 2017). An increased tea yield (10%) was also achieved with the application of BBNFs (He et al. 2019).

Table 3 Effects of BBNFs on crop yield
Full size table

In addition to crop yield, other plant growth processes such as germination rate and seedling growth parameters could also be enhanced by BBNFs. Cottonseed treated with BBNFs showed a higher germination rate (about 94%) than that treated with NH4Cl (about 84%) (Wen et al. 2017). On the other hand, turnip germination rate was lowered (Amaro et al. 2016). Further studies are required to determine the key effects of BBNFs on plant seed germination.

It is reported that the biomass and crop yield generally increased with the application of BBNFs to the soil, while belowground productivity exhibited no significant response to biochar addition, according to a meta-analysis (Biederman and Harpole 2013). The root length of cotton plants under BBNF treatments increased by 26% compared to the use of NH4Cl (Wen et al. 2017). Similarly, the biomass and volume of roots—under BBNFs increased by 26% and 38%, increased, compared to the use of related to urea fertilizers, possibly indicating enhanced plant growth in dryland agriculture (Shi et al. 2020) and under drought conditions (Bruun et al. 2014; Li and Tan 2021). Significantly less N fertilizer (58 kg N/t grain) was consumed in the BBNF treatments than conventional inorganic compound fertilizers (85 kg N/t grain), implying a higher plant NUE, lower N pollution, and higher economic output (Zheng et al. 2017).

5 Conclusions and perspectives

While there is strong evidence that BBNFs can improve N use efficiency, there is still a need for an improved understanding of their optimal application rates, and how these affect their economic feasibility compared to conventional fertilizers. Across about 20 studies in the literature, BBNFs showed 15–69% delay in cumulative N release and 25–65% improvement in fertilizer use efficiency over traditional chemical fertilizers. The future challenge is to enhance the N efficiency of BBNFs, with the associated reduction in environmental pollution, while maintaining their cost-effectiveness by minimizing cost of their production.

There are several mechanisms by which BBNFs can improve nutrient use efficiency and lower fertilizer N loss to the environment. Modelling approaches to understand the kinetics of fertilizer nutrient release, as well as additional studies using isotope techniques will allow a detailed understanding of the benefits of BBNFs. By linking this knowledge with an understanding of soil properties, crop fertilizers demand, and climate (rainfall and temperature) forecasts for the season, it may be possible to produce BBNFs formulations to provide optimum N release characteristics for a specific crops and environments. Essentially, it is better to design BBNFs based on the optimal N absorption rate by crops or plants to keep balance between N release rate and N absorption. Further work is required to establish the relationship between N-release control of BBNFs and N absorption by plants. The long-term effects of large-scale application of BBNFs on the environment, and the corresponding ecological risks to biodiversity and ecosystem balance also require consideration. In conclusion, evidence suggests that BBNFs can result in improved fertilizer use efficiency, while providing other benefits to soil, such as increasing SOC. The use of biochar as a carrier for fertilizers will have other benefits compared to conventional polymer coating, especially related to avoiding the entry of plastics and microplastics into the environment.

This review shows that BBNFs possess considerable potential as improved N fertilizers. The contribution of BBNFs to sustainable agriculture includes economic benefits, and reduced energy consumption and greenhouse gas emissions. Although some biochar modification methods seem to be very costly, BBNFs may still be economically feasible due to the relatively low mass fraction of biochar in BBNFs (~ 29%) (Jia et al. 2020), increased economic output (Cen et al. 2021), and ecological and economic benefits arising from lowered eutrophication. Biochar systems could deliver emission reductions of 3.4–6.3 PgCO2e (Lehmann et al. 2021), partly due to the energy gained during biochar production and also the stable C stored in soil. This could be further enhanced through other greenhouse gas (GHG) benefits when introduced into BBNFs. The GHG intensity can be effectively reduced (e.g., 10 kg CO2-eq emission per ton maize grain) after replacing chemical fertilizer with BBNF (Zheng et al. 2017). Roberts et al. (2010) estimated the energy, climate change impacts and the economics of biochar systems by life cycle assessment (LCA). They found that biochar returned to the soil could reduce 800–900 kg CO2 equivalent emissions per tonne of dry feedstock pyrolyzed. Using an LCA approach, González-Cencerrado et al. (2020) showed that BBNFs compared to conventional fertilizers have the lowest impact on acidification, terrestrial and aquatic eutrophication. Thus, the use of BBNFs has the potential to contribute to carbon sequestration, reduced N use and increased farmer income. Our review has highlighted the key benefits and challenges related to the development of enhanced efficiency N fertilizers based on biochar technologies. A framework for the systematic assessment of new products will allow the technology to be fast-tracked to commercial adoption, which will facilitate a wide range of economic and environmental benefits, including the broader goal of carbon neutrality in primary industries.