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. 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. 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 (** more effective enhanced biochar fertilisers for improvement of pepper yield and quality. Pedosphere 25:703–712. https://doi.org/10.1016/S1002-0160(15)30051-5 " href="/article/10.1007/s42773-022-00160-3#ref-CR87" id="ref-link-section-d277406080e3186">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.