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.

Sewage sludge biochar (SSB) exhibits higher phosphorus (P) concentrations than the original sewage sludge (SS) and can be used as a P fertilizer. However, SSB-associated P is strongly retained in chemical compounds, which hinders its release and subsequent plant uptake. The use of organic acids facilitates P solubilization from SSB. Herein, we evaluated the effect of oxalic acid on P release from SSB applied to soil over time. Biochar was produced at 300 °C (SSB300) and 500 °C (SSB500). P release from SSB increased with an increasing concentration of oxalic acid in the SSB incubation solution and in SSB-treated soil. P speciation in SSB showed that P was predominantly inorganic (P_i), which represented 81% and 92% of the total P in SSB300 and SSB500, respectively. P_i in SSB consisted mainly of non-apatite P, accounting for 91% and 96% of all P_i in SSB300 and SSB500, respectively. Because SSB is predominantly insoluble in water, oxalic acid is crucial for the release of P from SSB. Oxalic acid increased P release from SSB300 and SSB500 by 103- and 600-fold, respectively, compared to the control, from which P was extracted with water. Oxalic acid enhancement of P release from SSB increases the possibility of using SSB as a sustainable source of P for agriculture.