Biochar is a carbonized biomass that can be used as a soil amendment. However, the exclusive use of biochar may present some limitations, such as the lack of nutrients. Thus, biochar enrichment techniques have made it possible to obtain biochar-based fertilizers (BCFs), with great potential to improve soil fertility. Nevertheless, there is still a lack of information about the description, advantages, and limitations of the methods used for biochar enrichment. This review provides a comprehensive overview of the production methods of enriched biochar and its performance in agriculture as a soil amendment. Studies demonstrate that the application of BCF is more effective in improving soil properties and crop yields than the exclusive application of pure biochar or other fertilizers. The post-pyrolysis method is the most used technique for enriching biochar. Future studies should focus on understanding the mechanisms of the long-term application of BCFs.

The use of sewage sludge biochar (SSB) for agro-environmental purposes has been increasing. However, due to the strong influence of pyrolysis temperatures on its production, there is great variation in its final properties. In this regard, efforts to generate relationships among many correlated SSB properties may help to understand this influence. This study sought to evaluate the effect of pyrolysis temperature on agro-environmental physicochemical properties of SSB. Biochars from sewage sludge (SS) were produced at 300, 400 and 500°C and their physicochemical properties were evaluated in comparison to SS samples. The increase in pyrolysis temperature decreased C, N, and H contents and the H/C atomic ratio, while increasing the C/N ratio. The pyrolysis process increased pH values, the surface area and pore volume and enriched the SSB with macro and micronutrients. Considering all variables together, the biochar produced at 300°C was that which showed the greatest nutrients availability, such as N total, S, NO₃⁻, NH₄⁺, Ca²⁺ and Mg²⁺. Conversely, SSB produced at 500°C showed higher recalcitrant organic matter and alkalinity, important properties for C sequestration and the correction of acidic soils. The combined application of SSB produced at lower and higher pyrolysis temperatures should be furthered studied.

Sewage sludge (SS) is a by-product of wastewater treatment resulting from human and industrial activities. This waste is problematic because it is difficult to manage due to its volume and environmental impacts; therefore, new technologies for treatment are needed to make its final disposal feasible. Such technologies include the pyrolysis process that produces biochar, which can be used for agricultural purposes. The present study reports a two-year field experiment with SS biochar produced under different pyrolysis temperatures and its application to soil in combination with mineral fertilizer to evaluate its effect on soil chemical properties and on corn nutrition and yield. To conduct this evaluation, 15 Mg ha⁻¹ of SS biochar combined or not with mineral fertilizer (NPK) was applied to the soil in the 2015 and 2016 cropping seasons. SS biochar increased soil chemical attributes, especially phosphorus, magnesium, cation exchange capacity and base saturation. Biochar resulted in a greater absorption of nutrients by plants and higher corn yields. However, SS biochar alone was incapable of increasing K uptake by corn. It was concluded that the application of SS biochar can replace mineral fertilizers (nitrogen, phosphorus and micronutrients) for corn production.

Several methods exist for determining soil organic carbon, and each one has its own advantages and limitations. Consequently, a comparison of the experimental results obtained when these methods are employed is hampered, causing problems in the comparison of carbon stocks in soils. This study aimed at evaluating the analytical procedures used in the determination of carbon and their relationships with soil mineralogy and texture. Wet combustion methods, including Walkley-Black, Mebius and Colorimetric determination as well as dry combustion methods, such as Elemental and Gravimetric Analysis were used. Quantitative textural and mineralogical (kaolinite, goethite and gibbsite) analyses were also carried out. The wet digestion methods underestimated the concentration of organic carbon, while the gravimetric method overestimated. Soil mineralogy interfered with the determination of carbon, with emphasis on the gravimetric method that was greatly influenced by gibbsite.