Dynamics of potassium released from sewage sludge biochar fertilizers in soil.

The solid product of sewage sludge (SS) pyrolysis, called SS biochar (SSB), is rich in carbon and nutrients, such as phosphorus (P), nitrogen (N), calcium (Ca), and zinc (Zn). However, SSB has a low potassium (K) concentration because it is released with water during the final stage of sewage treatment. The enrichment of SSB with mineral sources of K can solve the low supply of K in SSB and produce an organomineral fertilizer with a slow release of K. However, the dynamics of K release from these enriched fertilizers in different soil types remain unclear. This study investigated the dynamics of K release from biochar-based fertilizer (BBF) in the form of pellets and granules in two soil types (clayey and sandy) and natural silica. An incubation experiment was conducted for 60 days, and replicates were evaluated at prescribed time intervals. After the incubation period, the levels of K available in the solid fraction were determined, and the dynamics of K release were evaluated using four nonlinear regression models. BBFs achieved a slower release of K than the mineral KCl. The dynamics of K release were affected by the physical form of BBF, such that the pelleted BBF exhibited the slowest K release. Furthermore, regarding the concentration detected in the solid phase, the total released was highest in clayey soil, followed by sandy soil and natural silica. The enriched BBFs reduced K release throughout the experimental period, behaving as slow-release fertilizers with the potential to optimize K uptake by plants throughout the growth cycle. Further studies are required to evaluate K leaching and retention in the soil profile when biochar-based fertilizers are applied.

A scoping review on biochar-based fertilizers: enrichment techniques and agro-environmental application.

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.