Biochar, known as “Black Gold”, has become a novel approach with potential benefits for soil amendment, such as improving soil physicochemical properties, reducing greenhouse gas emissions, and enhancing soil fertility. The previous research studies mainly focus on exploring different methods for the improvement of biochar enriched nutrients as fertilizers; however, the migration and transformation mechanisms of these nutrients induced by biochar are yet to be extensively investigated. This paper provides an overview of recent advances in the application and mechanisms of biochar for soil amendment focusing on soil properties and nutrients improvement. Biochar positively alters microbial-mediated reactions in the soil C and N cycles, i.e., mineralization of C and N, and N₂ fixation, thus enhancing maximizing C and N use efficiency and reducing the potential losses. Moreover, biochar provides reactive surfaces where P and K ions are retained in soil microbial biomass and in exchange sites, leading to increasing the availability of P and K to plants uptake. In addition, the toxic substances and potential ecological risks of biochar were also reviewed and discussed, thereby providing a baseline reference and guiding significance for future biochar applications as promising soil amendments.

Biochar with a higher total phosphorus concentration (⩾1.0%), total carbon concentration (⩾70%), and specific surface area (⩾50 m² g⁻¹) were optimal for improving crop yields. Greater yield increases were observed in the soil with acidic, sandy or clay soils than alkaline or loam soils. The addition of biochar boosted crop yields in low-fertility soils. Biochar application to soil improved the soil quality by increasing soil organic carbon, total N, and pH, while decreasing soil bulk density. Biochar has been widely used for soil improvement, but uncertain results persist due to diverse biochar characteristics, soil properties, and crop responses. Therefore, the effects of biochar on crop yields and soil quality were evaluated using effect size method from 1 011 paired data points from field trials, based on a global meta-analysis method. The results indicated that biochar with a higher total phosphorus concentration (⩾1.0%), total carbon concentration (⩾70%), and specific surface area (⩾50 m² g⁻¹) were optimal for improving crop yields. For improving crop yields, biochar made from manure (effect size, 42%) outperformed that made from ligneous (22%) or cereal (12%) material. Porous, acidic, or young soil types were optimal for biochar application, while sandy and clay soils were preferred over loam soil. Soils with lower available nitrogen (<80 mg kg⁻¹), phosphorus (<10 mg kg⁻¹), potassium (<120 mg kg⁻¹), pH (<4.5), and cation exchange capacity (<10 cmol kg⁻¹) were more effective on crop yield increases. The effect of biochar on yield was higher for cash crops (oil plants: 37%, vegetables: 28%) compared to food crops (legumes: 26%, maize: 20%, wheat: 12%, rice: 6%), but with no significant effect observed on rice (P=0.788). Finally, biochar increases crop yields by improving soil quality through enhanced levels of soil organic carbon, total nitrogen, ammonium-nitrogen, nitrate-nitrogen, and soil pH while reducing soil bulk density. Our research enhances understanding of the relationships between biochar, soil, and crops, aiding researchers, manufacturers, and farmers in making informed decisions regarding biochar selection, planting locations, and crop choices.

Pyrolysis presents a promising solution for the complete purification and recycling of waste salt. However, the presence of organic pollutants in waste salts significantly hinders their practical application, owing to their diverse sources and strong resistance to degradation. This study developed predictive models for the removal of organic pollutants from waste salt using three machine learning techniques: Random Forest (RF), Support Vector Machine, and Artificial Neural Network. The models were evaluated based on the total organic carbon (TOC) removal rate and the mass loss rate, with the RF model demonstrating high accuracy, achieving R² values of 0.97 and 0.99, respectively. Feature engineering revealed that the contribution of salt components was minimal, rendering them redundant. Feature importance analysis identified temperature as the most critical factor for TOC removal, while moisture content and carbon and nitrogen content were key determinants of mass loss. Partial dependence plots further elucidated the individual and interactive effects of these variables. The model was validated using both the literature data and laboratory experiments, and a user interface was developed using the Python GUI library. This study provides novel insights into the pyrolysis process of waste salt and establishes a foundation for optimizing its application.