Biochar and hydrochar from agro wastes: technological pathways, characteristics, and advances in soil amendment

The increasing generation of agro waste presents major environmental and management challenges, driven by improper disposal practices that contribute to soil degradation, pollution, and greenhouse gas emissions. Among available waste valorization pathways, thermochemical and hydrothermal conversion technologies enable the production of biochar and hydrochar with significant potential for soil amendment and carbon sequestration. This review focuses on the production and application of biochar and hydrochar derived from agro waste for soil amendment. The characteristics of agro waste are discussed in relation to their suitability for thermochemical and hydrothermal conversion; the physicochemical properties of biochar and hydrochar are discussed with respect to surface chemistry, stability, nutrient retention, and environmental performance. Recent advances in soil application are reviewed, highlighting effects on soil physicochemical properties, microbial biomass, crop productivity, pollutant immobilization, and carbon sequestration. Overall, biochar generally exhibits superior stability, surface area, and soil amendment performance compared to hydrochar; however, it is not suitable for all biomasses, particularly wet residues or sludges. In contrast, hydrochar can be effectively upgraded through post-treatments such as washing or secondary pyrolysis, enabling competitive soil amendment performance. Challenges for both chars include feedstock heterogeneity, scalability, long-term environmental impacts, and standardization for carbon accounting. Addressing these issues is critical to align agro waste–derived biochar and hydrochar with climate mitigation and net-zero targets.