Biochar and Hydrochar as Soil‐Based Carbon Sequestration Technologies: Implications for Greenhouse Gas Mitigation

ABSTRACT Biochar and hydrochar have emerged as promising carbon‐based amendments for enhancing soil carbon sequestration and mitigating greenhouse gas emissions. This review synthesizes and critically compares current knowledge on the production technologies, physicochemical properties, stabilization mechanisms, carbon persistence, and life‐cycle climate performance of biochar and hydrochar, emphasizing their comparative roles as negative emission technologies. Biochar, produced via pyrolysis, is characterized by highly aromatic and condensed carbon structures that confer long‐term stability in soils, frequently yielding mean residence times from centuries to millennia. Hydrochar, generated through hydrothermal carbonization, offers a more efficient path for processing wet feedstocks and waste valorization, yet generally exhibits lower intrinsic stability and greater variability in mineralization rates. The review demonstrates that carbon persistence is governed not only by molecular recalcitrance but also by soil‐mediated mechanisms, including organo‐mineral interactions, aggregate occlusion, and microbial feedbacks. Impacts on CO 2 , N 2 O, and CH 4 fluxes are strongly context‐dependent, reflecting interactions among char properties, soil texture, climate, and management practices. Life‐cycle assessments indicate that biochar systems more consistently deliver net climate benefits, whereas hydrochar performance depends on energy sources and process integration. Beyond climate mitigation, both materials provide agronomic and environmental co‐benefits, including improved soil fertility, water retention, and waste recycling. However, large‐scale deployment remains constrained by methodological heterogeneity, regulatory uncertainty, and economic barriers. The review concludes that biochar and hydrochar are complementary components of climate‐smart land management systems. Future progress depends on standardized methodologies, long‐term field experiment validation, and harmonized carbon accounting frameworks enabling reliable contributions to global net‐zero pathways. Policy alignment will be decisive for responsible and equitable global deployment.