Hydrothermal Humification of Biomass for Circular Carbon Management in Sustainable Agro‐Ecosystems

Hydrothermal humification (HTH) is emerging as a low-carbon approach for valorizing biomass into hydrothermal humic acid (HHA), a multifunctional soil amendment with potential to enhance soil carbon sequestration and climate-resilient agriculture. However, broader deployment is constrained by limited carbon-conversion efficiency and heterogeneity in HHA composition. This Review synthesizes recent advances in HTH process engineering, elucidating how operating conditions, catalytic strategies, and integrated designs govern HHA yield, molecular structure, reactivity, and stability. Comparative analysis of hydrolysis-, condensation-, and oxidation-dominated pathways reveals distinct carbon-transformation mechanisms and structure-function relationships underlying soil and plant responses. Life-cycle and technoeconomic assessments highlight pathways toward environmentally and economically viable deployment. Advancing HHA stability and field-scale integration are key frontiers for HTH. Progress toward scalable negative-emissions deployment in sustainable agro-ecosystems will require close collaboration between scientists and engineers.