Balancing energy inputs and carbon outcomes in hydrochar applications: Temperature-Dependent effects on soil carbon sequestration

Hydrochar, a carbon-rich material derived from hydrothermal carbonization of biomass, has emerged as a promising soil amendment for enhancing carbon sequestration and promoting sustainable agricultural practices. However, the trade-offs between its stability, degradability, and energy inputs during production remain underexplored, particularly across diverse soil types. This study systematically evaluated the effects of hydrochars produced at varying temperatures (150, 180, 200, and 240 °C; denoted as H150–H240) on soil carbon mineralization (C_m) and net carbon sequestration in four representative paddy soils from Ningxia (N), Jiangsu (J), Guangxi (G), and Yunnan (Y). Results revealed that low-temperature hydrochars (H150–H180) significantly stimulated C_m (up to 127 %) due to their higher proportion of labile carbon, enhanced microbial biomass, and increased hydrolytic enzyme activities. In contrast, high-temperature hydrochar (H240) showed greater biological stability, with reduced C_m and higher humification coefficients (88–97 %), indicating potential for long-term carbon storage. Energy assessments demonstrated that while lab-scale hydrochar production led to net positive CO₂ emissions (57–114 t CO₂ ha⁻¹), scaling up substantially reduced the energy footprint. Notably, application of hydrochar to low-SOC soils (N, J) resulted in net carbon sequestration (up to −6.80 t CO₂ ha⁻¹), whereas in high-SOC soils (G, Y), only high temperature hydrochar contributed to net carbon sequestration. These findings underscore the need to balance hydrochar production conditions and soil characteristics to optimize its climate mitigation potential. This study advances the cleaner production of soil amendments by integrating carbon stability, energy efficiency, and site-specific suitability, contributing to sustainable land management and climate-smart agriculture.