Mechanisms of SolidLiquid Component Regulation of Hydrothermal Humification Products on Straw Decomposition and Soil Carbon Sequestration under Freeze-Thaw Conditions

Straw return boosts soil organic carbon, yet frequent freeze thaw cycles in cold regions slow straw decay and lower carbon efficiency, weakening sequestration. This study compared liquid artificial humic acid and solid hydrothermal humic carbon from straw hydrothermal humification products for their effects on straw decomposition and soil carbon dynamics under simulated freeze thaw environment. The results showed that AHAL rapidly stimulated early stage straw decomposition by enhancing microbial activity and hydrolytic enzyme functions. In contrast, HHC exhibited superior longterm performance, promoting sustained decomposition through enriched lignin-degrading fungi and persistent peroxidase activity, leading to a 21.78% increase in straw decomposition rate compared to the control after nine FTC. Notably, HHC significantly enhanced the accumulation of stable mineral-associated organic carbon, while the positive effects of AHAL on soil organic carbon gradually diminished over time due to the depletion of labile carbon components. These findings highlight HHC's unique dual-functionality in synchronizing straw decomposition with carbon stabilization, offering a strategic pathway for sustainable straw return in freeze thaw systems.