Freeze–Thaw Cycle Events Enable the Deep Disintegration of Biochar: Release of Dissolved Black Carbon and Its Structural-Dependent Carbon Sequestration Capacity

Biochar is widely regarded as a recalcitrant carbon pool. However, the impact of freeze–thaw cycle events on its storage capacity, particularly on the release of dissolved black carbon (DBC), has remained poorly investigated. This study investigated the release behavior of DBC from biochar pyrolyzed at 300–700 °C during freeze–thaw cycles and their retention capacity in soil. Freeze–thaw cycles dramatically promoted DBC release (33.08–230.74 mg C L–1), exhibiting an order of magnitude higher than those without freeze–thaw process. The release kinetics of freeze–thaw-induced DBC varied depending on the pyrolysis temperature of biochar due to the different disintegration mechanisms. Interestingly, the retention capacity of freeze–thaw-induced DBC in soil showed a reduction ranging from 7.7 to 29.5% compared to DBC without the freeze–thaw process. This reduction can be attributed to numerous hydrophilic low-molecular-weight compounds (16.97–75.31%) in freeze–thaw-induced DBC, as evidenced by the results of size exclusion chromatography, fluorescence excitation/emission matrix, Fourier transform infrared spectroscopy, and nuclear magnetic resonance. These compounds tend to concentrate in the aqueous phase rather than being retained in the soil, potentially exacerbating the outflow of dissolved organic carbon. These findings clarify the release behavior of DBC during freeze–thaw cycles and reveal their contribution to the attenuation of carbon pools in cold regions.