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

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