Evolution of Physicochemical Properties of Low-Temperature Wheat Straw Biochar Under Long-Term Freeze–Thaw Cycles

This study targets biochar utilization in seasonally frozen Northeast China and addresses the insufficient research on aging characteristics and mechanisms of low-temperature wheat straw biochar under long-term freeze–thaw stress. A 60-day simulated freeze–thaw test with 12 h −20 °C freezing and 12 h 0 °C thawing per daily cycle was carried out on 300 °C wheat straw biochar (B300). We tracked dynamic shifts in pH and water absorption during aging, and comprehensively characterized particle size, micromorphology, pore structure, elemental composition and surface functional groups for fresh (CK-B300) and fully aged (FC-B300) biochar. Freeze–thaw cycling caused drastic aging: the average particle size dropped by 33.09%, specific surface area increased by 13.86%, while total pore volume and average pore size fell by 31.47% and 54.9%, respectively. Freeze–thaw oxidation raised the O/C ratio and enriched -OH, C=O functional groups; biochar pH declined by 12.94% alongside improved water absorption. This study confirms that biochar aging is jointly controlled by ice-crystal physical fragmentation and water-temperature oxidation, providing basic data and theoretical support for evaluating and applying biochar in cold freeze–thaw zones.