Soil CO2 and CH4 Fluxes in Birch Forests during Autumn Freeze-Thaw Period: Effects of Stand Age in the Daxing’an Mountains Permafrost Region

The autumn freeze-thaw period plays a critical role in regulating soil carbon dynamics in permafrost region. However, the dynamics of soil carbon dioxide (CO2) and methane (CH4) fluxes during the autumn freeze-thaw period in high latitude permafrost regions of China remain poorly understood. In our study, we measured soil CO2 and CH4 fluxes using the dark chamber method in three different stand ages of birch (Betula platyphylla Sukaczev) forests (30, 45, and 66 years old) in the Daxing’an Mountains permafrost region and explored their relationships with key soil factors. Mean soil CO2 and CH4 fluxes are 19.24 to 36.44 mg m–2 h–1 and –12.22 to –25.92 μg m–2 h–1, respectively during the study period. Both soil fluxes in 30 years forest stand were significantly lower than in the older stands (45 and 66 years, P < 0.05). Across stand ages, soil CO2 fluxes were strongly driven by soil temperature and dissolved organic carbon, whereas CH4 fluxes were jointly regulated by soil temperature and volumetric water content. These findings highlight the critical but underappreciated role of soil CO2 and CH4 dynamics during the autumn freeze-thaw period in permafrost carbon feedbacks, underscoring the need to explicitly include this period for ecosystem carbon budget in permafrost regions, especially under climate warming background.