Ecosystem-specific interaction effects on CO2 and CH4 fluxes during autumn freeze-thaw cycles in permafrost peatlands of the Great Hinggan Mountains, Northeast China.

Autumn freeze-thaw cycles (AFTCs) represent critical transitional periods influencing greenhouse gas emissions in permafrost peatlands, yet field-based evidence regarding how freeze-thaw variability interacts with peatland type to affect carbon flux dynamics remains limited. Here, we conducted a field observational study across three permafrost peatland types in the Great Hinggan Mountains, Northeast China, including Calamagrostis angustifolia peatland (CA), Larix gmelinii-Sphagnum peatland (LG-SP), and Eriophorum vaginatum peatland (EV). Carbon dioxide (CO2) and methane (CH4) fluxes were monitored during AFTCs under different freeze-thaw variability stages to evaluate relative influences of peatland type, freeze-thaw variability, and their interaction on greenhouse gas dynamics. CO2 fluxes ranged from -266.93 to 209.34 mg m-2 h-1, whereas CH4 fluxes ranged from 1.00 to 2.23 mg m-2 h-1 across peatland types. Most peatlands functioned as net CO2 sources during AFTCs; however, the LG-SP peatland exhibited temporary net CO2 uptake under severe freeze-thaw variability, indicating ecosystem-specific differences in carbon exchange responses during AFTCs. General linear model and effect-size analyses revealed contrasting response patterns between the two greenhouse gases. For CO2, the interaction between peatland type and freeze-thaw variability explained a greater proportion of flux variability (η2p = 0.057) than freeze-thaw variability alone (η2p = 0.004). In contrast, CH4 fluxes were primarily associated with peatland type (η2p = 0.557), followed by the interaction effect (η2p = 0.217), whereas freeze-thaw variability alone explained only a small proportion of variation (η2p = 0.018). These contrasting effect-size patterns indicate that CO2 and CH4 responded differently during AFTCs across permafrost peatland ecosystems. The results highlight the importance of considering peatland type and freeze-thaw variability when evaluating greenhouse gas dynamics during AFTCs. This study provides field-based evidence that autumn carbon dynamics in permafrost peatlands are characterized by both ecosystem-specific and gas-specific response patterns during AFTCs.