Factors Influencing Carbon and Nitrogen Emissions Induced by Freeze–Thaw Collapse in Altai Mountain Peatlands

Permafrost peatlands in high-altitude regions store substantial amounts of organic carbon, yet the biogeochemical consequences of thermokarst collapse remain poorly understood. Using a space-for-time substitution approach, we selected four habitats representing a thermokarst development sequence in the Altai Mountains peatlands—slightly collapsed peat mounds (P1), severely collapsed peat mounds (P2), thawed herbaceous peat (PB1), and thermokarst ponds (PB2)—and conducted in situ greenhouse gas flux monitoring, soil physicochemical analysis, enzyme activity assays, and structural equation modeling. We found that thermokarst development fundamentally altered the greenhouse gas source–sink balance through three interconnected mechanisms. First, CO2 fluxes shifted from net emission in P1 (684.1 mg m−2 h−1) to net uptake in PB2 (−25.6 mg m−2 h−1), driven primarily by the oxidative loss of mineral-associated organic carbon in the 40–60 cm layer (71.3% loss), whereas lateral dissolved organic carbon export accounted for only 12.3% of total carbon loss. Second, CH4 fluxes in PB2 (3.8 ± 0.7 mg m−2 h−1) reached approximately 43% of the theoretical maximum, with this suppression associated with phosphorus limitation (total phosphorus < 0.05 g kg−1) and a marked reduction in alkaline phosphatase activity. Third, N2O uptake increased along the thaw sequence to −28.6 μg m−2 h−1 in PB2, with the 40–80 cm layer contributing 42% more than the surface layer. This increase in N2O uptake occurred when the soil C/N ratio exceeded 300, a threshold that reflects the substantial stoichiometric imbalance between carbon and nitrogen following thermokarst development. These findings demonstrate that the transition from peat mounds to thermokarst ponds alters the net greenhouse gas source–sink balance through changes in MAOC stability, phosphorus availability, and carbon-to-nitrogen stoichiometry. Our results provide empirical constraints for evaluating carbon-climate feedbacks in cold-region peatlands.