Analyzing the Seasonal Dynamics of Organic Carbon Stability and Greenhouse Gas Emissions in the Vegetation Successional Sequence on the Southern Slope of the Altai Mountains, Northwest China

Seasonal freeze–thaw processes are important disturbances regulating soil carbon cycling in mid- to high-latitude mountain ecosystems, yet the mechanisms underlying carbon-fraction transformation and greenhouse gas emissions across vegetation types remain unclear. Here, we investigated five typical vegetation types along a 400–1500 m elevational gradient on the southern slope of the Altai Mountains. We measured topsoil organic carbon fractions, including particulate organic carbon (POC), mineral-associated organic carbon (MAOC), and dissolved organic carbon (DOC), extracellular enzyme activities, and CO2 and CH4 fluxes during the defrosting and melting periods to characterize vegetation-specific patterns of freeze–thaw-driven carbon stability. The results showed the following: (1) During the melting period, POC loss was significant in the low-elevation desert zone, riparian arbor forest, and floodplain meadow, with losses exceeding 90% in the latter two vegetation types. MAOC in the high-elevation mixed coniferous–broadleaf forest increased during the melting period. (2) The activities of carbon-acquiring enzymes, including CBH and AG, and the nitrogen-acquiring enzyme LAP generally decreased during the melting period, whereas the activity of the phosphorus-acquiring enzyme ALP increased in some vegetation types. This enzymatic shift may have partly constrained CO2 release per unit of mineralized organic carbon, but it did not reverse the overall increase in carbon emissions during the melting period. (3) Carbon dynamics during the defrosting period were mainly characterized by the temporary retention of mineralization-derived carbon in dissolved forms within the DOC pool under low-temperature conditions, followed by a shift during the melting period toward carbon emissions jointly regulated by hydrothermal conditions and extracellular enzyme activities. This study showed pronounced differences in carbon responses among typical vegetation habitats during freeze–thaw processes. These differences may be jointly influenced by vegetation composition, elevation, hydrothermal conditions, and soil properties, revealing an elevational pattern of carbon loss at low elevations and carbon-fraction reorganization at high elevations. These findings suggest that carbon-balance assessments in arid mountain regions should incorporate stratified identification and classified evaluation based on the distribution of dominant vegetation types.