Long-Term Effects of Freeze-Thaw Events on Ecosystem Carbon Exchange.

Freezing-thawing cycle (FTC) event plays a significant role in the regions with relative higher altitude and latitude, which is judged based on soil temperature. It affects soil environment and ecosystem productivity by altering soil physical structure, including physicochemical properties and water phase changes. Drawing on more than a decade of continuous measurements from 24 eddy-covariance towers, we quantified how freeze-thaw cycles affect net ecosystem exchange (NEE) and ecosystem respiration (RE) and assessed the capacity of these carbon fluxes to withstand such events. The average percentile of ecosystem NEE in the first, middle, and last three stages of freeze-thaw events is 66.57%, 61.77%, and 57.21%, respectively. Prior to the freeze-thaw event, ecosystem respiration accounted for 28.07% of the reference level; during the event, this proportion declined to 24.38%, whereas it subsequently rose to 31.75% after the freeze-thaw disturbance. Resistance denotes an ecosystem's ability to preserve its functional stability throughout a freeze-thaw cycle. It is quantified as the ratio of the change in ecosystem carbon exchange before and during the freeze-thaw event to the corresponding change in soil temperature. Mean annual temperature (MAT), duration of freeze-thaw events, and elevation emerged as the primary drivers governing the resistance of both NEE and RE. Collectively, our results underscore how freeze-thaw cycles modulate carbon dynamics and why this interaction merits heightened attention under a changing climate.