Seasonal dynamics of carbon exchange in a typical subalpine meadow ecosystem of the Qilian Mountains

The alpine grassland ecosystem is the largest terrestrial ecosystem type in the Qilian Mountains region and has a significant impact on the regional carbon cycle. This study analyzed the variation characteristics of CO₂ flux components in the subalpine meadow ecosystem in the middle of the Qilian Mountains based on continuous observational data from an eddy covariance system and meteorological data from the year 2021. Correlation and regression analysis were performed to explore the relationships between the flux components and environmental factors. The results showed that: 1) the ecosystem respiration (Reco), the net ecosystem carbon exchange (NEE), and the gross primary productivity (GPP) of the subalpine meadow all displayed unimodal curves in their average half-hourly flux changes throughout the year. On a daily scale, the peak carbon absorption of the ecosystem occurred during 11:00－13:00, whereas negative carbon absorption (i.e., carbon emission) occurred at nighttime; 2) Stepwise regression analysis showed that on a daily scale, NEE was controlled by the soil temperature (T_s) at a 5 cm soil depth, Reco showed a significant relationship with air temperature, soil temperature, and soil moisture content (SWC) at a 5 cm soil depth (P < 0.01), whereas GPP significantly correlated (P < 0.01) with T_s and SWC; 3) On a monthly scale, Reco, NEE, and GPP all demonstrated a significant relationship with leaf area index (LAI), with distinct seasonal variations. The annual NEE of the subalpine meadow ecosystem was –267.9 g·(m²·a)⁻¹, indicating a strong carbon sink. The results provide a scientific basis for the assessment of carbon sequestration services in the subalpine meadow ecosystems of Qilian Mountains.