Urban wetlands on the Qinghai–Tibetan Plateau are increasingly recognized as potentially important components of city-scale carbon budgets; however, their CO₂ flux dynamics and associated environmental drivers remain insufficiently quantified, particularly under high-altitude urban conditions. In this study, we addressed this knowledge gap by conducting continuous eddy covariance observations at Haihu Wetland Park in Xining City, China. Carbon fluxes were monitored throughout 2023 using the Huangshui Park Station flux tower. We quantified the temporal dynamics of gross primary productivity (GPP), ecosystem respiration (Re), and net ecosystem exchange (NEE), and systematically assessed their responses to key environmental drivers across multiple temporal scales. GPP and Re exhibited unimodal seasonal patterns, with substantially higher values during the growing season. NEE showed pronounced diel cycling, with nighttime CO₂ release and daytime uptake, and shifted seasonally between net source and net sink states. At the daily scale (n = 365), Pearson correlations showed that air temperature (T_a), 5 cm soil temperature (T_s5) and volumetric soil water content (SWC) exhibited the strongest associations with the flux components, whereas photosynthetic photon flux density (PPFD) showed moderate associations and precipitation was weak. At the monthly scale (n = 12), Mantel tests further highlighted a dominant thermal control on GPP and Re (T_a and T_s5), whereas precipitation showed additional associations with Re and NEE. Overall, the ecosystem acted as a net CO₂ sink in 2023 (annual NEE = −292.25 g C m⁻² yr⁻¹ under our sign convention), with uptake concentrated in the first eight months of the year. Under the combined effects of multiple environmental factors, plateau urban wetlands functioned as a strong carbon sink, and the results of this study provide a data basis for improving the accuracy of carbon budget estimates for this type of ecosystem.

To characterize ecosystem CO₂ exchange and its environmental responses in a high-elevation urban wetland, multi-year eddy covariance observations collected from 2021 to 2024 were used to examine the daily dynamics, seasonal diurnal patterns, and cumulative values during the valid observation period of each year for gross primary productivity (GPP), ecosystem respiration (Reco), and net ecosystem CO₂ exchange (NEE) in Haihu Wetland Park, Xining, China. A light-response model, an exponential temperature-response model, Pearson correlation analysis, and Mantel tests were further applied to evaluate the relationships between carbon fluxes and environmental factors. Both GPP and Reco showed a unimodal seasonal pattern, increasing concurrently during the growing season and decreasing during the non-growing season. NEE was negative on 88.24% of the valid observation days, indicating that the ecosystem predominantly acted as a net CO₂ sink. Cumulative NEE during the valid observation periods was −444.57, −525.36, −344.92, and −434.23 g C m⁻² yr⁻¹, respectively. The mean of these four cumulative NEE values was −437.27 g C m⁻² yr⁻¹, with an interannual coefficient of variation of 16.9%. Among the respective valid observation periods, net CO₂ uptake was greatest in 2022 and weakest in 2023. Sensitivity analyses of the prolonged data gaps indicated that 2022 retained the greatest magnitude of net CO₂ uptake under both reconstruction scenarios. Cumulative GPP exceeded cumulative Reco in all four years, suggesting that the persistent carbon sink was maintained by ecosystem carbon input remaining greater than carbon loss, whereas interannual variation in sink strength depended on the relative changes in GPP and Reco. GPP showed a nonlinear saturating response to photosynthetic photon flux density (PPFD). Fitting a rectangular hyperbolic model to the unbinned valid 30 min data yielded an apparent initial quantum yield (α) of 0.06296 and a GPP_max of 20.73 μmol m⁻² s⁻¹. Reco increased exponentially with soil temperature at 5 cm depth (T_s5), with a temperature sensitivity coefficient (Q₁₀) of 1.98. Correlation analyses further showed relatively strong associations between GPP and PPFD and between Reco and T_s5, while air temperature, soil temperature, radiation, and vapor pressure deficit exhibited clear seasonal covariation. Overall, Haihu Wetland remained a net CO₂ sink during the respective valid observation periods in all four years, although sink strength varied among years. The persistence of this sink reflects the combined effects of radiation-related carbon uptake and temperature-related carbon release.