Interannual Variability in Ecosystem Carbon Exchange and Mechanisms Sustaining a Stable Carbon Sink in a High-Elevation Urban Wetland

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 GPPmax of 20.73 μmol m−2 s−1. Reco increased exponentially with soil temperature at 5 cm depth (Ts5), with a temperature sensitivity coefficient (Q10) of 1.98. Correlation analyses further showed relatively strong associations between GPP and PPFD and between Reco and Ts5, 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.