Abstract: To investigate the characteristics and influencing mechanisms of ecosystem CO₂ exchange in salt marsh, we monitored net ecosystem CO₂ exchange (NEE) using the eddy covariance technique, and analyzed the effects of photosynthetically active radiation (PAR), temperature (T_a) and biomass on CO₂ exchange in Liaohe River estuary during the growing season. The results showed that the daily average curves of gross primary productivity (GPP), ecosystem respiration (R_eco) and NEE of each month showed inverted ‘U’, horizontal ‘S’ and ‘U’ shape, respectively. There was seasonal variation for CO₂ exchange. GPP and |NEE| reached the maximum cumulative values in June (273.5 and 210.5 g C·m^-2, respectively) and R_eco reached the maximum cumulative value in July (76.7 g C·m^-2). PAR was the dominant factor of NEE in daytime, with a rectangular hyperbola relationship. PAR explained 37.7% to 51.6% variation of NEE. The parameter of light fitting curve reached a maximum in June. T_a and soil water content at 10 cm depth (SWC₁₀) synergistically influenced nighttime respiration (R_eco,night). R_eco,night and T_a showed exponential correlations at different SWC₁₀ levels. When SWC₁₀≤35%, R_eco,night values were derived from early and late growing season, and the temperature sensitivity (Q₁₀) was 1.44. When SWC₁₀>35%, R_eco,night values were derived from middle (Q₁₀=1.97) and vigorous period (Q₁₀=2.03) of the growing season. In the growing season of 2020, the reed salt marsh at the Liaohe River estuary acted as a CO₂ sink, with the NEE, GPP, and R_eco being -693, 1043, and 350 g C·m^-2, respectively.

Key words: coastal wetland, carbon sink, CO₂ flux, eddy covariance