Temporal variations and temperature sensitivity of ecosystem respiration in three brackish marsh communities in the Min River Estuary, southeast China

Ecosystem respiration (Reco) is an important pathway for gaseous carbon loss in coastal wetland environments. However, little is known regarding the dynamics of Reco and its temperature sensitivity (Q10) under different tidal stages, and vegetation types. In this study, Reco was measured with dark chambers for nearly 2 years in three different brackish marsh stands dominated by Cyperus malaccensis, Phragmites australis, and Spartina alterniflora, respectively, in the Min River estuary of southeast China. The mean Reco across tidal stages and vegetation types exhibited strong temporal variations, and the Q10 value for Reco was higher in the warm months than in the cold months. The mean annual Reco was negatively correlated with electrical conductivity and soil water content, but positively correlated with both tidal water level and soil pH (P < 0.05). On the other hand, the seasonal variation in Reco and Q10 was largely driven by the thermal regimes and soil salinity. Also, we found that the mean Reco and Qs10 value were both relatively higher when the site was exposed before tidal inundation, as compared to that after tidal inundation for all the vegetation types studied (P > 0.05). Overall, the C. malacensis and S. alterniflora stands had the highest and lowest average rates of Reco, respectively, while the mean Qs10 values were lower in the Phragmites australis stand than the other two vegetation types. Our findings point to the need of strengthening the in situ measurements of gas fluxes at multiple time scales in order to develop a better understanding of the response of Reco to changing environmental conditions in the subtropical coastal wetlands. Our results also reiterate the importance of considering the influence of vegetation types in characterizing the magnitude and temperature sensitivity of Reco in estuarine tidal marshes for predicting ecosystem responses to future global change.