The variability in CO2 fluxes at different time scales in natural and reclaimed wetlands in the Yangtze River estuary and their key influencing factors

Highlights: • Reclamation significantly weakened the CO₂ uptake capacity of the wetland. • Plant CO₂ fixation made up 70.7% of the variability in CO₂ fluxes in two wetlands. • N deficiency was the main reason for poor plant CO₂ fixation in the reclaimed wetland. • N input promoted soil respiration, but was masked by high moisture and salinity. With the increasing pace of global warming, studies of the carbon cycle and carbon sink capacity of estuarine wetlands have received increasing attention. Estuarine wetlands are often located in economically developed and densely populated areas, and their reclamation has become an important way to acquire land resources. To explore the effect of reclamation on the carbon sink function of estuarine wetlands, the Chongming Dongtan reclaimed wetland and Jiuduansha natural wetland, which are located in the Yangtze River estuary, were selected to investigate their variabilities in carbon fluxes and the main influencing factors using the open path eddy covariance flux monitoring system. The CO₂ uptake capacity of the Dongtan reclaimed wetland was significantly weaker than that of the Jiuduansha natural wetland (P < 0.05). The difference in carbon fluxes between the two wetlands was mainly influenced by plant growth and carbon fixation, which accounted for 70.7% of the variation, with soil respiration being the second most important factor. The soil water content and nitrogen and phosphorus concentrations in the reclaimed wetland significantly decreased due to the barrier to water flow presented by a dam (P < 0.01). Nitrogen limitation was the main reason for the poor plant growth in the reclaimed wetland. Although nitrogen inputs in the natural wetland promoted soil respiration (40.6%), they were overshadowed by the inhibitory effect of soil moisture and salinity (55.8%). It was therefore possible to improve the carbon sink function of reclaimed wetlands if plant growth and carbon fixation efficiency could be enhanced without promoting soil respiration.