Dual effect of precipitation redistribution on net ecosystem CO₂ exchange of a coastal wetland in the Yellow River Delta

Hydrological regime is crucial in determining the carbon dioxide (CO₂) exchange between the atmosphere and wetlands. Seasonal redistribution of precipitation is one featured hydrological regime shift, but its impacts on ecosystem CO₂ exchange in coastal wetlands remain unclear. Here, based on the eddy-covariance technique, we examined how the net ecosystem CO₂ exchange (NEE) in a coastal wetland of Yellow River Delta in China differed between two years (2012 and 2013) with contrasting seasonal distribution of precipitation. The ecosystem absorbed more CO₂ during the growing stage in 2013 (−268.5 g C m⁻²) than 2012 (−174.7 g C m⁻²). This difference resulted from higher NEE in the fast and middle growth stages with different reasons. In the fast growth stage, the higher mean daily NEE occurred due to more precipitation coupled with lower salt stress in 2013 (−6.3 g CO₂ m⁻² day⁻¹) compared to that in 2012 (−2.2 g CO₂ m⁻² day⁻¹). During the middle growth stage, the mean daily NEE in 2013 (−4.2 g CO₂ m⁻² day⁻¹) was significantly higher than that in 2012 (−1.1 g CO₂ m⁻² day⁻¹) because the ecosystem in 2012 suffered more waterlogged stress. This dual effect of precipitation distribution on vegetation photosynthesis was also observed in a field manipulation experiment at the same site. Our results indicated that the redistribution of precipitation among seasons would play a critical role in regulating ecosystem CO₂ exchange in the coastal wetland. More research on the associated changes between dynamics of soil hydrology and salinity could promote the accuracy of the carbon-budget estimates in coastal wetlands.