Salt marshes are significant contributors to global "blue carbon" resources, and these habitats are sensitive to precipitation events due to periodically dry-wet alternation induced by tides.

However, whether annual marsh-atmospheric CO₂ flux responds more to annual or seasonal precipitation remains unclear.  

Based on the eddy covariance technique, a research team led by HAN Guangxuan from the Yantai Institute of Coastal Zone Research (YIC) of the Chinese Academy of Sciences has conducted automatic monitoring of CO₂ flux data on a long-time scale.

They made a series of discoveries in the carbon cycle and the precipitation driving mechanism of inter-annual variation of carbon budget in coastal wetlands. 

Based on the largest continuous carbon flux data of coastal wetlands in China, the team evaluated eddy covariance data from 2012 to 2019 to determine typical CO₂ budgets in a salt marsh of the Yellow River Delta, China.  

The result showed that the salt marsh was a sink for atmospheric CO₂ in each of the eight study years, with an 8-year average net ecosystem CO₂ exchange (NEE) of -51.7 ± 9.7 g C m^-2 a^-1 varying from -8 to -85 g C m^-2 a^-1.

The team assessed the effect of annual and seasonal precipitation on interannual net ecosystem CO₂ exchange (NEE) in the salt marsh. Using the eddy covariance technique, they found that annual NEE was mainly regulated by precipitation levels during the early growth stage of plants, which modulated maximal plant biomass accumulation via water-salt transport. 

These findings suggest that soil water-salt conditions induced by precipitation during the onset of the growing season are crucial for interannual variations of NEE in salt marshes. 

Fig. 1 Effects of precipitation distribution changes during the early growth season on ecosystem CO₂ exchange in a salt marsh (Image by YIC)

HAN's team also revealed the dual effect of precipitation redistribution on net ecosystem CO₂ exchange of a coastal wetland in the Yellow River Delta.  

The results showed that the higher precipitation promoted net ecosystem CO₂ absorption due to the increased soil water content (SWC) and reduced salt stress during the fast growth stage. While the higher precipitation suppressed net ecosystem CO₂ uptake due to the increased waterlogged stress during the middle growth stage.  

Besides, a five-year field precipitation manipulation experiment showed that the observed positive asymmetric responses of soil respiration along the precipitation gradient were associated with the decreased response of belowground biomass and leaf area index to high soil electric conductivity under decreased precipitation.

Fig. 2 Diagram the asymmetric response mechanism of soil respiration to precipitation changes in a coastal wetland (Image by YIC)