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, the team of HAN Guangxuan, Yantai Institute of Coastal Zone Research of the Chinese Academy of Sciences, has conducted automatic monitoring of CO₂ flux data on a long time scale, and made a series of discoveries in the carbon cycle and the precipitation driving mechanism of inter-annual variation of carbon budget in coastal wetlands.

Owning the longest continuous carbon flux data (2010- ) of coastal wetlands in China, the team evaluated eight years (2012-2019) of eddy covariance data 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 NEE of -51.7 ± 9.7 g C m^-2 a^-1 varying from -8 to -85 g C m^-2 a^-1.

Fig.1 Seasonal and inter-annual variations of ecosystem CO₂ excahnge for the period of 2012–2019. (Image by YIC)

The team assessed the effect of annual and seasonal precipitation on interannual net ecosystem CO₂ exchange (NEE) in a salt marsh of the Yellow River Delta, China. 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. 2 Effects of precipitation distribution changes during the early growth seson on ecosystem CO₂ exchange in a salt marsh (YIC)

Using the eddy covariance technique, 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.

Their results showed that the higher precipitation promoted net ecosystem CO₂ absorption due to the increased 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 5-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.3 Diagram the asymmetric response mechanism of soil respiration to precipitation changes in a coastal wetland (YIC)

Paper ：

[1] Chu XJ, Han GX^*, Wei SY, Xing QH, He WJ, Sun BY, Li XG, Hui DF, Wu HT, Wang XJ, Li PG, Song WM. 2021. Seasonal not annual precipitation drives 8-year variability of interannual net CO₂ exchange in a salt marsh. Agricultural and Forest Meteorology, 108557, 308-309.

https://doi.org/10.1016/j.agrformet.2021.108557

[2] Li XG, Han GX*, Eller F, Hui DF, Zhu LQ, Chen L, Chu XJ, Song WM, Xu JW. 2021. Acclimation of coastal wetland vegetation to salinization results in the asymmetric response of soil respiration along an experimental precipitation gradient. Agricultural and Forest Meteorology, 108626, 310. https://doi.org/10.1016/j.agrformet.2021.108626

[3] Chu XJ, Han GX, Xing QH, Xia JY, Sun BY, Li XG, Yu JB, Li DJ, Song WM. 2019. Changes in plant biomass induced by soil moisture variability drive interannual variation in the net ecosystem CO₂ exchange over a reclaimed coastal wetland. Agricultural and Forest Meteorology, 264, 138-148.

[4] Chu XJ, Han GX, Xing QH, Xia JY, Sun BY, Yu JB, Li DJ. 2018. Dual effect of precipitation redistribution on net ecosystem CO2 exchange of a coastal wetland in the Yellow River Delta. Agricultural and Forest Meteorology, 249, 286-296.