Frequent water table depth fluctuations due to warm-dry conditions can significantly trigger carbon emissions from coastal salt marshes

Coastal salt marshes are crucial for blue carbon storage, but their carbon emission patterns are uncertain due to frequent fluctuations in water table depth (WTD). This uncertainty is caused by the simultaneous impacts of global warming and rising sea levels. This study assessed carbon emission patterns in the Yellow River Delta under different hydrological fluctuation modes (high fluctuation of WTD scenario (HF-WTD) and low fluctuation of WTD scenario (LF-WTD)) and hydrothermal conditions through a 3-year in-situ observational experiment. The region is typically impacted by climatic warming and drying. Results showed that: (i) Intra-annual WTD fluctuations promote CO₂ emission flux, but they are not significantly related to the rising or falling state. On the other hand, methane (CH₄) emission flux are dominated by WTD that are more favourable when WTD continues to rise. (ii) The transition point for carbon emission flux with respect to WTD is between −51 cm and −54 cm, and is not affected by the fluctuation pattern of the WTD. (iii) CO₂ is more sensitive to temperature when the WTD fluctuates violently and is high, while CH₄ is more sensitive to temperature when the WTD is stable and low and the temperature is <17.71 °C. (iv) In the scenario of high-frequency WTD, the variation in WTD, combined with the sharp temperature change, contributes to the peak of CO₂ emission flux. In contrast, in the low-frequency WTD scenario, WTD fluctuations, coupled with extreme temperature fluctuations, contribute to the peak of CH₄ emission flux. The conclusions suggest that the high and frequently fluctuating WTD caused by climate warming and drying, as well as rising sea levels, will significantly increase CO₂ emission flux from coastal salt marsh wetlands. This increase in emission flux will be detrimental to the blue carbon function of coastal wetlands.