Wetlands are vital carbon sinks and play a key role in climate regulation. However, climate change and human disturbances are altering wetland hydrology with water level fluctuations profoundly influencing carbon sequestration. While previous syntheses and modeling studies have suggested the existence of water level thresholds in wetland carbon dynamics, the mechanisms underlying these thresholds remain poorly understood, and field-based evidence is lacking. Here, we present results from a long-term field experiment across six water level treatments, ranging from 100 to 40 cm above the soil surface. We identify a carbon sink threshold occurring between -20 and 0 cm, as water levels rise to the soil surface. This threshold emerges from water-level-induced soil-vegetation feedbacks. We further show that the dominant control of CO2 exchange shifts from soil properties (moisture and salinity) to vegetation traits (height and coverage) once the threshold is crossed. These findings provide in situ empirical evidence for a wetland carbon threshold and highlight the critical role of ecohydrological feedbacks. Our study offers new insights into wetland carbon dynamics under changing hydrological regimes and informs management strategies for enhancing wetland carbon storage.
Keywords: carbon sink; ecological threshold; soil property; vegetation trait; water level; wetland.