The high organic carbon accumulation in estuarine wetlands necessarily does not represent a high CO sequestration capacity.

Estuarine wetlands with high organic carbon (OC) accumulation rates due to their high plant biomass and interception of tide-derived OC are generally considered as large CO sinks. However, our previous study found that tidal OC input seems to stimulate soil CO emissions, potentially weakening CO sequestration in estuarine wetlands. To further verify this phenomenon, we first established a structural equation model, which confirmed a positive correlation between tidal OC input and soil organic carbon (SOC) and soil respiration. We then performed trace analysis to determine the stability of SOC derived from different sources and its effect on soil CO emissions by analyzing the input and retention of OC derived from tides and plants in the Yangtze Estuary wetlands. From upstream to downstream, as tidal OC input decreased, the relative retention ratio of the tidal OC in wetland soil increased from 1.259 to 2.148, whereas the relative retention ratio of plant OC in the soil decreased from 61.5% to 14.8%. Our findings indicated that the degradability of tidal OC was higher upstream than that downstream, but both inhibited plant OC degradation, thus providing an important reason for the higher CO emissions upstream of wetlands (with higher tidal OC input). In addition, the primarily contributor to CO (δ) emissions' transforming from plant SOC (81.35%) to tidal SOC (91.18%) was an increase in organic matter input from the tide in a microcosm system. Consequently, a higher CO output than CO input (plant OC) due to the ready degradation of tidal OC consequently weakens the CO sequestration capacity of the estuarine wetlands. This phenomenon is cause for concern regarding the CO sink function of estuarine wetlands intercepting large amounts of organic matter.