Abstract: Aims Rising sea levels and the associated increase in inundation heights will alter the carbon (C) cycle in tidal marsh wetlands. However, current research primarily focuses on the impact of increased inundation on total soil C stocks, while the effects on the balance of C budget processes remain unclear. Therefore, understanding how sea level rise affects the C sequestration capacity of tidal marshes is essential for predicting future impacts. Methods To address this, our study established a ‘marsh organ’ experimental platform in the tidal marshes of the Minjiang River Estuary. Three inundation treatments—CK (control), CK + 20 cm, and CK + 40 cm—simulated the current and the projected sea level rise scenarios for the next 50 and 100 years. We measured the effects of increased inundation on the net ecosystem carbon dioxide exchange (NEE), gross primary productivity (GPP), ecosystem respiration (ER), plant biomass, plant photosynthetic characteristics, and soil physicochemical properties of the Cyperus malaccensis tidal marshes. Important findings The results showed that increased inundation led to a decrease in aboveground biomass and an increase in belowground biomass. Compared to the control (CK), GPP decreased by 27% and 32%, while ER increased by 20% and 58% in the CK + 20 cm and CK + 40 cm treatments, respectively. The reduction in GPP was related to decreased aboveground biomass and declining plant photosynthetic characteristics, such as net photosynthetic rates, stomatal conductance, intercellular CO2 concentrations. The increase in ER was associated with higher soil oxidation-reduction potential (ORP) and dissolved organic carbon (DOC) content. Under the CK, CK + 20 cm, and CK + 40 cm treatments, NEE was ?539.8, ?102.7, and 185.6 g C·m?2·a?1, respectively. These findings indicate that a 20 cm increase in inundation height leads to an increase in NEE, demonstrating a weakened carbon sequestration capacity of the Cyperus malaccensis tidal marshes. Furthermore, a 40 cm increase in inundation height results in NEE shifting from negative to positive, indicating a transition of the ecosystem from a carbon sink to a carbon source. This research provides a scientific basis for predicting and mitigating the impacts of future sea level rise on the C cycle of tidal marsh.

Key words: enhanced inundation, net ecosystem CO2 exchange, gross primary productivity, ecosystem respiration, Cyperus malaccensis, tidal marsh