Effects of waterlogging and elevated salinity on the allocation of photosynthetic carbon in estuarine tidal marsh: a mesocosm experiment

Background and aim: Coastal marshes and wetlands hosting blue carbon ecosystems have shown vulnerability to sea-level rise (SLR) and its consequent effects. In this study, we explored the effects of waterlogging and elevated salinity on the accumulation and allocation of photosynthetic carbon (C) in a widely distributed species in marsh lands. Methods: The plant–soil mesocosms of Phragmites australis were grown under waterlogging and elevated salinity conditions to investigate the responses of photosynthetic C allocation in different C pools (plant organs and soils) based on ¹³CO₂ pulse-labeling technology. Results: Both waterlogging and elevated salinity treatments decreased photosynthetic C fixation. The hydrological treatments also reduced ¹³C transport to the plant organs of P. australis while significantly increased ¹³C allocation percentage in roots. Waterlogging and low salinity had no significant effects on ¹³C allocation to rhizosphere soils, while high salinity (15 and 30 ppt) significantly reduced ¹³C allocation to soils, indicating a decreased root C export in saline environments. Waterlogging enhanced the effects of salinity on the ¹³C allocation pattern, particularly during the late growing season. The responses of flooding and elevated salinity on C allocation in plant organs and rhizosphere soils can be related to changes in nutrient, ionic concentrations and microbial biomass. Conclusion: The adaptation strategy of P. australis led to increased C allocation in belowground organs under changed hydrology. Expected global SLR projection might decrease total C stocks in P. australis and alter the C allocation pattern in marsh plant-soil systems, due to amplified effects of flooding and elevated salinities.