Effects of salinity and inundation on carbon storage of halophytes in the tidal salt marsh of the Yangtze River Estuary, China

Halophytes, the key component of estuarine ecosystems, have a tremendous capacity to capture carbon dioxide (CO₂) from the atmosphere through photosynthesis, and then store the organic compounds in plant tissues, formi ng a temporary pool of fixed carbon. Soil carbon mostly originates from decayed aboveground and belowground plant tissues, constituting a long-term carbon pool, which holds considerable potential for climate change mitigation and adaptation. Estuarine salt marshes have been identified as important natural carbon sinks, which are highly susceptible to human- and climate-driven threats. Understanding how halophytes respond to environmental stresses becomes increasingly important under the anticipated sea-level rise and aggravated saltwater intrusion. In this study, we specifically focused on carbon storage of halophytes and highlighted the importance of salinity and inundation regimes as crucial abiotic drivers influencing the ability of halophytes to alter carbon input into the soil. Controlled outdoor pot experiments were conducted to quantify the independent impacts of flooding salinity (0, 5, 10, 15, 25, and 35), flooding depth (0, 10, 20, 40, 60, and 80 cm), and flooding frequency (every day, every 3 days, every 7 days, every 10 days, and every 15 days) on three dominant halophytes in the Yangtze River Estuary, China: the native species Phragmites australis and Scirpus mariqueter and the invasive species Spartina alterniflora. There were significant decreases in aboveground and total carbon storage of P. australis, S. alterniflora, and S. mariqueter with increasing flooding salinity from freshwater (0) to seawater (35). The contribution of soil salinity to variations in aboveground and total carbon storage of P. australis, S. alterniflora, and S. mariqueter was about 47.2%, 66.5%, and 72.7%, and 34.7%, 45.0%, and 62.0%, respectively. Elevated flooding depth exerted significant effects on aboveground and total carbon storage of P. australis, and total carbon storage of S. mari queter. Approximately 68.6%, 28.5%, and 71.1% of their variations were caused by gradient changes in flooding depths (10-80cm). In contrast, S. alterniflora still had high carbon storage at a flooding depth of 80cm, with less severe impact than the two native species. No significant differences were observed in the flooding frequency treatments and belowground car bon storage of each species among all treatments. Elevated flooding salinity and flooding depth levels caused by rising sea-le vels and saltwater intrusion might lead to significant decreases in carbon storage of these three halophytes, which could direct ly affect soil carbon pools through limited input of plant carbon into soil. For S. mariqueter, these stressful environmental conditions would potentially further weaken its low carbon storage capacity, thus making a "negligible" contribution to carbon sinks of estuarine salt marshes. Although carbon storage was higher in P. australis and S. alterniflora than in S. mariqueter, the plant negative responses to elevated salinity and inundation regimes should not be ignored.