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,forming 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 80cm),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.mariqueter.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 carbon storage of each species among all treatments.Elevated flooding salinity and flooding depth levels caused by rising sea-levels and saltwater intrusion might lead to significant decreases in carbon storage of these three halophytes,which could directly 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.%盐生植物是盐沼有机碳储存的“临时库”,也是土壤有机碳累积的主要来源,其碳储量大小对盐沼生态系统“碳汇”功能的发挥十分重要.以长江口潮滩本地种芦苇(Phragmites australis)和海三棱藤草(Scirpus mariqueter),及入侵种互花米草(Spartina alterniflora)为研究对象,采用单因素盆栽实验,模拟分析淹水盐度(0、5、10、15、25和35)、淹水深度(0、10、20、40、60cm和80cm)和淹水频率(每天、每3天、每7天、每10天和每15天)变化对各盐生植物地上、地下和总体碳储量大小的影响.研究结果表明,随着淹水盐度增加,芦苇、互花米草和海三棱藤草地上部分与总体碳储量均显著降低.土壤盐度可分别解释其地上部分碳储量变异的47.2％、66.5％和72.7％,与总体碳储量变异的34.7％、45.0％和62.0％.随着淹水深度增加,芦苇地上部分、总体碳储量和海三棱蔗草地上部分碳储量均显著降低,其变异的68.6％、28.5％和71.1％可由淹水深度变化(10-80cm)解释.互花米草在80cm淹水深度下仍有较高的地上部分碳储量和总体碳储量.3种盐生植物碳储量对淹水频率变化的响应差异均不显著,所有处理地下部分碳储量差异也未达到显著水平.总体而言,互花米草对水盐胁迫的耐受性要强于本地种芦苇和海三棱蔗草.尽管互花米草和芦苇具有相对较高的碳储量,但水盐胁迫对其碳储量的显著抑制作用不容忽视.海三棱蔗草碳储量本就不高,输入土壤的有机碳量较为有限,海平面上升及盐水入侵等逆境胁迫会使其对盐沼“碳汇”贡献更加微弱.
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