| Estuarine and coastal salt marshes have inspired widespread concern over their long-term sequestration of organic carbon,which hold considerable potential for climate change mitigation and adaptation.As the key component of estuarine ecosystems,halophytes 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 originates not only from decayed aboveground and belowground plant tissues,but also from suspended sediments during tidal inundation,constituting a long-term carbon pool.Due to the unique position at the interface between land and ocean,estuarine salt marshes are highly susceptible to pronounced,interactive,unpredictable,and irreversible damages from global climate change as well as anthropogenic activities.When the plant-soil system in estuarine salt marshes is perturbed by a series of abiotic stressors,large effects on ecosystem stability and carbon storage may result.Understanding how halophytes and marsh soils respond to elevated salinity and flooding stresses becomes increasingly important under the anticipated sea-level rise,aggravated saltwater intrusion and storm surges scenarios.Estuarine salt marshes are usually characterized by distinct plant zonation patterns and environmental gradients along elevation,which may provide ideal conditions for studying the effects of salinity and flooding gradients on organic carbon accumulation.However,it is difficult to evaluate the contributions of these environmental variables to the variations of carbon pools,because the carbon biogeochemistry,magnitude of carbon burial,and spatial distribution of soil organic carbon are regulated by the interaction between biotic and abiotic factors under field conditions.It is also virtually impossible to predicte the impacts of accelerated sea-level rise and saltwater intrusion on carbon storage capacity of salt marshes via field observations,with limited variations in salinity and flooding,which may hinder us to facilitate pre-emptive and effective management actions on cabon sinks to meet the challenges of global climate changes.In this study,we conducted controlled pot experiments to subdivide the salinity and flooding stresses into three aspects:flooding salinity,flooding depth,and flooding frequency,to quantify their independent impacts on the two native dominant species,Phragmites australis and Scirpus mariqueter,and on the invasive species Spartina alterniflora,as well as on their corresponding marsh soils in the Yangtze River Estuary,China.Our specific objectives were:1)to compare the tolerance of native and invasive species to elevated salinity and flooding;2)to investigate the responses of biomass accumulation and allocation to elevated salinity and flooding;and 3)to evaluate the variations in plant and soil carbon storage across salinity and flooding gradients.The present study provided insight into the potential consequences of sea-level rises and saltwater intrusion via controlled pot experiments in estuarine salt marshes as a beneficial and necessary tool for field observations,which could contribute to strengthen the case for protecting the value of'blue carbon' sinks.The main results were listed as follows:1.Invasive S.alterniflora was more tolerant to elevated salinity and flooding than the two native species,P.australis and S.mariqueter.For plant morphology and reproduction,height of S.alterniflora significantly decreased in response to high levels of flooding salinity,while the stem diameter,survival rates,number of new ramets,and seed setting rates of S.alterniflora showed no significant difference among the flooding salinity levels.Elevated flooding depths were conducive to enhance the survival rates and number of new ramets for S.alterniflora.As flooding frequency increased,significant increase was detected in the number of new ramets for S.alterniflora,whereas no significant difference was observed for its survival rate and seed setting rate.In contrast,there were significant decreases in the height,survival rates,number of new ramets,and seed setting rates for P.australis subjected to increasing flooding salinity and flooding frequency levels.Elevated flooding depths also significantly decreased the seed setting rates of P.australis.It could lead to the death of S.mariqueter in our controlled experiments when the flooding salt concentration increased to 25 and 35 ppt for 2 months,while the soil salinity was approximately 5.7 to 6.9 g·kg-1.No significant differences in plant morphology and reproduction for S.mariqueter were observed in the flooding depth and flooding frequency treatments.For plant physiology,we found that the increased flooding salinity,flooding depth and flooding frequency levels could exhibit varying degrees of inhibitory effects on the chlorophyll a,chlorophyll b,carotenoid,and malonaldehyde contents,as well as on the chlorophyll alb ratios and carotenoid/chlorophyll ratios for P.australis and S.mariqueter,while only the carotenoid contents were significantly affected by elevated flooding salinity and flooding frequency levels for S.alterniflora.For biomass,there were greater decreases in the aboveground biomass and total biomass for P.australis(69.5%,41.9%)and S.mariqueter(76.6%,47.9%)than for S.alterniflora(48.3%,40.3%)at high salt concentration(35 ppt)when compared to the responses in freshwater(0 ppt).Elevated flooding depths significantly decreased their live aboveground biomass of P.australis and S.mariqueter,while S.alterniflora still had high live aboveground biomass and total biomass even at the highest flooding depth(80 cm).For biomass allocation,the belowground to aboveground biomass ratios for P.australis and S.mariqueter were significantly increased by the increase of flooding salinity,while no significant difference was observed for S.alterniflora among the flooding salinity levels.Accordingly,S.alterniflora was more tolerant to experimental conditions than the two native species,which indicated the potential expansion of this non-native species under future scenarios of accelerated sea-levels and saltwater intrusion.2.Elevated salinity and flooding could decrease the aboveground biomass and total biomass for both invasive and native species.There were significant decreases in the stem biomass,leaf biomass,fruit or panicle biomass,aboveground biomass,and total biomass for P.australis,S.alterniflora,and S.mariqueter subjected to increasing flooding salinity.The contribution of soil salinity to variations in the aboveground biomass and total biomass for all three species was about 46.7%,65.0%,and 72.4%,and 26.1%,38.2%,and 44.0%,respectively.Elevated flooding depths significantly decreased the aboveground biomass of P.australis and S.mariqueter,while their total biomass were not significantly affected.About 68.5%and 69.8%of the variations in the aboveground biomass for P.australis and S.mariqueter were attributed to the increase of flooding depths(10-80 cm).No significant differences in the biomass of P.australis,S.alterniflora,and S.mariqueter were observed in the flooding frequency treatments.Despite no significant difference being detected in the belowground biomass among these treatments for all species,reductions in their aboveground biomass would potentially decrease the input of organic matter into the soils.Moreover,the relatively low aboveground biomass might weaken the ability of stems and leaves to slow water velocities,reduce erosion and enhance mineral sediment deposition,thus influencing the organic matter accumulation from both autochthonous and allochthonous resources,as well as altering the ability of estuarine salt marshes to maintain their position in the intertidal zone.However,the relatively high belowground to aboveground biomass ratios indicated phenotypic plasticity in response to stressful environmental conditions,which suggest that marsh species can adapt to the accelerated sea-level rise and maintain marsh elevation.3.Elevated salinity and flooding could decrease the carbon storage of halophytes and weaken their capacity of carbon fixation.There were significant decreases in the aboveground and total carbon storage of P.australis,S.alterniflora,and S.mariqueter,as well as in their capacity of carbon fixation with increasing flooding salinity from freshwater(0 ppt)to seawater(35 ppt).The contribution of soil salinity to variations in the 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.Similarly,soil salinity could explain 46.7%,65.0%,and 72.4%of the variations in their capacity of carbon fixation,respectively.Elevated flooding depths exerted significant effects on the aboveground and total carbon storage of P.australis,and total carbon storage of S.mariqueter.About 68.6%,28.5%,and 71.1%of their variations were caused by gradient changes in flooding depths(10-80 cm).In contrast,S.alterniflora still had high carbon storage and carbon fixation capacity even at the highest flooding depth(80 cm),with less severe impact than the two native species.No significant differences were observed in the flooding frequency treatments.No significant variations in the belowground carbon storage were detected for each species among all treatments either.Total carbon storage of three halophytes presented:S.alterniflora(2596.9 ± 589.7 g·m-2)>P australis(1532.6±317.6 g·m 2)>S.mariqueter(635.6 ± 168.3 g·m 2),while in their capacity of carbon fixation,the order demonstrated:S.alterniflora(2655.8 ± 661.0 g·m-2·yr-1)>P.australis(1142.9 ± 301.1 g·m-2·yr-1)>S.mariqueter(830.6 ± 286.8 g·m-2·yr-1)as well.Elevated flooding salinity and flooding depth levels caused by rising sea-levels and saltwater intrusion might lead to significant decreases in the carbon storage of three halophytes,which could directly affect soil carbon pools through the limited input of plant carbon into the soils.For S.mariqueter,these stressful environmental conditions would potentially weaken its low carbon storage,thus making a"negligible" contribution to carbon sinks of estuarine salt marshes.Although the carbon storage of P.australis and S.alterniflora was higher than that of S.mariqueter,their negative responses to elevated salinity and inundation regimes should not be ignored.4.Elevated salinity could decrease soil organic carbon contents and the carbon storage of plant-soil system.There were significant decreases in soil organic carbon contents and the soil carbon storage for P.australis and S.alterniflora at the end of controlled experiments.Elevated soil salinities caused by the increased flooding salinity levels could explain 29.2%and 40.3%of the variations in their soil organic carbon contents,as well as 26.8%and 31.8%of the variations in their soil carbon storage,respectively.Significant positive relationships between the aboveground biomass,total biomass and the corresponding soil organic carbon contents were also detected for P.australis(r = 0.501,P = 0.034;r = 0.550,P = 0.018)and S.alterniflora(r = 0.744,P<0.001;r =0.699,P = 0.001)at the end of controlled experiments,which suggested that the marsh soil organic carbon dynamics were determined by inputs of net primary productivity.In the flooding depth treatments,soil organic carbon contents for S.alterniflora were also significantly correlated to its aboveground biomass(r-0.531,P = 0.023)and total biomass(r = 0.534,P = 0.023).The carbon storage of plant-soil system for all three species significantly decreased in response to high levels of flooding salinity.As flooding depths increased,significant decrease was detected in the carbon storage of plant-soil system for P.australis,whereas no significant difference was observed for S.mariqueter.There was no obvious tendency in the carbon storage of plant-soil system for S.alterniflora among flooding depth treatments.No significant differences were detected in the carbon storage of plant-soil system for all three species in the flooding frequency treatments.Since soil organic carbon contents for P.australis,S.alterniflora and S.mariqueter during 3-4 months of controlled treatments were not overly influenced by the elevated salinity and flooding stresses,we suggested that the accelerated sea-level rises and aggravated saltwater intrusion associated with global climate change could not strongly decrease soil organic carbon contents over relatively short-term scales.However,the excessive accumulation of soil salt and high levels of flooding depth would significantly decrease the plant carbon storage of all three species,as well as affect the soil organic carbon contents through rhizodeposition to decrease the input of organic matter released by living roots into the soils.Therefore,we highlighted the importance of salinity and flooding as crucial abiotic drivers influencing the ability of plant-soil system to sequester carbon in estuarine salt marshes,and their long-term impacts deserve further attention. |
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