Study On Responses Of Carbon Flux In The Salt Marsh Vegetation-soil System To Hydrological And Saline Conditions In The Yangtze Estuary

Coastal salt marshes have high photosynthetic efficiency and low ecosystem carbon emissions;therefore,coastal saltmarsh ecosystems can capture large amounts of carbon dioxide（CO₂）and sequester it as soil organic carbon,often acting as a net carbon sink to suppress the atmospheric CO₂ concentration.However,coastal saltmarsh located at the land-sea interface is highly vulnerable to global changes,particularly to sea level rise（SLR）.The increase in the duration of inundation and frequency of saltwater intrusion accompanying SLR will change the photosynthetic CO₂ fixation,photosynthetic carbon allocation and soil biochemical cycle of coastal salt marsh vegetation,which will eventually lead to the change of carbon source-sink balance.In this thesis,we examined the response of multi-component carbon processes to the independent and synergistic projections of SLR in a reed（Phragmites australis）saltmarsh through a microcosm experiment（ms）under waterlogging and increased salinity conditions.This study also adopted eddy covariance（EC）measurements of the CO₂ fluxes in subtropical coastal marshes along natural inundation and salinity gradients in the Yangtze Estuary.In addition,this study explored the effect of multi-factor interaction between waterlogging and salinity on the allocation of photosynthetic carbon into different carbon pools in salt marsh plant-soil systems using stable ¹³C isotope labeling techniques.The main conclusions are as follows,（1）Photosynthetic processes of reed are more sensitive to waterlogging and salinity than respiratory processes.Waterlogging inhibited non-significantly the maximum net photosynthetic rate(P_max)of reed leaves and the maximum net photosynthetic rate based on the canopy of plants(P_max.C),while salinity significantly inhibited P_max and P_max.C（except for low salinity of 5 ppt）.Likewise,waterlogging non-significantly inhibited reed plant aboveground respiration(R_shoot)but slightly promoted reed leaf respiration(R_leaf).The degree of inhibition of salinity on respiration rate at both leaf and plant levels increased with salinity,and high salinity treatments（15-30ppt）significantly inhibited R_leaf.Photosynthetic rate is more sensitive to waterlogging and salinity stress than reed respiration rate,which may be one of the reasons for the decrease in the primary productivity of reed plants.Moreover,as the study scale increased from the leaf level to the plant level,the degree of response was amplified.For instance,P_max decreased by 11.6%while P_max.C decreased by 20.3%under waterlogging treatment conditions;P_max decreased by 11.4-55.8%while P_max.Cdecreased by 23.4-82.9%under salinity treatment conditions.The decrease in P_max.C of the plant was mainly associated with the decrease in leaf area,while the decrease in R_shoot was associated with the overall limited growth of the plant.When waterlogging is combined with high salinity,there occurred a greater inhibition in photosynthetic and respiration rates of reed plants.（2）Both waterlogging and salinity inhibited soil respiration in reeds,but re-aeration caused a surge in soil respiration and reduced the inhibition by salinity.This study explored the effects of waterlogging and elevated salinity on marsh soil respiration during inundation(non-drainage,R_soil.ND)and re-aeration(drainage,R_soil.D).R_soil.D under waterlogging treatments was significantly reduced by 80%compared to that under non-waterlogging treatments,but R_soil.D under waterlogging treatments significantly increased by 42%compared to that under non-waterlogging treatments.Salinity inhibited R_soil.ND and R_soil.D,and the degree of inhibition increased with increasing salinity,with a decrease between 9.1 and 56%when regardless of waterlogging treatments.Waterlogging strengthened the negative effects of salinity on R_soil.ND but offset the negative effects on R_soil.D likely due to the drastic CO₂ emission after drainage.Changes in soil respiration under hydrological treatment were mainly attributed to changes in root biomass,microbial and enzyme activities,ion concentrations and redox potentials.（3）Waterlogging and elevated salinity inhibited marsh photosynthetic capacity more than CO₂ emission,reducing net CO₂ ecosystem exchange.The net ecosystem CO₂ exchange(NEE_EC based on the EC dataset)in a medium-elevation oligohaline marsh was higher than that in a low-elevation mesohaline marsh,whereas the NEE_ECwas lower than that in a high-elevation freshwater marsh.The declines in NEE_EC at the marsh with higher salinity and lower elevation relative to that with lower salinity and higher elevation could be attributed to a greater decrease in gross primary production relative to ecosystem respiration.Waterlogging slightly increased the NEE_ms（NEE based on the microcosms）because of inhibited R_soil and slight changes in plant photosynthesis and R_shoot.However,the NEE_ms measured during the drainage period decreased significantly due to the stimulated R_soil.The NEE_ms decreased with increasing salinity（except under mild salinity）,and waterlogging exacerbated the adverse impacts of salinity.Under high salinity conditions,the reduction in NEE_ms is mainly because salinity inhibits the photosynthetic capacity of plants（e.g.photosynthetic rate and leaf area index）,although salinity also inhibits CO₂ emission of the whole plant-soil system as well,the inhibitory effect on CO₂ uptake capacity dominates.（4）Waterlogging and elevated salinity reduced photosynthetic carbon accumulation within reed plant and transport to the rhizosphere soil.The hydrological treatments reduced ¹³C transport to the plant organs of reed,while significantly increased ¹³C allocation percentage in roots,thus displaying the adaptation strategy of P.australis.Waterlogging and low salinity had no significant effects on ¹³C allocation to rhizosphere soils,while high salinity（15 and 30 ppt）significantly reduced¹³C 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 waterlogging and elevated salinity on carbon allocation in plant organs and rhizosphere soils can be related to changes in nutrient,ionic concentrations and microbial biomass.In conclusion,waterlogging and elevated salinity under the projection of SLR inhibit photosynthetic capacity with a scale-amplification effect from leaf to shoot level,plant respiration and soil respiration in salt marsh.The inhibition was more pronounced in the combined waterlogging and salinity treatments than in the single-factor treatments.The inhibition in plant photosynthetic capacity by altered hydrological conditions is the main reason for the reduction in net CO₂ uptake capacity of salt marsh.In addition,the altered hydrological conditions（increased waterlogging and salinity）in coastal marsh also reduced the transport of photosynthetic carbon to the soil carbon pool,thus likely weakening the carbon sink.