【作者】 李亚雷；

【导师】 葛振鸣；

【作者基本信息】 华东师范大学 ， 生态学， 2022， 博士

【摘要】 滨海盐沼植被具有较高的光合效率和较低的生态系统碳排放量,因此,滨海盐沼生态系统能够捕获大量二氧化碳（CO₂）并将其以土壤有机碳的形式封存在土壤中,常作为抑制大气CO₂浓度升高的“碳汇”。然而,滨海盐沼位于海陆交界地带,极易受到全球变化的影响,尤其是加剧的海平面上升。伴随海平面上升而来的盐水入侵以及淹水频次和持续时间的增加将会改变盐沼植被光合碳同化、碳分配以及土壤生物地化循环过程,最终改变盐沼的碳“源”/“汇”功能。本论文以长江口崇明盐沼为研究对象,通过微宇宙实验模拟芦苇（Phragmites australis）盐沼在未来海平面上升情景下,水盐多因子的交互作用对滨海盐沼多组分碳过程的影响,并结合野外涡度协方差通量观测方法,探究海平面上升情景下滨海盐沼净生态系统碳交换量（NEE）的变化以及引起该变化的原因。此外,本研究还采用稳定性碳13同位素(¹³C)标记技术探究了水盐多因子交互作用对盐沼植物光合碳在不同碳库分配的影响。本论文主要结论:（1）相较于植株地上呼吸,芦苇植株光合过程对淹水和盐度处理的响应更敏感。淹水对芦苇叶片最大净光合速率(P_max)和植株最大净光合速率(P_max.C)均有抑制作用但不显著,而盐度会显著抑制P_max和P_max.C（除5 ppt低盐度外）。同样,淹水对芦苇地上植株呼吸(R_shoot)也有轻微抑制作用,但略微促进了芦苇叶片呼吸(R_leaf);盐度对叶片和植株水平呼吸速率的抑制程度均随盐度增加而增强,而且高盐处理（15-30 ppt）会显著抑制R_leaf。与芦苇呼吸速率相比,光合速率对水盐胁迫的响应更敏感,这或许是芦苇植株净初级生产力下降的原因之一。而且随着研究尺度从叶片水平上升到植株水平,芦苇光合速率的响应程度被放大,比如淹水处理条件下,P_max下降11.6%而P_max.C下降20.3%;在盐度处理条件下,P_max下降11.4-55.8%而P_max。C下降23.4-82.9%。植株光合速率的下降主要与叶面积的降低有关,而植株呼吸速率的下降与植株整体生长受限有关。淹水与高盐度复合处理会对芦苇植株光合速率和呼吸速率产生更大的抑制。（2）淹水增强和盐度上升均会抑制芦苇盐沼土壤呼吸,但曝气会削弱盐度对土壤呼吸的抑制作用。与滨海盐沼生态系统的潮汐波动相对应,微宇宙实验探究了不同淹没（非排水,模拟涨潮后）和再曝气（排水,模拟退潮后）期间,淹水和盐度增加的单因素和水文交互因素对芦苇盐沼土壤CO₂排放(R_soil)的影响。在淹水条件下,R_soil较不淹水对照组显著降低了80%,但在再曝气状态下,淹水组和对照组的R_soil均出现激增,而且淹水组R_soil比对照组显著高42%;而盐度处理亦抑制R_soil,且随盐度升高抑制程度增强,降幅在9-56%之间。在非排水状态下,淹水条件会加剧盐度对R_soil的抑制作用;而在再曝气状态下,淹水处理会部分地抵消盐度对R_soil的抑制。水文处理条件下,土壤呼吸的变化主要与根系生物量、微生物和酶活性以及离子浓度和氧化还原电位的变化有关。（3）水盐因子对盐沼植物-土壤系统植被光合固碳能力的抑制程度高于碳排放能力,导致净生态系统碳交换量降低。基于野外通量塔观测,中滩寡盐盐沼的净生态系统碳交换量(Net Ecosystem Exchange based on Eddy Covariance,NEE_EC)高于低滩中盐盐沼,但低于高滩淡水盐沼湿地。不同盐沼间,稍低盐度盐沼湿地的NEE_EC相对稍高盐度盐沼湿地的下降可归因于总初级生产力的降幅相较于生态系统呼吸更大。芦苇植物-土壤微宇宙系统在淹水处理条件下,由于R_soil受到强烈抑制,而P_max.C和R_shoot的变化相对较小,因此净生态系统碳交换量(Net Ecosystem Exchange based on Microcosm System,NEE_ms)略有增加。然而,排水后（重新曝气）,在淹水处理组观测到R_soil的激增,导致NEE_ms显著下降。盐度处理（低盐度除外）强烈抑制NEE_ms,且淹水处理会加剧盐度处理对NEE_ms的抑制程度。在高盐度条件,净生态系统碳交换量的降低主要是由于盐度抑制了植物的光合固碳能力（如光合速率和叶面积指数）;尽管盐度也同样会抑制整个植物-土壤系统的CO₂呼吸,但对光合固碳能力的抑制程度更高。（4）在海平面上升情景下,淹水加剧和盐度升高会降低植物光合碳的积累及其向根际土壤的传输。淹水增强和盐度升高均降低了芦苇各器官中¹³C的量,并显著增加了地下根系组织中¹³C的分配百分比,体现了芦苇在环境胁迫下的适应策略。改变的淹水条件对根际土壤中剩余的¹³C量没有显著影响,而高盐处理则显著降低了根际土壤中¹³C剩余量,可能是由于盐渍环境中的芦苇减少了根部向土壤的¹³C输入。淹水增强了盐度对¹³C分配模式的影响,尤其是在生长后期芦苇将更高比例的¹³C分配到根部。在植物器官和根际土壤碳库间的光合碳分配对水盐因子的响应可能与养分、离子浓度和微生物生物量的变化有关。综上所述,未来海平面上升背景下,淹水和盐度条件的改变会抑制盐沼植物的光合固碳能力（且存在尺度放大效应）、盐沼地上植物呼吸和土壤呼吸。淹水与盐度复合处理比单一因素处理的抑制作用更明显。水盐条件的改变对植物光合固碳能力的抑制是导致盐沼净CO₂吸收能力降低的主要原因。此外,水盐处理还会减少光合碳向土壤碳库的传输,从而削弱滨海盐沼的“碳汇”能力。更多还原

【Abstract】 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.更多还原