Source Discrimination And Degradation Characteristics Of Sediment Organic Carbon In Coastal Wetlands Of The Yangtze River Delta

The"blue carbon"effect of coastal zones is of great significance in global carbon storage and response to global change,and plays an important role in achieving national carbon neutrality goals.In coastal wetland ecosystems,the potential of organic carbon storage from different sources varies greatly,so identifying and quantifying the"blue carbon"source is crucial to estimating the global carbon budget.At present,the contribution ratios of terrestrial,local plant,and marine organic carbon in coastal wetlands are unclear.Little is known about the response of coastal wetland carbon pools to different sources of organic carbon degradation under future global change scenarios,which will become a core difficulty in assessing and predicting wetland organic carbon conversion and storage changes in global change.Due to factors such as wetland siltation,vegetation succession,and high deposition and burial rates,coastal wetlands in the Yangtze River Delta have a high carbon burial and storage capacity.In addition,due to the interaction between land and sea,the Yangtze River estuary has significant spatial differences in ecological and environmental conditions,and the intertidal zone is significantly affected by seawater intrusion.It is an ideal area for studying the impact of global change on the organic carbon of coastal wetlands.Based on this,this thesis conducts in-depth research on the source identification and degradation characteristics of organic carbon in the sediments of coastal wetlands in the Yangtze River Delta,relying on the general project of the National Natural Science Foundation of China,"Carbon burial flux and carbon sink potential in the intertidal zone of the Yangtze River estuary:source identification and transformation mechanism".Based on field sampling and indoor analysis,using typical biomarkers and individual carbon isotope tracer techniques,explore the spatiotemporal and vertical distribution characteristics of individual biomarker content in coastal wetland sediments and their influencing factors,solve the problem of accurate identification and quantitative evaluation of terrestrial,local plant,and marine organic carbon,and combine online carbon dioxide（CO₂）and methane（CH₄）concentrations and their ¹³C isotope analyzer,conduct long-term series indoor incubation experiments to reveal the differences in the resistance to degradation of organic carbon from different sources of sediment,and evaluate the response of coastal wetland carbon sequestration capacity to key scenarios of global change,with a view to providing scientific basis for more accurate assessment and prediction of coastal wetland carbon sequestration potential.The specific research content and main conclusions are as follows:（1）Through field collection and analysis of n-alkane and lignin content in sediments from different seasons,profile depths,and vegetation zones,as well as wetland environmental factors,the spatiotemporal and vertical changes of biomarkers in coastal wetland sediments and their influencing factors were explored.There is a strong positive correlation between the total organic carbon content（TOC）,n-alkane content（Σn-Alk）,and absolute lignin content（Λ8）of coastal wetland surface sediments（p<0.05,p<0.05）,and all exhibit a high degree of spatiotemporall heterogeneity.Specifically,the sediment TOC andΣn-Alk content increases spatially from the inner estuarine region to the mouth region,and in time,it is higher in spring and summer than in autumn and winter,while the normalized content of lignin（Λ8）is the highest in spring.The contents of TOC,Σn-Alk,andΛ8 in the core sediments tend to decrease with the increase of depth,with the highest values appearing in the surface layer（0-5 cm）or subsurface layer（10-15 cm）.Wetland environmental factors such as water content,bulk density,salinity,p H,and particle size are the main factors affecting the spatiotemporal changes of sediment biomarkers.（2）Based on the composition characteristics of n-alkane monomer and lignin phenol monomer,the source and degradation degree of organic carbon in surface and column sediments are analyzed at the molecular level.The main source of n-alkane in surface and column sediments is long chain odd carbon,mainly from terrestrial plant inputs,while short chain n-alkane have an even carbon advantage,mainly from microbial activities.TheΣn-Alk content and molecular indicators(CPI_25³3,ACL,and TAR)of columnar sediments in vegetation areas tend to decrease with increasing depth,with the highest values appearing in the surface or subsurface.The PCA analysis results of wetland environmental factors and n-alkane molecular indicators indicate that there is a positive correlation between ACL,CPI_25³3,and TAR values,indicating that it is highly reliable to use n-alkane to identify the source of organic carbon in coastal wetland sediments.The S/V and C/V source indicators,as well as the（Ad/Al）s,（Ad/Al）v,and P/（S+V）degradation indicators,have high spatiotemporal heterogeneity.The lignin in autumn surface sediment mainly comes from the coastal woody angiosperm litter carried by rivers,and is transported over a long distance to the estuary,with a high degree of lignin degradation from this source.In summer,the lignin in surface sediments mainly comes from the input of local herbaceous angiosperms,and the degradation degree of lignin from this source is relatively low.The（Ad/Al）s,（Ad/Al）v,and P/（S+V）ratios of the column sediment are generally lower than 0.6,indicating that the degradation degree of lignin is at a medium to low level and there is no significant change in vertical distribution,revealing the relative stability of the lignin phenol monomer components in the sediment and the accuracy of source identification.Completed the carbon isotope analysis of lignin phenols,achieved a precise estimation of the contribution ratio of terrestrial C₃plants and C₄plants to sediment organic carbon,and usedδ¹³C andΛ8 indicators to conduct a three-terminal model analysis,accurately quantifying the relative contributions of organic carbon from marine（marine phytoplankton）,terrestrial（terrestrial soil）,and local plant sources（terrestrial C₃plants）.（3）Analyze the carbon isotope of lignin phenols,and further estimate the contribution ratio of terrestrial C₃plants and C₄plants to sediment organic carbon.δ¹³C andΛ8 indicators are analyzed using a three-terminal model to accurately quantify the relative contributions of organic carbon from marine（marine phytoplankton）,terrestrial（terrestrial soil）,and local plant sources（terrestrial C₃plants）.The carbon isotope composition of lignin phenol monomer shows that the input of sediment lignin was mainly from C₃plants,with a relative contribution ratio of 70.84～96.87%,which is much higher than the contribution of C₄plants.This result provides an important basis for selecting terrestrial C₃plants as local plant sources.Three-terminal model results ofδ¹³C andΛ8 indicators show that the contribution of sediment organic carbon mainly comes from land soil（48.32±8.84%）,showing a decreasing trend in space from the inner estuary area to the mouth area,while the spatial variation of the contribution ratio of marine phytoplankton（35.53±11.15%）is opposite.In vertical distribution,the high contribution ratio of terrestrial C₃plants appears in the subsurface layer of 10-15 cm,indicating that plant litter is the main carbon source of subsurface sediments.（4）Adding litter and changing salinity to simulate key scenarios of global change（saline intrusion and increased net primary productivity）,combined with online carbon dioxide（CO₂）and methane（CH₄）concentrations and their ¹³C isotope analyzer,conducte a 120 d indoor incubation experiment,explore the differences in the resistance to degradation of organic carbon from different sources,and evaluate the response of carbon sink potential to global change scenarios.The results show that,from the perspective of the variation characteristics of the content of different organic carbon molecular components,the addition of litter increases the TOC content of sediment by 5.37～206.7%,the averageΣn-Alk value increases by 26%,the average content of long chain odd carbon number normal alkanes(C₂₇+C₂₉+C₃₁)increases by60%,and the average content ofΛ8 increases by 26%,indicating that the addition of litter would enhance the carbon pool capacity of sediment.With the addition of litter,the content ofΛ8 decreases with the increase of salinity,while the degradation indicators of（Ad/Al）v and P/（S+V）are on the contrary,indicating that excessive salinity may promote lignin degradation.These results provide new insights into the degradation characteristics of organic carbon from different sources.The proportion of the increment of cumulative CO₂-C（or CH₄-C）emissions to the total cumulative CO₂-C（or CH₄-C）emissions（120 d）under litter addition is 92±5%（or 100%）,indicating that the newly added unstable organic carbon will be preferentially decomposed,weakening the carbon sink effect caused by litter input.Combined with the results ofδ¹³CO₂,it is shown that compared to no litter addition（-26.74±3.42‰）,the addition of litter results in a lowerδ¹³CO₂value generated by the sediment（-32.24±2.11‰）,confirming that the CO₂released by the sediment mainly comes from degradation of plant derived organic carbon.Adding 13.95±1.42 g of litter carbon,the net carbon sequestration capacity of the 120 d sediment of the low,medium,and high salinity groups was 7.42±1.17 g C,9.67±0.90 g C,and 10.49±1.24g C,respectively,indicating that the addition of litter enhanced the net carbon sequestration capacity of the sediment.With an appropriate increase in salinity,the increase in carbon sequestration capacity increased.