| Coastal wetlands are among the most productive and valuable ecosystems worldwide,and their carbon sink is 30-50 times than that of terrestrial forests.Soil organic carbon(SOC)pool fixed by the process of carbon accumulation and burial accounts for 49-98%of coastal wetlands’ carbon pools,which is the major carbon sink in coastal wetlands and is considered as a natural climate solution with high effectiveness and low cost.However,intensive anthropogenic activities in coastal area has caused extensive coastal wetlands losses over the past 50 years,such as more than 60%of global mangroves lost.The vulnerable blue carbon pools in coastal wetlands expose to severe climate risks with increased sea level rise(SLR)and climate warming,threating the climate adaptation of coastal blue carbon.Therefore,it is a hotspot of academic concern and major strategic needs to explore the spatio-temporal evolution of SOC stock and its drivers,clarify key factors affecting the carbon accumulation rate,and evaluate the climate resilience of carbon pool in coastal wetlands.In this thesis,bibliometric method was used to analyze blue carbon literature and propose relevant scientific questions and design research framework firstly(Chapter 1).China’s coastal wetlands(mangrove,salt marshes and mud flats)in 1990 and 2020 were interpreted by random forest algorithm based on the Google Earth Engine platform,and their spatial change characteristics over recent 30 years were analyzed(Chapter 2).SOC spatial prediction model in coastal wetlands was constructed by boost regression tree method to map high-resolution(30 m)and pixel-level SOC stock in China’s coastal wetlands(Chapter 3).The carbon flow network integrated land-sea-scape was built based on interpreted China’s reclamation datasets to explore SOC change and its drivers in China’s coastal wetlands over last three decades,and the concept of irrecoverable carbon was quoted to calculate the recoverable and irrecoverable SOC stocks in coastal wetlands under reclamation(Chapter 4).Finally,climate resilience index was established to reveal the mechanism of climate resilience to future climate change in China’s coastal wetlands,proposing future carbon sequestration and pathways to climate-resilient enhancement in coastal wetland restoration(Chapter 5).This thesis proposed the following scientific questions:(1)What are the SOC stock and spatial distribution characteristics of blue carbon in China’s coastal wetlands?What are the key drivers affecting the spatial change of SOC stock,and what role does reclamation play in this process?(2)What are the key drivers affecting SOC sequestration capacity in China’s coastal wetlands under future climate changes?How to identify the climate-resilient carbon pool in China’s coastal wetlands?The major conclusions in this thesis are as follows:(1)The SOC stock in China’s coastal wetlands was 89.5 Tg C,with a spatial distribution characteristic of "High in the south-Low in the north".The SOC stock in mangrove,salt marshes and mud flats were 3.6 Tg C,8.8 Tg C and 77.1 Tg C,respectively,accounting for 86.1%of the total coastal SOC stock,demonstrating great potential in future climate change mitigation and carbon sink increase.(2)The area of coastal wetlands losses driven by reclamation(urban development and aquaculture)in mainland China was 2306.7 km2 over recent 30 years,accounting for a quarter of the existing coastal wetlands,and 80%of the losses were in mud flats.As a result,19.4 Tg C of the SOC in coastal wetlands were lost,accounting for 21.7%of the total SOC stocks,and 71.2 Tg of CO2 emitted.Through mangrove restoration and exotic wetland species introduction,the area of mangrove and salt marshes increased by 1055.7 km2,which offset the loss of coastal wetlands by 45.8%,but the increased of SOC stock only offset the loss by 6.5%.(3)The irrecoverable SOC stock in China’s coastal wetlands was 30.1 Tg C,accounting for 33.6%of total SOC stock.The characteristic of "high stock and low accumulation" in mangrove determined 58.3%of its SOC pool were irrecoverable.Conversely,the characteristic of "low stock and high accumulation" in salt marshes determined only 13.6%of SOC pool were irrecoverable.(4)Only 9%of irrecoverable carbon in China’s coastal wetlands was protected,therefore it’s urgent to adopt different interventions according to the relevant climate risks and socioeconomic backgrounds in different regions.The analysis of trade-off and synergy relationship between biodiversity and carbon sink was suggested to expand existing protection area in a region with high-density irrecoverable SOC(e.g.,Luoyuan Bay in Fujian,Huizhou Bay in Guangdong,Nanliu River estuary and Qin River estuary in Guangxi,Qinglan Bay in Hainan).(5)The key factors affecting the SOC accumulation in mangrove and salt marshes were mean annual temperature,mean annual sea surface temperature,relative sea level rise,and tidal range.Higher recoverability and resilience of coastal wetlands to human disturbances and climate changes was in Jiulong River estuary,Liao River estuary and Yangtz River estuary.These regions were suggested to be considered as the potential priority area of blue carbon sink increase in future development plans.However,low recoverability and resilience were found in Pearl river estuary,Nanliu River estuary,Qin River estuary and Dongzhai Harbor.These regions should be promoted as the potential priority area of blue carbon conservation to avoid anthropogenic destruction and natural degradation,enhancing adaptation and resilience to warming and sea level rise.In this thesis,a high-resolution,pixel-level map of SOC stock in China’s coastal wetlands was firstly developed using integrated method of remote sensing and machine learning.A carbon flow network integrated land-sea-scape was innovatively built to identify the spatial drivers of SOC stock change in China’s coastal wetlands.An improved climate resilience quantitative method in coastal wetlands was proposed to reveal the mechanism of coastal wetlands resilience in response to future climate changes.Conclusions from this thesis could provide scientific evidence for informing coastal restoration and spatial planning,improving the function of ecosystem carbon sinks and implementing China’s goals of CO2 emission reduction and sink enhancement. |
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