Study Of The Spatial Characteristics Of Carbon Flux At Regional Scale In Typical Coastal Wetlands Using UAV-based Eddy Covariance Observation

Studies show that global warming will continue to increase with the excessive emission of greenhouse gases,mainly CO₂.Coastal wetlands are located between land and ocean,mainly including three types of ecosystems:salt marsh wetlands,seagrass beds,and mangroves.They have the characteristics of both land and ocean ecosystems and have a high carbon sink,which is an effective blue carbon sink to mitigate global warming.As an emerging observation tool,the airborne eddy correlation turbulent flux observation technology,can directly observe the CO₂fluxes of terrestrial ecosystems at the regional scale and realize the accurate estimation of regional carbon balance.Using airborne eddy-related systems to directly observe carbon fluxes in typical ecosystems of coastal wetlands and calculate the carbon flux intensity of different surface types,we can provide accurate data support for the assessment of carbon source/sink functions in coastal wetlands,carbon accounting and carbon neutral planning,and serve the"dual carbon"strategy of China.In this study,an unmanned aerial vehicle（UAV）eddy-related flux observation system was used as a means to study the core area of Jiangsu Yancheng Wetland Rare Bird National Nature Reserve（Yancheng Reserve for short）and the sea area of Caofeidian in Tangshan,Hebei Province,and the salt marsh wetland ecosystem and seagrass bed ecosystem in the two study areas were taken as typical research objects of coastal wetlands.Based on the land use cover data interpreted from remote sensing images,the corresponding flight routes were designed respectively,and the reliability of this system was evaluated by ground eddy observation data.The regional-scale surface carbon flux observation data are demixed using a combination of footprint model and multiple linear regression to obtain the component carbon flux intensities of different land types.The carbon sink in the study area were estimated based on the results of carbon flux intensities of different land types.Finally,the influence of environmental factors on the net ecosystem CO₂exchange was explored.The main research contents and results are as follows:1.The UAV eddy-related flux observation system has good applicability in turbulent flux observation.The time series of sensible heat flux,latent heat flux,carbon dioxide flux and friction wind speed observed by the UAV eddy-related flux observation system and the ground eddy system are compared,and the results show that the UAV eddy-related flux observation system can effectively identify the time variation trend of the surface flux;The R²between sensible heat flux,latent heat flux,carbon flux and frictional wind speed are 0.97,0.92,0.66 and 0.70,respectively,indicating that the observation results between the two systems are in good agreement.The differences between the two observations are mainly related to the observation instruments,observation methods,surface source areas and surface heterogeneity.2.The CO₂ fluxes in the core area of Yancheng reserve and the sea area of Caofeidian in Tangshan have obvious spatial heterogeneity.In December 2020 in the core area of Yancheng reserve,the carbon flux intensity of Phragmites australis,cropland,Spartina alterniflora was-5.45±1.00μmol·m^-2·s^-1,-3.20±0.98μmol·m^-2·s^-1,-1.64±1.30μmol·m^-2·s^-1,all three land types showed carbon sinksand high reliability of observations（P（27）0.05）,and the carbon flux intensity of water was1.1±2.20μmol·m^-2·s^-1,which behaved as a carbon source（P（29）0.05）.The integrated sum of carbon flux intensities of the four major land types was-9.17±1.08μmol·m^-2·s^-1.The carbon sink in the core area of Yancheng Reserve in December2020 is 327.5 kg;In September 2022 in Tangshan Caofeidian sea area,the carbon flux intensity of seagrass bed,aquaculture was-4.08±1.82μmol·m^-2·s^-1,-5.68±1.24μmol·m^-2·s^-1,both of which behaved as carbon sinks（P（27）0.05）,and the carbon flux intensity of seawater was 1.32±0.69μmol·m^-2·s^-1,which behaved as carbon sources（P（27）0.05）.The integrated sum of carbon flux intensities of the three major land types was-8.44±3.06μmol·m^-2·s^-1.The carbon sink in the monitoring area of Caofeidian in September 2022 is 920.5 kg.3.The net ecosystem carbon exchange of salt marsh wetland ecosystem in the core area of Yancheng reserve during daytime hours in December 2020 is mainly driven by temperature and light radiation;the net ecosystem carbon exchange of seagrass bed ecosystem in Caofeidian sea area during daytime hours in September2022 is mainly driven by temperature,light radiation,and wind speed.In summary,the CO₂ fluxes in the core area of Yancheng reserve and the sea area of Tangshan Caofeidian have obvious spatial heterogeneity,and the CO₂source/sink intensities of different land types differ significantly,and the CO₂source/sink intensities within the same land type also have some variability.Therefore,in order to obtain accurate carbon balance of coastal wetlands,we should pay attention to the monitoring and accounting of carbon source/sink of different types of coastal wetlands at the regional scale.The observations used in this study can not only provide technical reference for the direct observation of carbon fluxes at the regional scale,but also provide reliable data support for the observation and assessment of carbon source/sink in coastal wetlands at the regional scale by monitoring the carbon source/sink of each site type and the regional carbon sinks.