A three-class carbon pool system for normalizing carbon mapping and accounting in coastal areas

Climate change is a key issue receiving increasing global attention. Coastal areas play an important role in mitigating climate change because of enormous potentials for carbon sequestration. Simulating and evaluating the carbon storage, increment, and value of coastal areas is of considerable importance in achieving carbon neutrality. In this study, the South Coast of Hangzhou Bay in China was selected as the study area. A three-class carbon pool system was established based on vegetation and soil and considering the coastline as the boundary for comprehensively revealing the carbon sink in coastal areas and accurately distinguishing the roles of different carbon pools. Then, the maps of carbon storage and increment were generated by integrating the field and remote sensing data through “from point to area” spatial simulations. An accounting list was developed for systematically summarizing, precisely quantifying, and visually displaying the carbon storage, increment, and value following the three-class carbon pools. Results indicated that the carbon sink showed a gradient change from sea to land. Salt marshes had low storage but high increment, whereas areas above the coastline had the opposite characteristics. The total carbon storage of vegetation and soil were 1417.94 Gg and 3359.07 Gg, respectively, and their carbon increments during 2011–2022 were 309.17 Gg/yr and 29.46 Gg/yr, respectively. The carbon storage per area of vegetation and soil were 27.66 Mg/ha and 53.09 Mg/ha, respectively, and their carbon increments during 2011–2022 were 6.03 Mg/(ha yr) and 0.47 Mg/(ha yr), respectively. As the main body of blue carbon, salt marshes have achieved distinctly higher increment than the adjacent areas above the coastline. The carbon increment per area of soil was lower than 1/5 of vegetation in salt marshes. Mudflats had low carbon increment but large storage because of the extensive area. Alien species had higher increment in terms of vegetation but similar one from the perspective of soil compared with native species, denoting the low decrease of soil carbon sink when eliminating alien species. Considering the hazard of alien species to the health of coastal wetlands, this indicates the feasibility and necessity to develop the high-quality carbon sink in the study area. The carbon sink value based on the carbon stored in the soil summed to 1.68 million yuan, and were increased by 33%, 7%, 48%, and 45% in scenarios of baseline scenario, economic development, ecological protection, and comprehensive optimization, respectively. The present study validated the high carbon increment and immense increase potential and provided an applicable system to normalize carbon mapping and accounting in coastal areas.