Objective In view of the urgent necessity to achieve the carbon peaking and carbon neutrality goals and establish an effective ecological compensation system, it is of crucial significance to evaluate the carbon sink value With the increasing concerns about climate change and the pursuit of sustainable development, accurately evaluating the carbon sink value becomes essential for understanding the role of different ecosystems in carbon cycling. However, it is highly remarkable that in the current research on ecological compensation of carbon sink, the urban carbon sink value has been frequently overlooked, which is rather concerning given the rapid expansion of urban areas and their growing impact on the environment. Therefore, this research endeavors to address this issue by proposing a method for calculating the composite carbon sink value of urban areas, which is based on the distinct characteristics of urban ecosystems, with the ultimate aim of comprehensively uncovering the true composite carbon sink value of cities.

Methods On the basis of the analysis of carbon sink elements in urban spaces, a carbon storage estimation model for vegetation, soil and buildings is established by combining remote sensing imagery with plot data. This model is used to calculate the natural and artificial carbon storage within the Shenyang Fourth Ring area from 2003 to 2023. The afforestation cost method, carbon tax method, and carbon trading method are adopted to evaluate the composite carbon sink value of the research area. Using different methods, the research analyzes the spatiotemporal variations of the composite carbon sink value as well as the changing contributions of various carbon sink elements. Additionally, the research conducts preliminary discussions on the relationships between different carbon pricing calculation methods, changes in carbon sink value, and urban development, and compares the carbon sink values of urban spaces with those of other ecosystems.

Results The results indicate that from 2003 to 2023, within the Shenyang Fourth Ring area, the composite carbon sink value calculated by the afforestation cost method ranges from RMB 599 million to RMB 2.5 billion, while that calculated by the carbon tax method ranges from RMB 646 million to 864 million, and that calculated by the carbon trading method ranges from 623 million to RMB 833 million. There are significant differences in the evaluations of carbon sink value among different methods, suggesting that there is still room for increasing the carbon trading price and carbon tax price in China according to related research. Over the period from 2003 to 2023, the urban composite carbon sink value kept increasing. The vegetation carbon sink value and soil carbon sink value presented a fluctuating trend, while the building carbon sink value had been consistently rising. The proportion of carbon sink value contributed by vegetation and soil decreased from 43.61% and 46.15% in 2003 to 34.70% and 36.62% respectively, whereas the proportion contributed by buildings increased from 10.24% to 28.70%.

Conclusion The important contribution of urban space to carbon sinks in terrestrial ecosystems should not be ignored. Although the total composite carbon sink value of urban areas is slightly inferior to that of other terrestrial ecosystems such as forests, it has a certain advantage in average annual growth. This implies that urban areas, despite their different characteristics compared to natural ecosystems, still play a meaningful role in carbon sink. The evaluation of urban composite carbon sink value can reveal the situation of urban spatial carbon assets, clarify the values of various carbon sinks in urban areas, and provide support for further exploring the impacts of the conflict between natural and artificial spaces and urban environmental changes on carbon sink value. It also provides new theories and methods for enhancing urban composite carbon sink efficiency and serves as a reference for land use decision-making and planning for comprehensive benefits. The impact of various human activities on urban composite carbon sink value is complex, and identifying the key impact factors should be the focus of future research. This is because human activities such as urban construction, transportation, and industrial production can have both positive and negative impacts on carbon sink. Understanding these impacts and identifying the key impact factors will enable more effective management of urban carbon sink. Regarding the choice of different methods, the afforestation cost method is relatively unsuitable for evaluating urban carbon sink value. Meanwhile, the carbon tax method and the carbon trading method also have limitations in the application process. In the future, when regulating the carbon trading price and formulating the carbon tax rate, reference should be made to the afforestation cost of carbon sinks to make it more adaptable to the evaluation of urban composite carbon sink value. This will ensure that the methods used for evaluating urban carbon sink value are more accurate and reflective of the actual situation, thereby facilitating better decision-making in the context of urban carbon management.