Objective Ecosystem services are the link between ecosystems and social systems. While effectively coordinating regional ecological, social and economic needs and promoting carbon sequestration and emission reduction, ecosystem services can be transmitted to surrounding areas to boost regional ecological space optimization. Under the guidance of the carbon peaking and carbon neutrality goals, clarifying the positive impact of ecosystem services on net carbon sink efficiency in metropolitan areas and the spillover effect of ecosystem services can effectively contribute to regional ecosystem service enhancement, and realize efficient carbon sequestration and reduction in ecological space.

Methods Supported by multi-source panel data spanning the period from 2010 to 2020, this research takes the Shanghai Metropolitan Area as the research object and divides the research area into 40 research units. Based on the multiple benefits of ecosystem services in synergistically promoting urban sink enhancement and emission reduction, this research constructs a net carbon sink efficiency indicator system. Then, utilizing the undesirable slacks-based measurement (SBM) model, the research evaluates the net carbon sink efficiency of each unit during the period from 2010 to 2020 by setting up inputs, desired outputs, and undesirable outputs, and further explores the distributional characteristics and spatial-temporal changes of carbon sinks, carbon emissions, and net carbon sink efficiency from the geospatial perspective. In combination with the guiding content of spatial synergistic planning for the Shanghai Metropolitan Area, four important ecosystem services, namely water retention, water purification, soil retention and biodiversity maintenance, are quantitatively characterized with the InVEST model. Subsequently, based on the spatial decomposition effects (direct, indirect and total effects) obtained with spatial econometric model, the influencing mechanisms of ecosystem services and their interrelationship on the net carbon sink efficiency of 40 research units are analyzed. In addition, the spatial spillover effects of ecosystem services are innovatively revealed according to ecosystem service flow conduction mechanisms.

Results Research results are summarized as follows. 1) During the 11 years from the 2010 to 2020, the growth of carbon dioxide emissions in the Shanghai Metropolitan Area gradually slowed down, while the net primary productivity of vegetation continued to increase, and the areas with high carbon emissions and high carbon sinks were partially overlapped; in addition, the net carbon sink efficiency of some core nodes, such as Shanghai City, maintained a steady improvement, effectively driving neighboring cities to reduce carbon emissions and increase carbon sinks; meanwhile, the areas with improved net carbon sink efficiency have some similar characteristics and can be divided into 2 types: Areas with high production value, high carbon emissions, and high carbon sinks, and those with medium-high production value, low carbon emissions, and medium-high carbon sinks. 2) The four ecosystem services have significant spatial heterogeneity and relatively stable changes over the 11-year period, with the high values mainly distributed in the southwestern part of the area with high vegetation cover and the area around the Taihu Lake with concentrated water resources, while the low values mainly distributed in the concentrated urban construction areas and near the regional traffic arteries, and the total amount of the four ecosystem services has shown fluctuating characteristics. 3) Regarding the spatial decomposition effects of ecosystem services on net carbon sink efficiency, there are differences in the coefficients, directions and significance of the spatial effects of different ecosystem services. The direct and total effects of population density are significantly positive, the direct effect of built-up area ratio is significantly negative, the total effect of water retention services is significantly positive, the direct and total effects of water purification services are significantly negative, the direct and total effects of biodiversity maintenance and soil retention services are significantly negative, and the indirect effects are significantly positive. For the ecosystem service trade-off index and relationship index, the direct effects are significantly positive, while indirect effects significantly negative.

Conclusion The research clarifies that water-related ecosystem services such as water retention and water purification services can significantly affect carbon reduction and sink enhancement in the Shanghai Metropolitan Area, and attention should be paid to water network system and its coupling effects with green and grey spaces, so as to further stimulate the ecological vitality of Jiangnan water vein. As there are differences in the spillover effects of different ecosystem services, it is necessary to differentiate the optimization and enhancement strategies for each type of ecological space and its ecosystem services according to local conditions, and the conservation of important ecological spaces in the metropolitan area should be continuously strengthened, followed by joint protection and control of ecological red lines in neighboring areas, so as to promote territorial spatial carbon reduction and sink enhancement activities, thus contributing to the steady improvement of the net carbon sink efficiency of the Shanghai Metropolitan Area in general. The research clearly demonstrates the positive effects of enhancing water-related ecosystem services and conserving important ecological spaces on regional carbon sinks and reduction, and effectively reveals an effective path for synergistic carbon reduction in the region, which may provide certain reference for improving territorial spatial management.