Objective  The urban area serves as the main carrier of carbon emission. In China, the area of urban spaces accounts for 1.2% of the total territorial area, contributing to nearly 80% of national total carbon emissions. At present, there are two practical problems with respect to the carbon budget accounting of urban space. First, in terms of accounting scale, most of the existing researches are conducted at the national, regional, provincial, watershed or any other regional macroscale. Second, in terms of carbon budget accounting method, existing researches mainly adopt such methods as inventory measurement, vorticity correlation, ecosystem process simulation and emission factor accounting. Therefore, at present, there is still not a comprehensive carbon budget accounting method that considers carbon source and carbon sink factors at the urban scale. In this regard, this research hopes to establish a theoretical model of urban carbon budget by clarifying the cyclic metabolic relationship between carbon emission and carbon sequestration capacity of different land cover types within the scope of urban area. Based on the high-precision land cover data obtained, the research explores a urban-scale carbon budget accounting method.

Methods  Firstly, at the urban scale, the research collects the data on fossil energy consumption related to land cover, power consumption and industrial land that can reflect urban carbon sources. Secondly, the research superimposed land cover data reflecting ecological carbon sinks to form a data base for urban carbon budget accounting that is based on land cover type and associated with direct and indirect energy consumption data, and at the same time clarifies the carbon emission and carbon sink coefficients of elements involved in the urban carbon budget system. Thirdly, according to the action mechanism of direct carbon emission, indirect carbon emission and carbon sink in the process of urban carbon budget, the research calculates the carbon sinks of each type of land cover and the total urban carbon sinks, as well as the direct and indirect carbon emissions at each carbon source and the total urban carbon emissions and, based on the offset conversion relationship therebetween, obtains the accounting results of the overall urban carbon budget. Finally, the research analyzes the urban-scale spatial distribution characteristics of carbon sources and sinks by associating the accounting results of carbon emissions with corresponding urban geographical locations.

Results  The research studies and constructs a theoretical model of carbon budget at the urban scale, and proposes relevant methods for carbon budget accounting at the urban scale by taking land cover in the urban area as the basic factor of carbon emission accounting. Nanjing is selected as an example to demonstrate the aforesaid method, with the total emissions of carbon sources and carbon sinks in the city being calculated respectively, based on which the total amount of the city’s net carbon emissions is obtained through the overall carbon budget accounting.

Conclusion  Based on the theoretical model of urban carbon budget and related accounting methods mentioned above, the research conducts an overall carbon budget accounting based on the data on basic land cover and indirect energy consumption at the mesoscale, which improves existing carbon budget accounting methods that mainly focus on regional macroscale or single-building microscale to a certain extent, while providing a methodological support for low-carbon urban planning and sustainable development.