Global climate change has aroused widespread interest in reducing carbon emissions and increasing carbon sequestration.  Thus, an urban carbon inventory must consider both emissions and sequestration. In this context, we analyzed the main  contributors to the flows that comprise a city's carbon metabolic processes employing methods and concepts from  ecological science. The carbon emissions and sequestration by urban carbon metabolic processes can be compared to  ecological catabolism and anabolism, respectively. We used empirical coefficients to estimate the rates of carbon  catabolism and anabolism and calculate the resulting carbon imbalance index. Our analysis reveals the contributions of  individual metabolic actors and the distribution of the metabolic flows among them. Taking Beijing as  case study, we  found that the catabolic rate of the metabolic actors was more than five times the anabolic rate from 1995 to 2010, leading  to a carbon imbalance index that was twice the average Chinese level. The major catabolic actors were the other services  and domestic sectors. These catabolic rates were primarily influenced by the flows of electricity, heating energy  consumption, and mobile energy consumption. The overall carbon imbalance resulted from greatly reduced metabolic flows  in farmland anabolism due to conversion of farmland into urban land. Identifying the metabolic actors and flows in this  manner will inform government mitigation efforts by identifying where reduction is required and guiding planning of  appropriate mitigation actions. Our study also provides directions for conservation of the urban ecological environment.