Global change was emerged as a scientific issue since 1980s, and hitherto has become a political, economic and diplomatic issues affecting worldwide, beyond the scientific aspect. Under the support of the National Key Research and Development Program of China during the 13rd five-year period, 31 scientists from six institutions (including Institute of Applied Ecology of CAS, Northwest AF University, Institute of Botany of CAS, etc.) have been performing a project “Coupling of carbon and nitrogen, source and sink of carbon in forest and grassland ecosystems of northern China” since July 2016. The objectives of the project in the study region were: 1) to assess the role of succession of plant communities in the balance of uptake and emission of three greenhouse gases (CO2, N2O and CH4), particularly the role as a potential carbon sink in the future; 2) to identify and map the sensitive and vulnerable areas as the transient status of shift from carbon sink to source or from the source to sink; 3) to develop the methodologies assessing the historical changes in carbon and nitrogen cycling in response to nitrogen deposition, rising atmospheric CO2 concentration and climate changes at different time scales using mulitple stable isotope techniques; 4) to reveal the response patterns of coupled carbon and nitrogen cycles to nitrogen deposition, global warming, fire disturbance and vegetation succession, and their biological mechanisms involved in the response patterns. After five-year efforts through the project, it will be expected to enhance the understanding on biological mechanisms of coupled carbon and nitrogen cycles, and to reduce the uncertainties in predicting the responses of forest and grassland ecosystems in the study region to global changes in the future.