The open ocean, coastal ocean and terrestrial ecosystem are three major components modulating atmospheric CO₂ and thereby the earth climate system. The complexity of carbon cycling in the coastal ocean under the impact of both land inputs and dynamic exchanges with the open ocean make it a huge challenge to be included in any realistic prognostic climate simulations. CHOICE-C II is a five-year (January 2015 to August 2019) interdisciplinary research project involving 19 PIs/CoIs from 4 institutes in Mainland China and Hong Kong, renewed by the Ministry of Science and Technology of China based on the CHOICE-C (Carbon Cycling in China Seas: Budget, Controls and Ocean Acidification) project. Through comparative studies of carbon cycling in River-dominated Ocean Margins (RiOMar, the northern South China Sea shelf being a case) and Ocean-dominated Margins (OceMar, the South China Sea basin being a case), CHOICE-C II focuses on the carbon cycle in the South China Sea in terms of its budget, controls and global implications. The key question we sought to answer is: “Why are some marginal seas a source of atmospheric CO₂, while others are a sink?” So far, we have conducted extensive interdisciplinary mapping measurements and process studies and have developed a novel coupled physical-biogeochemical modelling system to diagnose factors controlling the CO₂ source and sink in the South China Sea. This poster session presents our current achievements from four interlinked tasks in CHOICE-C II to determine (1) CO₂ source and sink patterns and its major biogeochemical controls; (2) Carbon fixation of phytoplankton and its controls in the carbon cycle; (3) Recycling and export of organic carbon and coupling between C, N and Si; and (4) Physical controls, synthesis and future trends.