The recent global climate change is presumably linked to increased atmospheric CO_2 caused by anthropogenic activities. Therefore, the research on global carbon cycling attracts tremendous attention not only because it is related to science but also related to economy and society. This kind of research involves interactions among atmosphere, geosphere and the ocean. The carbon flux exported from land to ocean is equivalent to the land-sea interface and air-sea interface. However the major organic carbon derived from land is modified sharply when they are exported from rivers to coastal oceans, which convert the carbon sink into carbon source in the estuary. In such a scenario, the carbon cycling research between land and sea is therefore critically important in order to understand the processes and mechanisms behind it. The framework of microbial carbon pump (MCP) sheds light on the source and fate of recalcitrant dissolved organic carbon, which also is applicable to the soil system. Meanwhile, a new scientific understanding is thatcarbon fixation is not equal to carbon storage and carbon pool is not equal to carbon sink, which could provide insight into strategies to increase carbon sink. This scientific understanding provides practical and consistent foundation for the research and potential ecology management on carbon cycling between land and sea. Future endeavor in deciphering the mechanisms of carbon sink requires an interdisciplinary approach and the cooperation of industry, researchers, policy maker and customer.