Abstract: The reclamation of wetlands for agricultural uses affects carbon cycling directly, how-ever, previous studies ignored carbon emissions caused by human activities such as fertilizer production and use of agricultural machinery. To more thoroughly understand the changes in ecosystem carbon emission caused by land reclamations, we used flux observation and life cycle assessment to estimate total ecosystem carbon emission with and without accompanying carbon emissions due to human activities on Chongming Island. The results showed that croplands acted as carbon sink if only carbon fluxes between the ecosystems and atmosphere were considered. However, carbon emission of croplands increased by 10.47 t (CO₂-eq)·hm^-2 compared to wetlands. When carbon emissions caused by human activities were considered, total carbon emission of wetlands and reclaimed croplands respectively were -15.38 and 6.54 t (CO₂-eq)·hm^-2, and the carbon emission due to wetland reclamations increased by 21.92 t (CO₂-eq)·hm^-2, the carbon emissions caused by human activities were 11.45 t (CO₂-eq)·hm^-2. The carbon emissions of agricultural material production and rice cultivation accounted for 84.6% of the total emission in their life cycles, and thus the production and application of fertilizer were largely responsible for the carbon emission. In sum, we need to reassess reclamation effects because it changed carbon sink/source properties at ecosystem and even regional scales. And we should reduce fertilizer use and improve fertilizer use efficiency to achieve low-carbon agriculture.