In this study, two saline mangrove artificial wetlands, Datang Saline Constructed Wetland (DSCW) created for treating mariculture wastewater and sewage, vegetated with Avicennia marina, and Mangrove Wetland Park (MWP) created for mangrove conservation, vegetated with Rhizophora stylosa, were selected for assessment of carbon sequestration and carbon budget based on measuring greenhouse gas (GHG) emissions and net primary productivities. The average GHG flux and net carbon sequestration flux as carbon dioxide equivalent (CO2 eq.) were measured. The results showed that the GHG flux emitted from DSCW and MWP were 2,128 and 2,148 g CO2 eq./m2-yr, respectively, while the flux of net sequestered carbon was 2,909 and 3,178 g CO2 eq./m2-yr, respectively, which achieved carbon budget values of −676 and −230 g CO2 eq./m2-yr, respectively, exhibiting carbon source effects. Some amounts of N2O, with a high global warming potential of 265, emitted from both artificial wetland systems might cause high GHG flux as CO2 eq. emitted from the wetland systems. It was concluded that both the nitrogenous contents and environmental conditions suitable for microbial production of N2O might be the main factors to change the wetland systems from carbon sinks to carbon sources.