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To evaluate the impacts of organic cropping system on global warming potentials (GWPs), field measurements of CH and NO were taken in conventional and organic rice ( Oryza sativa L.) cropping systems in southeast China. Rice paddies were under various water regimes, including continuous flooding (F), flooding-midseason drainage-reflooding (F-D-F), and flooding-midseason drainage-reflooding and moisture but without waterlogging (F-D-F-M). Nitrogen was applied at the rate of 100 kg N ha, as urea-N or pelletized, dehydrated manure product in conventional or organic rice paddies, respectively. Seasonal fluxes of CH averaged 4.44, 2.14, and 1.75 mg m h for the organic paddy plots under the water regimes of F, F-D-F and F-D-F-M, respectively. Relative to conventional rice paddies, organic cropping systems increased seasonal CH emissions by 20%, 23%, and 35% for the plots under the water regimes of F, F-D-F, and F-D-F-M, respectively. Under the water regimes of F-D-F and F-D-F-M, seasonal NO-N emissions averaged 10.85 and 13.66 μg m h in organic rice paddies, respectively, which were significantly lower than those in conventional rice paddies. The net global warming potentials (GWPs) of CH and NO emissions from organic rice paddies relative to conventional rice paddies were significantly higher or comparable under various water regimes. The greenhouse gas intensities were greater, while carbon efficiency ratios were lower in organic relative to conventional rice paddies. The results of this study suggest that organic cropping system might not be an effective option for mitigating the combined climatic impacts from CH and NO in paddy rice production. [ABSTRACT FROM AUTHOR]