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Methane and N2O are two important greenhouse gases, which are the main emission in paddy ecology system. China is the highest rice planting country, so the greenhouse gas emission is huge in China. Application of biochar in paddy field can decrease the emissions of greenhouse gases. In this study, we investigated the greenhouse gas (GHG) emissions, carbon sequestration and economics of different rates of biochar application in a double rice cropping system. The experiment was carried out in rice variety area promotion experiment field in Huizhou of Guangdong from March 2012 to September 2015. Four treatments were designed as CK (famer traditional operation without biochar application), BC1 (biochar amendment at 5 t/hm²), BC2 (biochar amendment at 10 t/hm²), and BC3 (biochar amendment at 20 t/hm²), each treatment has 3 repeat. Biochar used in this study was obtained from wheat straw and produced by Henan Sanli New Energy Company. LCA (life cycle assessment) method was employed to calculate GHG emissions from the collection and transportation of raw materials, the production, transportation, and broadcasting of biochar, and from the avoidance of the straw burning and the soil carbon stocks. Static chamber-GC (gas chromatography) method was used to monitor the CH4 and N2O emissions under different rates of biochar application from 2012 to 2015. Then, the net GHG emissions and the percentage of emission reduction in the paddy field with different biochar amendment rates were calculated. The total GHG emissions from the double rice cropping system were 19.5, 15.6, 16.1, and 12.4 t/hm2 under CK, BC1, BC2, and BC3 treatments respectively in four years. Moreover, the total GHG emission reduction percentage of BC1, BC2 and BC3 were 19.70%, 17.46%, and 36.40%, respectively. Considering the GHG emissions of biochar in all stages of life cycle, the net GHG emissions of CK, BC1, BC2 and BC3 in the four years were 19.5, 20.3, 10.9, and 4.2 t/hm², respectively, the treatment of BC1 increased the net emissions of 4.3% in the four years. BC2 and BC3 reduced the net GHG emissions by 44% and 78.6%, respectively in four years. The economic analysis indicated that the BC1treatment had more economic benefits compared to other treatments. The different biochar amendment rates significantly reduced the GHG emissions from the double rice cropping system, the highest application rate of biochar had the best emission reduction effect. The LCA of biochar application indicated that from medium to high application levels, biochar reduces emissions, for higher application rates of biochar, the effects are stronger. Moreover, the economic analysis indicated that the more amounts of biochar application, the least profitable to gain economic benefits. [ABSTRACT FROM AUTHOR]