Conservation tillage is an important practice to improve agricultural soil fertility. However, the effects of this practice on crop nitrogen uptake and grain yield remain unknown. Here, a 2-year field experiment was conducted to investigate the effects of different tillage (conventional intensive tillage [CT] and no-tillage [NT]) and straw returning methods (preceding crop straw returning [S] and removal [NS]) on soil N_2O emission, root nitrate reductase and glutamine synthetase activities, nitrogen uptake of rice plants and grain yield in the 2016 and 2017 rice growing seasons at Huaqiao town, Wuxue county, Hubei province. The tillage practices and straw returning methods had significant effects on the N_2O emission from paddy soil. Compared with CT treatment, NT treatment enhanced the N_2O emission by 12.5%-18.2% in 2016 and 21.1%-38.6% in 2017. S treatments increased the soil N_2O emission by 38.5%-45.5% in 2016 and 13.1%-29.5% in 2017 as compared with NS treatments. Straw returning methods had significant effects on root nitrate reductase and glutamine synthetase activities, as well as on nitrogen uptake of rice plants and grain yield. Compared with NS treatments, S treatments had 6.7%-45.9% higher root nitrate reductase activity and 9.0%-46.7% higher root glutamine synthetase activity, resulting in 12.5%-26.0% higher nitrogen uptake of rice plants and 9.4%- 12.6% greater grain yield. Our results indicate that straw returning significantly increases nitrogen uptake and grain yield, and also promotes soil N_2O emissions, suggesting that the effects of long-term NT and straw returning on global warming potential and soil carbon sequestration should be taken into account when assessing the global warming potential of these practices.