Global Warming Potential From Paddy Field Under Rice-wheat Rotation As Affected By Biochar Amendment At Different Crop Seasons

Global warming is considered as one of the global environmental issues,which was attributed to the ongoing increased concentration of atmospheric greenhouse gases?GHGs?.Global warming will lead to China's agriculture facingmany problems and temperature increases will seriously aggravate the risk of food safety.It's urgent to develop a strategyat a high level of crop productivity for facing both the future climate change and food security.So,how to ensure both crop productivity and greenhouse gas mitigations has important theoretical and practical significance.Recent scientific studies showed that biochar benefited soil structure and nutrient retention and cycling,greenhouse gas reduction,and thus being considered an effective way of GHGs mitigation while maintaining crop production.Thus,the field experiment was designed to last an entire cycle of crop rotation,that is,two cropping seasons,rice season and wheat season,and eight treatments were adopted in triplicate,i.e.Treatment RBO-NO?no N fertilization without biochar amendment in the rice season?,Treatment RB0-N1(250 kg ha^-1 N fertilizer applied and zero biochar amended in the rice season),Treatment RB1-N1(250 kg ha^-1 N fertilizer applied and 20 t ha^-1 biochar amended in the rice season),Treatment RB2-N1(250 kg ha^-1 N fertilizer applied and 40 t ha^-1 biochar amendment in the rice season),Treatment WB0-N0?no N fertilization without biochar amendment in the wheat season?,Treatment WB0-N1(250 kg ha^-1 N fertilizer applied and biochar amended in the wheat season),Treatment WB1-N1(250 kg ha^-1 N fertilizer applied and 20 t ha^-1 biochar amended in the wheat season),and Treatment WB2-N1(250 kg ha^-1 N fertilizer applied and 40 t ha^-1 biochar amended in the wheat season).Biochar was amended before rice transplanting on June 17,2012 and wheat seeding on November 24,2012.CH₄ and N₂O gas emission fluxes were monitored with the static chamber and gas chromatography method.Environmental conditions and rice-wheat rotation crop yields were observed simuataneously to evaluate greenhouse gas intensity of these different ecological systems.Main results were as following:Relative to the RB0-N1 treatment,the RB2-N1 treatment significantly improved crop yield by 17.2%,significantly reduced the total CH₄ emissions and GWP by 8.6%and 9.3%,respectively.Biochar incorporation at wheat season with 20 t ha^-1?WB1-N1?significantly improved crop yield by 21.6%,and significantly reduced GHGI by 21.7%compared with the WB0-N1.Biochar incorporation at wheat season with 40 t ha^-1?WB2-N1?significantly reduced CH₄ and N₂O emissions by 11.3%and 20.9%,respectively,and significantly reduced GWP and GHGI by 15.7%and 23.5%,respectively.Biochar incorporation at rice season with 20 t ha^-1?RB1-N1?significantly increase the N fertilizer agronomic efficiency,while biochar incorporation at wheat season with 20 t ha^-1?WB1-N1?and 40 t ha^-1?WB2-N1?both significantly increased N fertilizer agronomic efficiency.N₂O emission factors for the annul rice-wheat rotation cycle were 0.65%and 0.75%for RB0-N1 and WB0-N1 treatments respectively.Biochar incorporation at rice season or wheat season can reduce N₂O emission factors,and the emission factors reduced with the increase of biochar application rate.In terms of total GWP on a 100-year horizon,the treatments followed an order of RB0-N1>RB1-N1>WB0-N1>WB1-N1>RB2-N1>W1B2-N1>RB0-N0>RB0-N0,while in terms of GWPs per unit crop grain yield,they followed as RB0-N1>WB0-N1>RB1-N1>RB2-N1>RB0-N0>WB1-N1>WB0-N0>WB2-N1.Obviously,biochar application at wheat season is more effective than at the rice season,in reduciing CH₄ and N₂O emissions,lowering the GWP and GHGI and increasing crop yield of the annual rotation system.Biochar application at wheat season produced better results than applied at rice season.Therefore,it is proper to apply biochar at wheat season other than at rice season to improve crop production,reduce CH₄ and N₂O emissions,while simultaneously to lower the GWP and GHGI in the rice-wheat rotation system.