Effect Of Biochar On CH₄ And N₂O Emissions From Paddy Soil Under Elevated Air Temperature And Atmospheric CO₂ Concentration

The globe is experiencing the climate change characterized by rising air temperature and atmospheric carbon dioxide(CO2)concentration.Methane(CH4)and nitrous oxide(N2O)have greater global wanning potential(GWP)than CO2.The concentrations of CH4 and N2O in the atmosphere have continued to rise since the 21st century.Paddy ecosystem is the important anthropogenic emission source of CH4 and N2O.Elevated air temperature and atmospheric CO2 concentration may lead to the emissions increase of CH4 and N2O from paddy ecosystem,which would result in climate change intensifying.Soil amendment with biochar derived from the agricultural waste biomass is considered to be a promising approach for carbon sequestration and mitigation in the agroecosystems.Whether biochar would play the role in greenhouse gases(GHGs)reduction is of great significance for mitigating global warming under the condition that the air temperature and atmospheric CO2 concentration will continue to rise in the future.In response to this question,a rice pot cultivation experiment with rice straw derived biochar was conducted in the microclimate system platform where the conditions of elevated air temperature(+3℃)and elevated atmospheric CO2 concentration(700 ppm)were set for the simulation.The effect of biochar on CH4 and N2O emissions from paddy soil under elevated air temperature and atmospheric CO2 concentration was revealed through the analytical techniques of chemistry and molecular biology.The results provided a theoretical basis for the research and development of new technologies for controlling GHGs emissions in paddy ecosystem under climate change or within some different latitudes regions or climate zones.The primary results of this study were summarized as follows:(1)Biochar amendment had the mitigation effect on CH4 and N2O emissions from paddy soil under elevated temperature and CO2.The closed chamber technique was used to systematically investigate the responses of CH4 and N2O emissions to biochar amendment during the rice growth season under elevated temperature and CO2.Under ambient temperature and CO2 during the peak periods of CH4 and N2O emission from paddy soil with biochar amendment,the cumulative CH4 emissions decreased significantly by 89.0%,the cumulative N2O emissions increased significantly by 173.4%.The GWP and GHGI of CH4 and N2O during the whole rice growth season were reduced by 87.2%and 90.8%with biochar amendment under the ambient condition,respectively.Under simultaneously elevated temperature and CO2 during the peak periods of CH4 and N2O emission from paddy soil with biochar amendment,there was a significant decrease of 38.5%in the cumulative CH4 emissions but no significant increase in the cumulative N2O emissions.The GWP and GHGI of CH4 and N2O during the whole rice growth season were reduced by 36.2%and 48.4%with biochar amendment under the simultaneously elevated condition,respectively.(2)Improving the internal methane oxidation in rice rhizosphere soil during the peak period of CH4 emission was the key to reducing CH4 emissions from paddy soil with biochar amendment under elevated temperature and CO2.Through using fluorescence quantitative PCR and Roche 454 sequencing technology,it was found that biochar amendment could significantly decrease the genes abundance ratio of methanogenic archaea to methanotrophic bacteria(16S rRNA/pmoA)and increase the relative abundance of type I methanotrophic bacteria(MOB)as well as ensure the methane oxidation rate of type II MOB in rice rhizosphere soil during the peak period of CH4 emission.Combining analysis of soil physical and chemical properties were used to investigate that bichar amendment resulted in a increase in soil moisture content and a decrease in soil DOC content during the peak period of CH4 emission,which chould contribute to the potential basis for changing the microbial community structure of MOB in rice rhizosphere soil.On this basis,the reduction effect of biochar on CH4 emissions might be attributed to the increase extents of soil pH and NO3--N content induced by biochar amendment under elevated temperature and CO2.(3)Weakening ammonia oxidation in rice rhizosphere soil during the peak period of N2O emission is the key to controlling N2O emissions from paddy soil with biochar amendment under elevated temperature and CO2.Fluorescence quantitative PCR combining analysis of soil physical and chemical properties were used to investigate that biochar amendment could increase N2O emissions generated through nitrification by enhancing ammonia oxidation with increasing soil pH and amoA gene abundance,as well as soil moisture content and NO2--N content in rice rhizosphere soil during the peak period of N2O emission.Under simultaneously elevated temperature and CO2,the liming effect of biochar was neutralized,which might contribute to the attenuated stimulating effect on N2O emissions.(4)Biochar in paddy soil could have the good stability in the short term under elevated temperature and CO2.Based on the analysis of elemental composition,TG-DTG,FTIR and XPS,it was shown that biochar had obvious aromatization structure and the thermal stability in paddy soil under elevated temperature and CO2 The ratio of H/C and O/C of biochar was less than 0.45 and 0.48,respectively.However,the oxidation rate of biochar was likely to be important for the potential role of biochar in GHGs mitigation under elevated temperature and CO2.The oxidation rate of biochar might be accelerated by the promotion of rice plant growth under elevated CO2,which could weaken the potential capacity of biochar for GHGs mitigation.