Effects Of Straw Return On Greenhouse Gas Emissions From Wheat Soil Under Elevated CO₂ Concentration And Temperature

Since the industrial revolution,the emissions of a variety of greenhouse gases（GHG）（CO₂,CH₄,N₂O,etc.）have increased significantly under the joint action of human activities and nature,resulting in global warming and an increase in the frequency of extreme weather.The greenhouse gases nitrous oxide（N2O）and methane（CH₄）have a greater warming potential than carbon dioxide（CO₂）,and their atmospheric concentrations have continued to increase since the 21 st century.Agricultural activities are one of the important sources of GHG emissions,and the wheat field ecosystem is an important anthropogenic source of N₂O emissions.With the atmospheric CO₂ concentration and temperature continue to increase in the future,straw return is considered to be a promising measure for carbon sequestration and mitigation in cropland ecosystem.However,under the background of elevated atmospheric CO₂ concentration and temperature,whether straw return can effectively play a role in reducing greenhouse gas emission is of great significance to mitigate global warming.In this study,the controled chambers were used to simulate the effect of straw return under elevated atmospheric CO₂ concentration and temperature（600μmol·mol-1,+2 ℃）on greenhouse gas emissions from wheat fields.To investigate the effects of different straw treatments（straw removal,straw incorporation,straw mulching）to the wheat soil on the CH₄ and N₂O emissions and accociated microbiological mechanism under elevated atmospheric CO₂ concentration and temperature by the combination of the chemical and molecular biological analysis technology.The main results of this study were as follows:（1）Straw return increased the average emission flux of soil N₂O and CH₄ under CK,EC,ET,and ECT by increasing soil C and N substrates,soil moisture,soil temperature and denitrification gene abundance,as well as the abundance of denitrifying bacteria Pseudomonas and methanogenic bacteria g_Methanobacterium,reducing the abundance of the N₂O reducing bacteria Bradyrhizobium and the anaerobic methane oxidizing bacteria g_Candidatus_Methylomirabilis and the aerobic methane oxidizing bacteria g_Methylocella.The average emission flux of soil N₂O increased by straw mulching was greater than that of straw incorporation,but there was no significant difference between the two straw return methods on soil greenhouse gas emission.Metagenomic analysis also showed that straw return increased the abundance of bacterial eutrophy-bacteria community,decreased the abundance of oligotrophic bacteria community,and increased the functional contribution of the abundance of Proteobacteria and Bacteroidetes to carbon and nitrogen metabolism,and decreased the functional contribution of the abundance of Actinobacteria.Therefore,straw return promoted the decomposition and utilization of original soil organic matter and newly added substrate,accelerated soil carbon and nitrogen cycle,and increased N₂O and CH₄ emissions in wheat soil.（2）Under elevated CO₂ concentration,the two straw return treatments increased the aboveground biomass of wheat,promoted nitrogen uptake by wheat,decreased soil NO3-content,reduced the competitive inhibition between NH₄+ and CH₄ oxidation in soil in 2021,increased soil CH₄ uptake.Metagenomic analysis showed that elevated CO₂ concentration decreased the abundance of denitrifying bacteria Pseudomonas and methanogenic bacteria g_Methanobacterium,increased the abundance of N₂O reducing bacteria Bradyrhizobium,the aerobic methane oxidizing bacteria（g_Methylocaldum,g_Methylocella,g_Methylosarcina,g_Methylohalobius）,and the anaerobic methane oxidizing bacteria g_Candidatus_Methylomirabilis,indicated that elevated CO₂ concentration reduced soil denitrification and methanogenesis processes,promoted soil N₂O reduction to N2 and soil methane oxidation process,leading to the reduction of N₂O emissions from wheat soil in the two years and CH₄ emissions from soil in 2021.Elevated [CO₂] had little effect on the average CH₄ emission flux of the wheat season in 2020.Straw mulching significantly reduced the average N₂O emission flux under elevated [CO₂] both in the two years,and the reduction degree was greater than that of straw incorporation.Both the two straw return methods decreased the average CH₄ emission flux,but there was no significant difference.（3）Elevated temperature decreased the content of soil moisture（SMC）,soil organic carbon（TOC）and total nitrogen（TN）,and decreased the ratio of nitrogen functional genes（nir K + nir S）/nos Z by 17.6%,thereby decreasing soil N₂O emissions.In addition,compared with the two straw returning methods under CK,the reduction range of SMC,TN,（nir K + nir S）/nos Z ratio was greater under straw mulching than that of straw incorporation under elevated temperature,so the reduction range of straw mulching on the average N₂O emission flux was greater than that of straw incorporation.Elevated temperature decreased SMC content both in the two years,significantly increased the abundance of pmo A genes involved in methane oxidation pathway in 2020,thus the total CH₄ emissions were decreased in wheat soil in 2020 and 2021.（4）The average N₂O emission flux in wheat soil under the combination of elevated [CO₂] and temperature was higher than that of single elevated [CO₂] and elevated temperature in the two years.The combination of elevated [CO₂] and temperature increased TOC content in the two years,and increased SMC,NIR activity,and NO3-content in 2021,provided substrate and suitable environment for soil microbial methanobacteria,resulting in an increase in N₂O emissions in 2021 and CH₄ emissions in wheat soil in the two years.In 2020,the combination of elevated CO₂ concentration and temperature significantly decreased the content of SMC and NH₄+,and NIR activity,which ultimately led to a slight decrease in soil N₂O emissions under ECT in 2020.（5）Straw return increased soil organic carbon sequestration（SOCS）in wheat soil under climate change,with an increase range of 0.11-1.08 t C ha-1 and 0.07-0.62 t C ha-1 in the two years,respectively.Compared with straw removal,straw return increased soil greenhouse gas（N2O,CH₄）emissions in the two years,but significantly increased soil organic carbon sequestration,improved grain yield,and ultimately reduced net global warming potential（NGWP）and greenhouse gas intensity（GHGI）.Under the elevated[CO₂] and temperature,straw return stimulates soil greenhouse gas（N2O and CH₄）emissions,but climate change can be mitigated by increasing soil organic carbon sequestration and improving grain yield.Among them,straw incorporation has a greater increment of soil SOCS under EC and ECT,and the decrease in NGWP and GHGI is greater than that of straw mulching.Therefore,straw incorporation method is more effective under future warming and elevated CO₂ concentration.In conclusion,straw return will stimulate soil GHG emissions in wheat cropping systems,whereas it reduce NGWP and GHGI by increasing soil SOCS and improving grain yield under future climate change especially straw incorporation.Straw return is conducive to mitigating climate change under future warming and elevated CO₂ concentration.