Effect Of Nitrogen Application Rate And Panicle Removel On Methane Emissions Form Paddy Fields And Underlying Mechanism

Rice is one of the main food crop and plays an important role in ensuring food security in China and the world.However,paddy fields are also a main source of atmospheric methane emissions.Realizing the synergization of high yield with low methane emissions from paddy fields is of great significance for ensuring China’s food security and achieving the“dual carbon”target.Previous studies have shown that high-yield varieties often have low methane emissions,but the underlying mechanism is still unclear.Nitrogen fertilizer application can increase yield by regulating the source-sink relationship of rice.In addtion,diffrent nitrogen application rates also affect the methane emissions from paddy fields.In this study,two experiments were conducted under field conditions.The source-sink relationship and assimilate distribution were regulated by nitrogen fertilizer application rate and panicle removal.Furthermore,the relationship of methane emissions from paddy fields with source-sink characteristics and rice yield was investigated,with an aim to clarify the effects of source-sink characteristics on methane emission from paddy fields and yield formation.In experiment 1,three nitrogen application treatments were set,consisting of no nitrogen（0N）,medium nitrogen treatment(150N,150 kg N ha^-1)and high nitrogen treatment(210N,210 kg N ha^-1).In experiment 2,two panicle removal treatments were set,namely,removal of half panicle（S1）and removal of whole panicle（S2）.The main results are as follows:1.The 150N treatment significantly reduced the cumulative methane emissions from paddy fields,meanwhile,the yield was also the highest under 150N.The 210N treatment did not result in more yield but significantly increased methane emissions.Two panicle removal treatments（reduction of sink）significantly reduced grain yield and increased methane emissions.Compared with 0N treatment,there was no significant difference in cumulative methane emissions under 150N.However,the grain yield was significantly increased by 16.7%,resulting in 12.6%decrease in cumulative methane emissions per unit yield.Compared with 210N treatment,the cumulative emissions of methanes from paddy fields under 150N treatment decreased by 31.4%,while the yield increased by 7.2%,resulting in a decrease of 32.2%in the cumulative emissions of methane per unit yield.Compared with the control,S1 resulted in a 52.9%decrease in grain yield.However,the cumulative methane emissions increased by305.5%,and the cumulative methane emissions per unit yield also increased by 761.4%.When the panicle were completely removed,the rice yield was zero,and the cumulative methane emissions was 340.0%higher than control.These data indicated that appropriate nitrogen application may increase rice yield and reduce methane emissions from paddy fields.Increasing rice sink capacity is conducive to reducing methane emissions from paddy fields.2.High nitrogen application rate and two panicle removal treatments reduced the root-shoot ratio and grain-leaf ratio of rice plants,and significantly increased methane emissions.Compared with 0N treatment,150N and 210N treatments reduced the root-shoot ratio by 9.9%and 34.2%,respectively.The ratio of spikelet number to leaf area,ratio of full grain number to leaf area,and ratio of full grain weight to leaf area decreased by 26.6%,30.4%,29.1%under 150N treatment and 46.6%,49.6%,48.6%under 210N treatment,respectively.Compared with the control（without panicle removal）,the root-shoot ratio decreased by 3.5%under S1 and 11.2%under S2.The ratio of spikelet number to leaf area,the ratio of full grain number to leaf area,and the ratio of full grain weight to leaf area decreased by 49.9%,49.5%and 69.0%under S1.Under different nitrogen application rates and panicle removal treatments,the root-shoot ratio was negatively correlated with the cumulative methane emissions from paddy fields.The ratio of full grain number to leaf area was significantly and negatively correlated with the cumulative methane emissions from paddy fields.These data indicated that source-sink characteristics affected the methane emissions from paddy fields.Increasing the root-shoot ratio and grain-leaf area ratio may reduce methane emissions.3.High nitrogen application rate and two panicle removal significantly increased the total organic carbon and carbohydrates in root exudates.Compared with 0N treatment,there was no significant change in total organic carbons and carbohydrates in root exudates under 150N treatment.However,the total organic carbon and carbohydrates in root exudates under 210N treatment increased significantly by 23.7%and 48.7%,respectively.Compared with the control（without panicle removal）,the total organic carbon and carbohydrates in root exudates increased by 73.7%and 281.5%under S1,respectively.Compared with the control（without panicle removal）,the total organic carbon and carbohydrates in root exudates increased by 108.0%and 579.7%under S2,respectively.Under different nitrogen application rates and panicle removal treatments,the total organic carbon and carbohydrates in root exudates were significantly and positively correlated with cumulative methane emissions.These data indicated that 210N treatment and panicle removal increased sources and reduce sinks,so more assimilates that were allocated to roots and secreted into rhizosphere soils as root exudates,thereby increased methanogenic substrates and promoted methane emissions.4.High nitrogen application rates and two panicle removal treatments significantly increased the content of soluble organic carbon in rhizosphere soil.Compared with 0N the content of soluble organic carbon in rhizosphere soil under high nitrogen treatment increased by 30.4%under 150N treatment and 59.2%under 210N treatment.Compared with the control,removals of panicle increased the dissolve organic carbon in rhizosphere soil by 46.8%under S1 and 107.3%under S2.Under different nitrogen application rates and panicle removal treatments,the soil soluble organic carbon content was positively correlated with the cumulative methane emissions.These data indicated that increasing sources and reducing sinks increased root exudates and soluble organic carbon content in rhizosphere soil,thereby promoting methane emissions from paddy fields.