The Effects Of Alternate Wetting And Drying Irrigation On Water And Nitrogen Use Efficiency Of Rice And CH₄ And N₂O Emissions From Paddy Soil

Alternate wetting and drying（AWD）irrigation is one of the most important water-saving irrigation methods for rice in recent years,and it has been widely applied in China and Southeast Asia countries.The previous researches have reported that the alternate wetting and drying irrigation is beneficial to saving water and could significantly reduce nitrogen leakage and runoff loss from paddy fields.However,the temporal and spatial distribution of water resources is uneven in south China,and the region is mainly distributed with complex and fragmented terrain units such as downland and hills.These have significant effects on water and nitrogen use efficiency in paddy fields.Therefore,the effects caused by the differences in meteorological conditions and field types should be considered when alternate wetting and drying irrigation is applied.Additionally,CH₄ and N₂O are the main greenhouse gases emitted from rice cultivation,and their emissions are controlled by soil moisture.Alternate wet and dry irrigation may change CH₄ and N₂O emissions due to it cause periodic changes in soil moisture.Exploring the law of CH₄ and N₂O emission under alternate wetting and drying irrigation during rice cultivation is significant in reducing the warming potential of the paddy field.According to previous researches,the effects of alternate wetting and drying irrigation on rice production,growth status,water and nitrogen use efficiency have been summarized in this study.Two typical paddy fields（hillside-field and ridged-field）were selected for field scaled irrigation experiment to explore the effects of different water and nitrogen management on rice yield,growth status,water and nitrogen use efficiency in different rainfall years（normal-flow year and the high-flow year）and typical paddy fields.Then,the CH₄ and N₂O emissions under alternate wetting and drying irrigation during rice cultivation were investigated according to the pot experiment,and the soil substrates and abundance of microbial functional genes related to CH₄ and N₂O emissions were analyzed to reveal the mechanism of CH₄ and N₂O emissions.Furthermore,the incubation experiment was conducted to explore the law and mechanisms of CH₄ and N₂O emissions from paddy soil in different degrees of alternate wetting and drying.The main findings of this study are as follows:（1）AWD irrigation does not reduce rice yield,and the rice growth of AWD irrigation also has no difference between with flooding irrigation.The results of field research indicate that rainfall years,field types and nitrogen levels have significant effects on rice yield and growth.Ridge-field was significantly increased 11.8%rice yield compared with hillside-field.N90 and N180 nitrogen levels were significantly increased 17.1%and 28.6%,respectively,compared with NO.Wet year results in a 17.0%reduction of rice yield compared with normal year.In addition,the rice yield of hillside-field and ridge-field in normal year have no diference,but hillside-field was significantly reduced 17.9%in wet year compared with ridge-field.With N90 nitrogen level,the yield of ridge-field was increased by 15.2%compared with hillside-field,but there was no difference with N180 nitrogen level.In hillside-field,N180 nitrogen level could increased 15.5%rice yield compared with N90,but there was no difference in ridge-field.AWD irrigation could improve rice yield and promote rice growth in normal year while more nitrogen should apply in hillside-field during wet year.（2）AWD irrigation could significantly reduced 19.7%irrigation water while improved 26.1%irrigation water production（IWP）.Furthermore,irrigation water was reduced by 28.3%in wet year and the IWP was increased by 22.3%,but the total water productivity（WP）was reduced by 43.1%.The ridge field was reduced 26.6%irrigation water while the IWP and WP were improved 57.0%and 22.3%,respectively.Nitrogen level has no significant effect on rice irrigation water use efficiency.Application efficiency（AE）was varies significantly during rice season.The AE of re-greening and tillering stage was significantly higher than other period.In addition,wet years,ridged fields,and AWD irrigation can effectively reduce the loss of irrigation water and improve AE.The AE in ridge-field with AWD irrigation during wet year was significantly higher than other treatments.Combined water productivity（IWP and WP）and application efficiency,AWD irrigation could significantly improve irrigation water use efficiency of paddy fields.（3）AWD irrigation does not affect the nitrogen uptake of rice while increased nitrogen recovery efficiency（REN）by 32.5%,but does not affect the nitrogen agronomic use efficiency（AEN）,nitrogen partial factor productivity（PFPN）,and nitrogen harvest index（HIN）.Rainfall years,field types and nitrogen levels have significant effects on rice nitrogen uptake and nitrogen use efficiencies.Normal year,ridge-field,N90 and N180 nitrogen level significantly increased,rice nitrogen uptake by 12.7%,13.2%,35.8%,and 49.9%,respectively.Rice nitrogen uptake was significantly inhibited in wet years（especially in the fields）.Rice nitrogen use efficiencies were reduced with the increase of nitrogen level.Rice nitrogen use efficiencies among different rainfall years and field types were significant different,and the variation showed different rule.Generally,AWD irrigation could improve the absorption and utilization of nitrogen by rice compared with flooding irrigation,and the impact of rainfall and field type should be considering to improve rice nitrogen use efficiency.（4）AWD irrigation could significantly reduce CH₄ emissions while increased N₂O emissions compared with flooding irrigation,but the warming potential of the paddy field does not increase.AWD irrigation reduced 87.1%CH₄ cumulative emissions on average,but the N₂O cumulative emissions increased by 181.8%.Especially,the N₂O emission peak of AWD irrigation was 5.6 times that of flooding irrigation after the first topdressing.Low nitrogen levels could significantly reduce CH₄ and N₂O emissions.Compared with the N180 nitrogen level,N90 reduced 25.0%and 54.3%CH₄ and N₂O cumulative emissions,respectively.Mid-season drainage just reduced 47.8%CH₄ cumulative emissions.According to the law of CH₄ and N₂O emission during rice cultivation,it is suggesting that keeping filed flooded during apply nitrogen fertilizer,and AWD irrigation or intermittent flooding should be applied during other periods to reduce the warming potential of the paddy soil.（5）The variation of soil substrates（NH₄⁺,NO₃^-,and DOC）and the abundance of microbial functional genes（McrA,pmoA,amoA,nirS,nirK and nosZ）related to CH₄ and N₂O emissions during rice cultivation were tallying to CH₄ and N₂O emissions.The soil substrates and microbial functional genes were significantly affected by irrigation methods and nitrogen levels,and they also varied significantly in different rice growth stages.The gene abundance of pmoA in soil with AWD irrigation was significant higher than McrA both in tillering and heading-filling stage.It results in no CH₄ emissions owing to that methane oxidation was dominant under AWD irrigation.The gene abundance of McrA in soil with flooding irrigation was significant higher than pmoA in heading-filling stage,and the accumulation of soil DOC also provided enough substates for CH₄ emissions.The N₂O emissions during tillering stage from AWD irrigation and flooding irrigation are mainly produced by nitrification（amoA）,and it was stronger in AWD irrigation.The difference in the abundance of nirK gene indicates that the denitrification of nitrifying bacteria is also the main approach of N₂O production at the tillering stage.The concentration of soil substrates（especially NH₄⁺）and the abundance of microbial functional genes related to N₂O emission during the heading and filling period were significantly reduced,which was the main reason that there was no N₂O emission during this period.（6）Paddy soil hardly emitted CH₄ with different degrees of AWD（AWD30,AWD 50 and AWD 100）,but the N₂O emissions of AWD30 and AWD50 were significantly increased compared with flooding.The variation of soil substrates（NH₄⁺,NO₃^-,and DOC）and the abundance of microbial functional genes（McrA,pmoA,amoA,nirS,nirK and nosZ）related to CH₄ and N₂O emissions under different water conditions were tallying to CH₄ and N₂O emissions.The gene abundance of pmoA in soil was always higher than McrA under different degrees of AWD,and methane oxidation dominates.However,the gene abundance of McrA increased significantly with the increase of flooding time and the add nitrogen while the pmoA decreased,these results in that methanogenesis gradually increases.The addition of nitrogen produced the N₂O emission peak in all water treatments.The significant increase in the gene abundance of nirS indicated that denitrification was the main approach of N₂O generation under flooding conditions.The abundance of amoA gene increased significantly in different degrees of AWD,and NH₄⁺was consumed and NO₃^-was accumulated,these indicated that nitrification（ammoxidation）is the main approach of N₂O emissions.In addition,the variation of nirS,nirK and nosZ genes in AWD30 indicate that the denitrification of nitrifying bacteria is also the main approach of N₂O production,and more nitrogen is lost in the form of N2,which results in less N₂O emissions compared with AWD50.