Characteristics And Mechanism Of Nitrous Oxide Emission From Ratoon Rice Fields Under Different Water Treatments

N₂O is one of the major greenhouse gases,and rice fields are considered as a major source of nitrous oxide emissions.At present,a number of studies have shown that different irrigation treatments significantly affect the CH₄ emissions of the paddy.The alternate wetting and drying treatment（AWD）irrigation mode is mostly used in rice production,but the effect of AWD and N₂O emission is still unclear.Therefore,exploring the effect of irrigation on characteristics of N₂O emission in ratoon rice field and revealing its mechanism are beneficial to reduction of paddy greenhouse gas emissions.Two cultivars,Liangyou 6326 and Huanghuazhan,were planted in farm fields under continuous flooding（CF）and AWD,and N₂O emission and soil characteristics were investigated in both the main season and ratoon season.The main results are as follows:1.Compared with CF,AWD increased the N₂O emission flux by 13.1%and 37.3%in main and ratoon season in Liangyou 6326,and by 1.5%and 3.8%in Huanghuazhan,respectively.Compared with CF,AWD increased the N₂O cumulative emission by 18.6%and 19.8%in main and ratoon seasons in Liangyou 6326,and by 7.0%and 10.9%in Huanghuazhan,respectively.N₂O emission flux in the main and ratoon seasons of Huanghuazhan were higher than that of Liangyou 6326 under CF and AWD,respectively.These results indicated that N₂O emission flux and cumulative emissions in the main and ratoon seasons under AWD were higher than that under CF.2.Compared with CF,AWD decreased the mean soil moisture content over whole main season and ratoon season by 17.4%and 12.0%in Liangyou 6326,respectively,and by 29.5%and 29.6%in Huanghuazhan.Compared with CF,AWD increased the mean soil redox potential over main season and ratoon season by 118.0%and 284.1%in Liangyou6326,and by 146.9%and 319.1%in Huanghuazhan,respectively.Compared with main season,the mean soil redox potential in ratoon season increased by 29.1%and 623.3%in Liangyou 6326 under CF and AWD,and by 5.9%and 338.9%in Huanghuazhan,respectively.Correlation analyses showed that there was a negative correlation between N₂O emission flux and soil moisture content,and a positive correlation between N₂O emission flux and redox potential.The results showed that AWD enhanced N₂O emission characteristics byreducing soil water moisture and increasing soil redox potential.3.NH₄⁺-N contents of soil in all growth stages were lower under AWD than those under CF,and correlation analyses showed that there was a negative correlation between N₂O emission flux and NH₄⁺-N content,but the difference was not significant.Compared with CF,soil NO₃^--N content under AWD were lower during panicle initiation stage,heading stage,maturity stage,heading stage in ratoon season and maturity stage in ratoon season,and correlation analysis showed that there was a negative correlation between N₂O emission flux and NO₃^--N content,but the difference was not significant.The results showed that AWD decreased soil inorganic nitrogen content of and promoted paddy N₂O emission during ratoon season.4.Soil urease and nitrite reductase activities gradually increased from tillering stage,and reached the highest activity during heading stage in main season,then gradually declined from heading stage in main season to maturity of ratoon season under both CF and AWD,respectively.Soil catalase activity decreased from tillering stage to heading stage in main season and then gradually increased from heading stage of main season to ratoon season maturity,the changes were similar under CF and AWD.The results showed that AWD and CF had no influence on the activities of soil urease,nitrite reductase and soil catalase.Correlation analyses showed that N₂O emission flux was no significantly correlated with soil urease activity,nitrite reductase and catalase activity during the whole growth period under two water treatments.In conclusion,N₂O emission in paddy was reduced under CF;N₂O emission in the ratoon season was significantly lower than that in the main season;N₂O emission of Huanghuazhan was significantly lower than that of Liangyou 6326.AWD treatment enhanced N₂O emission by increasing the soil redox potential,reducing the content of soil ammonium and nitrate nitrogen.