Effects Of Urease/Nitrification Inhibitors On Rice Yield And Greenhouse Gas Emissions

Rice is one of the main food crops in China,the cultivated area of which accounts for about 27%of the total cultivated area in China.Many problems occurred in rice production in China,such as excessive fertilization,high water consumption,low water and nitrogen（N）utilization efficiency and high greenhouse gas emission.Improving water and N use efficiency and reducing greenhouse gas emissions are of great significance for high-yield,high-efficiency and environmental-friendly rice production in China.The effects of nitrifying inhibitors（DMPP）and urease inhibitors（NBPT）on agronomic traits such as rice yield,N use efficiency,soil N conversion,soil urease activity and greenhouse gas emissions were investigated.The effects of nitrifying inhibitors on rice agronomic traits,root morphology and physiology,soil N,soil urease activity,and greenhouse gas emissions were studied under alternate drying and wetting irrigation（AWD）to clarify the synergistic and emission reduction mechanisms of the two inhibitors.The results of this study may provide a theoretical and practical basis for high-yield,high-efficiency and low-carbon rice cultivation.The main results are as follows:1.Compared with urea application（CK）,urea combined with nitrifying inhibitors（NI）or urease inhibitors（UI）could improve the productive tillers,leaf area index,and SPAD value of sword leaves after the heading stage of rice.The total number of spikelets and setting rate of rice increased in various degrees by NI and UI,leading to a yield increase of 5.57%～8.35%and 9.08%～11.4%,respectively.Compared with CK,urea combined with NI could significantly inhibit soil nitrification,increase soil ammonium nitrogen（NH4+）content and decrease soil nitrate nitrogen（NO3-）content,but had no significant effect on soil total N and soil urease activity.Application of NI increased the agronomic efficiency,N uptake efficiency,partial N productivity and physiological efficiency by 13.4%～19.4%,12.8%～13.6%,5.57%～8.35%and 0.50%～5.11%,respectively.Application of UI significantly inhibited soil urease activity within 5 days after fertilization,maintaining a relatively durable soil NH4 supply due to the inhibited urea hydrolysis,and decreasing the soil NO3-content.The agronomic,N uptake,N partial productivity and physiological utilization of rice were increased by 21.8%～26.6%,16.6%～17.2%,9.08%～11.4%and 4.48%～8.01%by UI,respectively.2.Under conventional irrigation,two peaks of methane（CH4）emission occurred during the rice whole growth period.The first peak appeared at the tillering stage,and the second peak appeared at the booting stage.Compared with no N application,N application reduced the second CH4 emission peak and the total CH4 emission significantly.Compared with CK,urea application combined with NI and UI had no significant effect on the first CH4 emission peak but significantly reduced the second CH4 emission peak.The application of NI and UI decreased the total CH4 emissions by 6.45%～7.50%and 8.80%～9.30%,respectively.Nitrogen fertilizer increased the N2O emissions,while the application of NI and UI could inhibit N2O emissions notably.Compared with CK,NI and UI reduced N2O emission by 49.4%-49.5%and 17.7%～20.9%,respectively.Nitrification inhibitors had a better reduction effect on N2O emissions.The global warming potential（GWP）of CH4 contributed more than 93.1%to the comprehensive warming potential of rice fields,indicating CH4 was the main greenhouse gas in paddy fields.There were no significant differences between the two inhibitors in GWP and greenhouse gas emission intensity（GHGI）in paddy fields.3.Compared with conventional irrigation,AWD decreased panicle number per unit area and the total number of spikelets but increased the grain number per panicle and setting rate under the same N application rate.The AWD regime had no prominent effect on rice yield but increased the irrigation water use efficiency by 37.4%～37.8%.The combination of AWD and NI could promote rice tillering,increase grain number per panicle,the total number of spikelets,dry matter accumulation and stem non-structural carbohydrate translocation at maturity,leading to a yield increase by 5.35%～8.08%.AWD promoted soil nitrification and increased soil urease activity.Compared with conventional irrigation,AWD significantly reduced soil NH4 content and increased soil NO3-content.AWD also improved the activities of N metabolism enzymes in rice roots and leaves.The combination of AWD and NI could maintain a high soil NH4+ content and low soil NO3-content,which was beneficial to the absorption and accumulation of N,and significantly improved the N use efficiency of rice.4.AWD enhanced soil permeability,oxidative capacity and root dry weight.Compared with conventional irrigation,AWD reduced total CH4 emission by 41.7%-44.4%under the same N application rate.The combination of AWD and NI could further reduce CH4 emissions.Compared with conventional irrigation,AWD stimulated the N2O emissions of paddy fields under the same N application rate,increasing the total N2O emissions by 74.7%-88.9%.The N2O emission flux increased during the soil drying and rewetting process.Compared with conventional irrigation,AWD combined with NI reduced the total N2O emission by 6.81%～9.68%.Under conventional irrigation,the warming potential of CH4 accounted for 91.9%～99.1%of GWP,while the warming potential of N2O contributed little to GWP.Under AWD conditions,CH4 accounted for 78.4%～87.5%of the GWP,and N2O contributed 12.5%～21.6%of the GWP.Compared with conventional irrigation,AWD reduced GWP by 29.9%～33.0%and GHGI by 32.3%～33.0%under the same N application.AWD combined with NI reduced GWP by 42.8%～45.0%and GHGI by 47.0%～47.8%,indicating the combination of AWD and NI had the potential to further mitigate the greenhouse gas emissions in paddy fields.