Study On Nitrogen Loss And Greenhouse Gas Emissions From Paddy Field With Modified DNDC Model

Paddy rice is one of the main staple crops in the world and widely cultivated in Asia.China is the largest rice producer around the world and produced 30%of global rice production with 20%of rice cultivation area.Investigation showed that the high application rates of nitrogen?N?fertilizer contributed a lot to the high rice yields.However,the relatively lower N use efficiency in Chinese paddy fields means that a great majority of fertilizer N has lost into environment via different pathways,and then cause a lot of environment pollution issues.For examples,N loss through surface runoff and subsurface leaching will lead to water body pollutions like eutrophication of surface water and nitrate contaminations in ground water.N loss via nitrous oxide?N₂O?emissions contributed highly to global warming.The impacts of rice production on local and global environments are not negligible in consideration of the increasing food demand for the rapidly growing population in China.In this study,a 2-year field experiment was conducted with a lysimeter system at a paddy field in the suburb of Shanghai,China.N loss and greenhouse gas?GHG?emissions from the paddy field were simultaneously observed with different fertilizer managements.In addition,a biogeochemical model,Denitrification-Decomposition or DNDC model was adopted in this study to explore the best fertilizer management practice in paddy field.1.A 2-year lysimeter experiment was conducted in a paddy field to study on the impact of different fertilizer managements on N loss in surface runoff and subsurface leaching.The results indicated that N loss from paddy field was sensitive to fertilization and precipitation.The relationship between surface runoff?Y?and precipitation?X?can express with equation Y=0.91X-12.78.Compared with subsurface leaching,N loss through surface runoff was the main pathway of N loss from paddy field.During the 2-year field observation,application of urea?CT?has led to the highest N loss of 18.79 kg N/ha from paddy field.Compared with CT,MT and OT,which applied with organic manure has reduced N loss by 21.86%and 30.41%,respectively.However,N loss across the treatments only accounted for 3.12%⁵.02%in total fertilizer N,which means N loss via water movements not the major pathway of N loss from paddy field.In addition,the observed data in lysimeter study has used for DNDC model calibration and validation.2.GHG emissions from paddy field with different fertilizer managements were collected with static chambers and analyzed with gas chromatograph?GC?system.The field data indicated that fertilization has increased CO₂ emissions from paddy field,but no significant difference was observed among different treatments.CH₄ and N₂O emissions were sensitive to fertilization and water managements in paddy field.Application of organic manure?MT and OT?has increased CH₄ emissions from paddy field,while application of urea?CT?significantly enhanced N₂O emissions.OT led to the highest seasonal CH₄ emissions of283.02 kg/ha,while CT output the highest seasonal N₂O emissions of 1.81 kg/ha.Global warming potential?GWP?is an index used for the evaluation of combined effects of GHG emissions on global warming.The study results showed that fertilization significantly increased GWP in paddy field,especially in which organic manure was applied.During the experimental period,OT treatment output the highest GWP of 7.24×10³ kg CO₂/ha.3.Calibration and validation are the pre-conditions to apply DNDC model in agriculture ecosystems.According to the configurations and water managements in paddy fields,several parameters were modified in original DNDC model.The observed data of N loss and GHG emissions in lysimeter study were used for the calibration and validation tests.The validation results indicated that the revised DNDC model well simulated water movements in surface runoff and subsurface leaching from paddy field.Due to the water movements were the driven forces of N loss from paddy fields,the revised DNDC model not only captured the N loss dynamics,but also accurately estimate seasonal N loading from paddy field.In addition,the modification on DNDC model in this study also enhanced the simulation on rice yields and GHG emissions with different fertilizer managements.The simulated results showed that application of organic manure could reduce N loss from paddy field,which in consistent with field observations.In general,the revised DNDC model proved to be an effective tool for carbon and nitrogen cycle simulations in paddy fields.4.With revised DNDC model,several simulation tests including fertilization scenarios simulation,region simulation and sensitive analysis were conducted to explore the best fertilization method in Shanghai paddy fields.The simulated results indicated that the best application rate of urea in rice cultivation was 250 kg N/ha.The best application rate of organic manure was 300 kg N/ha,but could obtain the optimal rice yields only after 20 years'cultivation.The combination use of urea?150 kg N/ha?and manure?100 kg N/ha?proved to be the best fertilization method in paddy fields,which could significantly reduce N loss when maintain the optimal rice yields.The region simulation results showed that the rice cultivation in Shanghai output a large amount of contamination loadings including 1.14×10³t N loss,3.10×10⁴ t CH₄ emissions and 1.61×10² t N₂O emissions in 2013.If apply the best fertilization method in Shanghai paddy fields,the rice production in Shanghai could reduce N loading by 4.58×10² t while maintain the optimal rice yields.In addition,the sensitive analysis results indicated that fertilization,precipitation and soil characteristics were the major sensitive factors which significantly impact C and N cycles in paddy fields.